Low Pin Count Demo Board
User’s Guide
© 2005 Microchip Technology Inc.
DS51556A
Note the following details of the code protection feature on Microchip devices:
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Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
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dsPIC, KEELOQ, microID, MPLAB, PIC, PICmicro, PICSTART,
PRO MATE, PowerSmart, rfPIC, and SmartShunt are
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dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR,
FanSense, FlexROM, fuzzyLAB, In-Circuit Serial
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Printed on recycled paper.
Microchip received ISO/TS-16949:2002 quality system certification for
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October 2003. The Company’s quality system processes and
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and manufacture of development systems is ISO 9001:2000 certified.
DS51556A-page ii
© 2005 Microchip Technology Inc.
LOW PIN COUNT DEMO BOARD
USER’S GUIDE
Table of Contents
Preface ........................................................................................................................... 1
Chapter 1. Low Pin Count (LPC) Demo Board Overview
1.1 Introduction ..................................................................................................... 7
1.2 Highlights ........................................................................................................ 7
1.3 Devices Supported by the LPC Demo Board ................................................. 7
1.4 LPC Demo Board Overview ........................................................................... 8
1.5 Running the PICkit™ 2 Flash Starter Kit Default Demonstration ................... 8
Chapter 2. Mid-Range PICmicro® Architectural Overview
2.1 Introduction ..................................................................................................... 9
2.2 Memory Organization ................................................................................... 10
2.3 Instruction formats ........................................................................................ 10
2.3.1 Assembler Basics ...................................................................................... 11
Chapter 3. LPC Demo Board Lessons
3.1 Introduction ................................................................................................... 13
3.2 LPC Demo Board lessons ............................................................................ 13
3.2.1 Lesson 1: Hello World (Light a LED) ....................................................................... 14
3.2.2 Lesson 2: Delay Loop (Blink a LED) ....................................................................... 15
3.2.3 Lesson 3: Rotate (Move the LED) ........................................................................... 17
3.2.4 Lesson 4: Analog-to-Digital ..................................................................................... 19
3.2.5 Lesson 5: Variable Speed Rotate ........................................................................... 22
3.2.6 Lesson 6: Switch Debouncing ................................................................................ 23
3.2.7 Lesson 7: Reversible Variable Speed Rotate ......................................................... 25
3.2.8 Lesson 8: Function Calls ........................................................................................ 26
3.2.9 Lesson 9: Timer0 .................................................................................................... 27
3.2.10 Lesson 10: Interrupts ............................................................................................ 29
3.2.11 Lesson 11: Indirect Data Addressing .................................................................... 31
3.2.12 Lesson 12: Look-up Table (ROM Array) ............................................................... 33
Appendix A. Hardware Schematics
A.1 Introduction .................................................................................................. 37
Worldwide Sales and Service .................................................................................... 38
© 2005 Microchip Technology Inc.
DS51556A-page iii
Low Pin Count Demo Board User’s Guide
NOTES:
DS51556A-page iv
© 2005 Microchip Technology Inc.
LOW PIN COUNT DEMO
BOARD USER’S GUIDE
Preface
NOTICE TO CUSTOMERS
All documentation becomes dated, and this manual is no exception. Microchip tools and
documentation are constantly evolving to meet customer needs, so some actual dialogs
and/or tool descriptions may differ from those in this document. Please refer to our web site
(www.microchip.com) to obtain the latest documentation available.
Documents are identified with a “DS” number. This number is located on the bottom of each
page, in front of the page number. The numbering convention for the DS number is
“DSXXXXXA”, where “XXXXX” is the document number and “A” is the revision level of the
document.
For the most up-to-date information on development tools, see the MPLAB® IDE on-line help.
Select the Help menu, and then Topics to open a list of available on-line help files.
INTRODUCTION
This chapter contains general information that will be useful to know before using the
Low Pin Count (LPC) Demo Board. Items discussed in this chapter include:
•
•
•
•
•
•
•
About this Guide
Warranty Registration
Recommended Reading
Troubleshooting
The Microchip Web Site
Development Systems Customer Notification Service
Customer Support
DOCUMENT LAYOUT
This document describes how to use the Low Pin Count Demo Board User’s Guide as
a development tool to emulate and debug firmware on a target board. The manual
layout is as follows:
• Chapter 1. “Low Pin Count (LPC) Demo Board Overview” – An overview of
Microchip’s Low Pin Count Demo Board.
• Chapter 2. “Mid-Range PICmicro® Architectural Overview” – An overview of
the Mid-range PICmicro® Architecture.
• Chapter 3. “LPC Demo Board Lessons” – Contains a variety of lessons that
demonstrate how to utilize and experiment with the Low Pin Count Demo Board.
© 2005 Microchip Technology Inc.
DS51556A-page 1
Low Pin Count Demo Board User’s Guide
CONVENTIONS USED IN THIS GUIDE
This manual uses the following documentation conventions:
DOCUMENTATION CONVENTIONS
Description
Arial font:
Italic characters
Initial caps
Quotes
Underlined, italic text with
right angle bracket
Bold characters
N‘Rnnnn
Text in angle brackets < >
Courier font:
Plain Courier
Represents
Referenced books
Emphasized text
A window
A dialog
A menu selection
A field name in a window or
dialog
A menu path
MPLAB® IDE User’s Guide
...is the only compiler...
the Output window
the Settings dialog
select Enable Programmer
“Save project before build”
A dialog button
A tab
A number in verilog format,
where N is the total number of
digits, R is the radix and n is a
digit.
A key on the keyboard
Click OK
Click the Power tab
4‘b0010, 2‘hF1
Italic Courier
Sample source code
Filenames
File paths
Keywords
Command-line options
Bit values
Constants
A variable argument
Square brackets [ ]
Optional arguments
Curly brackets and pipe
character: { | }
Ellipses...
Choice of mutually exclusive
arguments; an OR selection
Replaces repeated text
Represents code supplied by
user
DS51556A-page 2
Examples
File>Save
Press <Enter>, <F1>
#define START
autoexec.bat
c:\mcc18\h
_asm, _endasm, static
-Opa+, -Opa0, 1
0xFF, ‘A’
file.o, where file can be
any valid filename
mcc18 [options] file
[options]
errorlevel {0|1}
var_name [,
var_name...]
void main (void)
{ ...
}
© 2005 Microchip Technology Inc.
Preface
WARRANTY REGISTRATION
Please complete the enclosed Warranty Registration Card and mail it promptly.
Sending in the Warranty Registration Card entitles users to receive new product
updates. Interim software releases are available at the Microchip web site.
RECOMMENDED READING
This user’s guide describes how to use the Low Pin Count (LPC) Demo Board. Other
useful documents are listed below. The following Microchip documents are available
and recommended as supplemental reference resources.
Readme for Low Pin Count (LPC) Demo Board
For the latest information on using the Low Pin Count (LPC) Demo Board, read the
“Readme for Low Pin Count Demo Board.txt” file (an ASCII text file) in the
PICkit 2 installation directory. The Readme file contains update information and known
issues that may not be included in this user’s guide.
Readme Files
For the latest information on using other tools, read the tool-specific Readme files in
the Readmes subdirectory of the MPLAB IDE installation directory. The Readme files
contain update information and known issues that may not be included in this user’s
guide.
PICkit™ 2 Microcontroller Programmer User’s Guide (DS51553)
Consult this document for instructions on how to use the PICkit 2 Microcontroller
Programmer hardware and software.
PIC16F685/687/689/690 Data Sheet (DS41262)
Consult this document for information regarding the PIC16F685/687/689/690 20-pin
Flash based, 8-bit CMOS Microcontroller device specifications.
MPLAB® IDE, Simulator, Editor User’s Guide (DS51025)
Consult this document for more information pertaining to the installation and features
of the MPLAB Integrated Development Environment (IDE) Software.
© 2005 Microchip Technology Inc.
DS51556A-page 3
Low Pin Count Demo Board User’s Guide
THE MICROCHIP WEB SITE
Microchip provides online support via our web site at www.microchip.com. This web
site is used as a means to make files and information easily available to customers.
Accessible by using your favorite Internet browser, the web site contains the following
information:
• Product Support – Data sheets and errata, application notes and sample
programs, design resources, user’s guides and hardware support documents,
latest software releases and archived software
• General Technical Support – Frequently Asked Questions (FAQs), technical
support requests, online discussion groups, Microchip consultant program
member listing
• Business of Microchip – Product selector and ordering guides, latest Microchip
press releases, listing of seminars and events, listings of Microchip sales offices,
distributors and factory representatives
DEVELOPMENT SYSTEMS CUSTOMER CHANGE NOTIFICATION SERVICE
Microchip’s customer notification service helps keep customers current on Microchip
products. Subscribers will receive e-mail notification whenever there are changes,
updates, revisions or errata related to a specified product family or development tool of
interest.
To register, access the Microchip web site at www.microchip.com, click on Customer
Change Notification and follow the registration instructions.
The Development Systems product group categories are:
• Compilers – The latest information on Microchip C compilers and other language
tools. These include the MPLAB C18 and MPLAB C30 C compilers; MPASM™
and MPLAB ASM30 assemblers; MPLINK™ and MPLAB LINK30 object linkers;
and MPLIB™ and MPLAB LIB30 object librarians.
• Emulators – The latest information on Microchip in-circuit emulators.This
includes the MPLAB ICE 2000 and MPLAB ICE 4000.
• In-Circuit Debuggers – The latest information on the Microchip in-circuit
debugger, MPLAB ICD 2.
• MPLAB® IDE – The latest information on Microchip MPLAB IDE, the Windows®
Integrated Development Environment for development systems tools. This list is
focused on the MPLAB IDE, MPLAB SIM simulator, MPLAB IDE Project Manager
and general editing and debugging features.
• Programmers – The latest information on Microchip programmers. These include
the MPLAB PM3 and PRO MATE® II device programmers and the PICSTART®
Plus and PICkit® 1 development programmers.
DS51556A-page 4
© 2005 Microchip Technology Inc.
Preface
CUSTOMER SUPPORT
Users of Microchip products can receive assistance through several channels:
•
•
•
•
•
Distributor or Representative
Local Sales Office
Field Application Engineer (FAE)
Technical Support
Development Systems Information Line
Customers should contact their distributor, representative or field application engineer
(FAE) for support. Local sales offices are also available to help customers. A listing of
sales offices and locations is included in the back of this document.
Technical support is available through the web site at: http://support.microchip.com
In addition, there is a Development Systems Information Line which lists the latest versions of Microchip’s development systems software products. This line also provides
information on how customers can receive currently available upgrade kits.
The Development Systems Information Line numbers are:
1-800-755-2345 – United States and most of Canada
1-480-792-7302 – Other International Locations
DOCUMENT REVISION HISTORY
Revision A (May 2005)
• Initial Release of this Document.
© 2005 Microchip Technology Inc.
DS51556A-page 5
Low Pin Count Demo Board User’s Guide
NOTES:
DS51556A-page 6
© 2005 Microchip Technology Inc.
LOW PIN COUNT DEMO BOARD
USER’S GUIDE
Chapter 1. Low Pin Count (LPC) Demo Board Overview
1.1
INTRODUCTION
This chapter introduces the Low Pin Count (LPC) Demo Board and describes the LPC
Demo Board features.
1.2
HIGHLIGHTS
This chapter discusses:
• Devices supported by the LPC Demo Board
• The LPC Demo Board Overview
• Running the PICkit™ 2 Starter Kit Default Demonstration
1.3
DEVICES SUPPORTED BY THE LPC DEMO BOARD
For a list of supported devices, see the LPC Demo Board README file on the PICkit™ 2
Starter Kit CD-ROM.
8-pin DIP Flash Devices:
PIC12F508
PIC12F509
PIC12F510
PIC12F629
PIC12F675
PIC12F635
PIC12F683
14-pin DIP Flash Devices:
PIC16F505
PIC16F506
PIC16F630
PIC16F676
PIC16F684
PIC16F688
20-pin DIP Flash Devices:
PIC16F685
PIC16F687
© 2005 Microchip Technology Inc.
PIC16F689
PIC16F690
PIC16F785
DS51556A-page 7
Low Pin Count Demo Board User’s Guide
1.4
LPC DEMO BOARD OVERVIEW
The Low Pin Count Demo Board Works with the PICkit™ 2 Microcontroller
Programmer to help the user get up to speed quickly using PICmicro® microcontrollers.
This user’s guide is written in the form of Lessons intended for a person with some
exposure to assembly language but has never used a PICmicro® microcontroller.
The LPC Demo Board overview is shown in Figure 1-1.
FIGURE 1-1:
LPC DEMO BOARD
PICkit™ 2 Programming Header
14-pin Expansion
Header
20-pin
DIP Socket
Generous
Prototyping
Area
Potentiometer
Push Button
LEDs
1.5
RUNNING THE PICkit™ 2 STARTER KIT DEFAULT DEMONSTRATION
The Low Pin Count Demo Board comes preprogrammed with a demonstration program.
To use this program, connect the PICkit™ 2 Starter Kit to the PC’s USB port using the
USB cable. Start the PICkit™ 2 Microcontroller Programmer application and check the
target power box. The demo program will blink the four red lights in succession. Press
the Push Button Switch, labeled SW1, and the sequence of the lights will reverse. Rotate
the potentiometer, labeled RP1, and the light sequence will blink at a different rate. This
demo program is developed through the first 7 lessons in this guide.
DS51556A-page 8
© 2005 Microchip Technology Inc.
LOW PIN COUNT DEMO BOARD
USER’S GUIDE
Chapter 2. Mid-Range PICmicro® Architectural Overview
2.1
INTRODUCTION
This chapter describes the Mid-range PICmicro® Architectural Overview for the LPC
Demo Board.
SIMPLIFIED MID-RANGE PICmicro® BLOCK DIAGRAM
FIGURE 2-1:
INT
13
Flash
4k x 14
Program
Memory
8
Data Bus
Program Counter
RAM
256 bytes
File
Registers
8-Level Stack (13-bit)
Program 14
Bus
RAM Addr
9
Addr MUX
Instruction Reg
Direct Addr
Indirect
Addr
7
8
FSR Reg
Status Reg
8
3
Instruction
Decode and
Control
OSC1/CLKI
OSC2/CLKO
MUX
ALU
8
Timing
Generation
W Reg
Internal
Oscillator
Block
© 2005 Microchip Technology Inc.
DS51556A-page 9
Low Pin Count Demo Board User’s Guide
2.2
MEMORY ORGANIZATION
PICmicro® microcontrollers are designed with separate program and data memory
areas. This allows faster execution as the address and data busses are separate and
do not have to do double duty.
Data Memory is held in file registers. Instructions referring to file registers use 7 bits,
so only 128 file registers can be addressed. Multiple file registers are arranged into
“pages”. Two extra bits RP0 and RP1 (in the Status register) allow accessing multiple
pages. These two bits effectively become the top two bits of the file register address.
The additional pages may or may not be implemented, depending on the device.
Mid-range devices reserve the first 32 addresses of each page for Special Function
Registers (SFRs). SFRs are how the program interacts with the peripherals. The
controls and data registers are memory mapped into the SFR space. Addresses above
0x20 to the end of each page are General Purpose Registers (GPRs), where program
variables may be stored.
Some frequently used registers may be accessed from any bank. For example, the
Status register is always available no matter which bank is selected via the RP bits. The
last 16 bytes (0x70-0x7F) may also be accessed from any bank.
Program Memory is accessed via a 13-bit Program Counter (PC). The lower 8 bits are
accessible via SFR (PCL), and the upper 5 are at a PCLATH. See the
PIC16F685/687/689/690 Data Sheet’s (DS41262) Section on PCL and PCLATH for
more details on the PC. PCLATH becomes important when program memory size
exceeds 1k instructions, and also for the table look-up in Lesson 12.
Mid-range PICmicro® MCUs may be clocked by a number of different devices. Unless
otherwise noted, the lessons in this manual use the Internal Oscillator running at 4 MHz.
2.3
INSTRUCTION FORMATS
Most instructions follow one of three formats: Byte oriented instructions, Bit oriented
instructions and Literal instructions.
Byte instructions contain 7-bit data address, a destination bit, and 6-bit op code. The
data address plus the RP0 and RP1 bits create a 9-bit data memory address for one
operand. The other operand is the Working register (called W or Wreg). After the
instruction executes, the destination bit (d) specifies whether the result will be stored in
W or back in the original file register. For example:
ADDWF
data,f
adds the contents of Wreg and data, with the result going back into data.
Bit instructions operate on a specific bit within a file register. They contain 7 bits of data
address, 3-bit number and the remaining 4 bits are op code. These instructions may
set or clear a specific bit within a file register. They may also be used to test a specific
bit within a file register. For example:
BSF
STATUS,RP0
set the RP0 bit in the Status register.
Literal instructions contain the data operand within the instruction. The Wreg becomes
the other operand. Calls and GOTO’s use 11 bits as a literal address.
MOVLW'A'
Moves the ASCII value of ‘A’ (0x41) into Wreg.
DS51556A-page 10
© 2005 Microchip Technology Inc.
Mid-Range PICmicro® Architectural Overview
2.3.1
Assembler Basics
Numbers in the Assembler
Unless otherwise specified, the assembler assumes any numeric constants in
the program are hexadecimal (base 16). Binary (base 2), Octal (base 8), Decimal
(base 10), and ASCII coding are also supported.
Hexadecimal:
12 or 0x12 or H'12'
Decimal
.12 or D'12'
Octal
O'12'
Binary
B'00010010'
ASCII
A'c' or 'c'
Org (Origin)
Org tells the Assembler where to start generating code. Normally we start coding
at address ‘0000’, but it could be anywhere. Baseline devices have a Reset
vector at the last location in program memory, so it’s good practice to have a
GOTO instruction pointing to the beginning of the program.
End
End tells the assembler to stop assembling. There must be one at the end of the
program. It does not necessarily have to be at the end of the file, but nothing after
the end statement will be assembled.
Defining Data Memory Locations
There are three ways to name a location (see Example 2-1).
EXAMPLE 2-1:
DEFINING DATA MEMORY
#define Length
0x20
;c-like syntax
Length
0x20
;equate 0x20 with the symbol
0x20
;start a block of variables
;this will be at address 0x20
;this will be at address 0x21
;this is 2 bytes long, starting at
;address 0x22
;this will be at address 0x24
equ
cblock
Length
Width
Area:2
Girth
endc
Unless there is a reason to want a name to a specific location, the cblock/endc
method is preferred. The advantage is that as variables come and go through the
development process, the cblock keeps the block to a minimum. Using one of the other
methods, you may have to go back and find an unused location.
© 2005 Microchip Technology Inc.
DS51556A-page 11
Low Pin Count Demo Board User’s Guide
NOTES:
DS51556A-page 12
© 2005 Microchip Technology Inc.
LOW PIN COUNT DEMO BOARD
USER’S GUIDE
Chapter 3. LPC Demo Board Lessons
3.1
INTRODUCTION
The following lessons cover basic LPC Demo Board features. Refer to applicable
documents as needed. Any updates to the applicable documents are available on
Microchip’s web site.
The code and hex files are installed in C:\Microchip\PICkit 2 Lessons\. They
may also be found on the PICkit™ 2 CD-ROM under directory \PICkit 2 Lessons\.
3.2
LPC DEMO BOARD LESSONS
•
•
•
•
•
•
•
•
•
•
•
•
Lesson 1: Hello World (Light a LED)
Lesson 2: Delay Loop (Blink a LED)
Lesson 3: Rotate (Move the LED)
Lesson 4: Analog-to-Digital
Lesson 5: Variable Speed Rotate
Lesson 6: Switch Debounce
Lesson 7: Reversible Variable Speed Rotate
Lesson 8: Function Calls
Lesson 9: Timer0
Lesson 10: Interrupts
Lesson 11: Indirect Data Addressing
Lesson 12: Look-up Table (ROM Array)
© 2005 Microchip Technology Inc.
DS51556A-page 13
Low Pin Count Demo Board User’s Guide
3.2.1
Lesson 1: Hello World (Light a LED)
The first lesson shows how to turn on a LED. This is the PICmicro® microcontroller
version of “Hello World” and discusses the I/O pin structures.
New Instructions
BSF
Bit set
BCF
Bit clear
The LEDs are connected to I/O pins RC0 through RC3. When one of these I/O pins
drive high, the LED turns on. The I/O pins can be configured for input or output. On
start-up, the default is input. The TRIS bits use the convention of ‘0’ for output and ‘1’
for input. We want digital output so these must be configured.
EXAMPLE 3-1:
PICkit 2, LESSON 1: “HELLO WORLD”
; PICkit 2 Lesson 1 - 'Hello World'
;
#include <p16F690.inc>
__config (_INTRC_OSC_NOCLKOUT & _WDT_OFF & _PWRTE_OFF &
_MCLRE_OFF & _CP_OFF & _BOD_OFF & _IESO_OFF & _FCMEN_OFF)
org 0
Start
BSF
STATUS,RP0
;select Register Page 1
BCF
TRISC,0
;make I/O Pin C0 an output
BCF
STATUS,RP0
;back to Register Page 0
BSF
PORTC,0
;turn on LED C0
GOTO
$
;wait here
end
Now lets look at the program that makes this happen.
;
Starts a comment. Any text on the line following the semicolon
is ignored.
#include
Brings in an include file defining all the Special Function
Registers available on the PIC16F690. Also, it defines valid
memory areas. These definitions match the names used in the
device data sheet.
__Config
Defines the Configuration Word. The labels are defined in the
p16F690.inc file. The labels may be logically ANDed
together to form the word.
Org 0
Tells the assembler where to start generating code. Code may
be generated for any area of the part. Mid-range PICmicro®
microcontroller devices start at address ‘0’, also called the
Reset vector.
BCF TRISC,0
Tells the processor to clear a bit in a file register. TRISC is the
Tri-state register for pin 0 of PORTC. A ‘1’ in the register
makes the pin an input; a ‘0’ makes it an output. We want to
make it an output, so the bit must be cleared.
BSF PORTC,0
Tells the processor to set pin 0 of PORTC. This will force the
I/O pin to a high condition turning on the LED.
GOTO $
Tells the processor to go to the current instruction.
For more information, refer to the I/O Ports Section of the PIC16F685/687/689/690
Data Sheet (DS41262).
DS51556A-page 14
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.2
Lesson 2: Delay Loop (Blink a LED)
The first lesson showed how to turn on a LED, this lesson shows how to make it blink.
While this might seem a trivial change from Lesson 1, the reasons will soon become
apparent.
New Instructions
CLRF
Clear file register
INCF
Increment file register
DECF
Decrement file register
INCFSZ
Increment file register, Skip next instruction if zero
DECFSZ
Decrement file register, Skip next instruction if zero
GOTO
Jump to a new location in the program
EXAMPLE 3-2:
Loop
BSF
BCF
GOTO
PICkit 2, LESSON 2: BLINK
PORTC,0
PORTC,0
Loop
;turn on LED C0
;turn off LED C0
;do it again
While adding a BCF instruction and making it loop will make it blink, it will blink so fast
you won’t see it. It will only look dim. That loop requires 4 instruction times to execute.
The first instruction turns it on. The second one turns it off. The GOTO takes two instruction times, which means it will be on for 25% of the time.
As configured, the PICmicro executes 1 million instructions per second. At this rate, the
blinking needs to be slowed down so that the blinking can be seen, which can be done
by using a delay loop.
Note:
© 2005 Microchip Technology Inc.
Counting cycles – Relating clock speed to instruction speed. The processor
requires 4 clocks to execute an instruction. Since the internal oscillator as
used in these lessons runs at 4 MHz, the instruction rate is 1 MHz.
DS51556A-page 15
Low Pin Count Demo Board User’s Guide
Increment or Decrement a File Register
The INCFSZ and DECFSZ instructions add or subtract one from the contents of
the file register and skips the next instruction when the result is zero. One use is
in the delay loop as shown in Example 3-3.
CLRF
Clears the counter location.
DECFSZ
Decrements the location, and if the result is zero, the next
instruction is skipped.
EXAMPLE 3-3:
DELAY LOOP
Short Loop
CLRF
Loop
DECFSZ
GOTO
Delay
Delay,f
Loop
Long Loop
CLRF
CLRF
Loop
DECFSZ
GOTO
DECFSZ
GOTO
Delay1
Delay2
Delay1,f
Loop
Delay2,f
Loop
The GOTO Loop (in Example 3-3) backs up and does it again. This loop takes 3
instruction times; one for the decrement and two for the GOTO (see note) and the
counter will force it to go around 256 times, which takes it a total of 768 instruction times
(768 μs) to execute.
Even that is still too fast for the eye to see. It can be slowed down even more by adding
a second loop around this one.
The inner loop still takes 768 μs plus 3 for the outer loop, but now it’s executed another
256 times, 771 * 256 = 197376 μs = 0.197s.
Note:
GOTO instructions take two instructions due to the pipelined design of the
processor. The processor fetches the next instruction while executing the
current instruction. When a program branch occurs, the fetched instruction
is not executed.
Open Blink.asm and build the lesson. Next, import the hex file into the PICkit 2 and
program the device. Note the LED now flashes at about a 2 Hz rate.
DS51556A-page 16
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.3
Lesson 3: Rotate (Move the LED)
Building on Lessons 1 and 2, which showed how to light up a LED and then make it
blink with a delay loop, this lesson adds rotation. It will light up DS4 and then shift it to
DS3, then DS2, then DS1 and back to DS4.
New Instructions
MOVLW
Loads Wreg with a literal value
MOVWF
Moves the contents of Wreg to a file register
MOVF
Moves the contents of a file register, either to Wreg or back
into the file register (see note)
RRF
Rotate file register right
RLF
Rotate file register left
Note:
Moving a file register to itself looks like a NOP at first. However, it has a
useful side effect in that the Z flag is set to reflect the value. In other words,
MOVF fileregister,f is a convenient way to test whether or not the
value is zero without affecting the contents of the Wreg.
Rotate Program Flow
•
•
•
•
First, initialize the I/O port and the Display,
Copy the Display variable to the I/O Port, then
Delay for a little while
Rotate the display
FIGURE 3-1:
ROTATE PROGRAM FLOW
Initialize I/O Port
Put Up Display
Delay
Rotate Display
Did it overflow?
Yes
No
Reset Display
© 2005 Microchip Technology Inc.
DS51556A-page 17
Low Pin Count Demo Board User’s Guide
Rotate
The rotate instructions (RRF or RLF) shift all the bits in the file register right or left by
one position, through the Carry bit. The Carry bit is shifted into the byte and receives
the bit shifted out of the byte. The Carry bit should be cleared before rotation so
unwanted bits are not introduced into the display byte. The Carry bit also indicates
when the display byte is empty. When it is, reinsert the ‘1’ at bit 3.
PICmicro MCUs have two rotate instructions: Rotate Left (RLF) and Rotate Right (RRF).
These instructions rotate the contents of a file register and Carry bit one place.
FIGURE 3-2:
ROTATE LEFT
Carry
EXAMPLE 3-4:
ROTATE EXAMPLE
Start
BSF
CLRF
BCF
MOVLW
MOVWF
STATUS,RP0
TRISC
STATUS,RP0
0x08
Display
;select Register Page 1
;make I/O PORTC all output
;back to Register Page 0
MainLoop
MOVF
MOVWF
Display,w
PORTC
;Copy the display to the LEDs
OndelayLoop
DECFSZ
GOTO
DECFSZ
GOTO
Delay1,f
OndelayLoop
Delay2,f
OndelayLoop
BCF
RRF
BTFSC
BSF
GOTO
DS51556A-page 18
File Register
;Delay .197S
STATUS,C
Display,f
STATUS,C
Display,3
MainLoop
;ensure the carry bit is clear
;Rotate Display right
;Did the bit rotate into the carry?
;yes, put it into bit 3.
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.4
Lesson 4: Analog-to-Digital
This lesson shows how to configure the ADC, run a conversion, read the analog voltage
controlled by the potentiometer (RP1) on the board, and display the high order 4 bits
on the display.
The PIC16F690 has an on board Analog-to-Digital Converter (ADC) with 10 bits of
resolution on any of 11 channels. The converter can be referenced to the device’s VDD
or an external voltage reference. The LPC Demo Board references it to VDD as
provided by the USB cable. The answer from the ADC is represented by a ratio of the
voltage to the reference.
ADC = V/VREF * 1023
Converting the answer from the ADC back to voltage requires solving for V.
V = ADC/1023 * VREF
Two of the three factors on the right side of the equation are constants and may be
calculated in advance. This eliminates the need to actually divide, but still requires fixed
or floating point multiply to solve the equation on the fly.
However, sometimes, such as when reading a sensor, calculating the voltage is only
the first step. There may be additional math to calculate the meaningful data from the
sensor. For example, when reading a thermistor, calculating the voltage is only the first
step on the way to getting the temperature.
There are other means to convert ADC values, including a straight table look-up or a
piece-wise linear interpolation. Each of these represents different speed/memory
trade-offs.
The schematic (Appendix A. “Hardware Schematics”) shows the wiper on the
potentiometer is connected to pin RA0 on the PIC16F690.
Here’s the checklist for this lesson:
• Configure PORTA as an analog input, TRISA<0> = 1, ANSEL<0> = 1
• Select clock scaling in ADCON1.
• Select channel, justification and VREF source in ADCON0.
© 2005 Microchip Technology Inc.
DS51556A-page 19
Low Pin Count Demo Board User’s Guide
3.2.4.1
ADCON1
ADCON1 selects the ratio between processor clock speed and conversion speed. This
is important because the ADC needs at least 1.6 μs per bit. Accuracy degrades if the
clock speed is too high. As the processor clock speed increases, an increasingly large
divider is necessary to keep the conversion speed. Four MHz is fastest at 8:1 ratio with
a conversion speed of 2 μs per bit. Refer to the “TAD vs. Device Operating
Frequencies” Table in the Analog-to-Digital Section of the PIC16F685/687/689/690
Data Sheet (DS41262) for recommended configurations.
REGISTER 3-1:
ADCON1 – A/D CONTROL REGISTER 1 (ADDRESS: 9Fh)
U-0
R/W-0
R/W-0
R/W-0
U-0
U-0
U-0
U-0
—
ADCS2
ADCS1
ADCS0
—
—
—
—
bit 7
bit 0
bit 7
Unimplemented: Read as ‘0’
bit 6-4
ADCS<2:0>: A/D Conversion Clock Select bits
000 = FOSC/2
001 = FOSC/8
010 = FOSC/32
x11 = FRC (clock derived from a dedicated internal oscillator = 500 kHz max)
100 = FOSC/4
101 = FOSC/16
110 = FOSC/64
bit 3-0
Unimplemented: Read as ‘0’
Legend:
DS51556A-page 20
R = Readable bit
W = Writable bit
U = Unimplemented bit, read as ‘0’
- n = Value at POR
‘1’ = Bit is set
‘0’ = Bit is cleared
x = Bit is unknown
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.4.2
ADCON0
ADCON0 controls the ADC operation. Bit 0 turns on the ADC module. Bit 1 starts a
conversion and bits <5:2> selects which channel the ADC will operate. VCFG bit< 6>
selects the ADC reference, which may be either VDD or a separate reference voltage
on VREF. ADFM bit <7> selects whether the 10 bits are right or left justified in the 16 bits.
For purposes of this lesson, the ADC must be turned on and pointed to RA0. Choose
the internal voltage reference and 8TOSC conversion clock.
The ADC needs about 5 μs, after changing channels, to allow the ADC sampling
capacitor to settle. Finally, we can start the conversion by setting the GO bit in ADCON0.
The bit also serves as the DONE flag. That is, the ADC will clear the same bit when the
conversion is complete. The answer is then available in ADRESH:ADRESL.
This lesson takes the high order 4 bits of the result and copies them to the display LEDs
attached to PORTC.
See the Analog-to-Digital section in the PIC16F685/687/689/690 Data Sheet
(DS41262) for more details on the ADC module.
REGISTER 3-2:
ADCON0 – A/D CONTROL REGISTER (ADDRESS: 1Fh)
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
ADFM
VCFG
CHS3
CHS2
CHS1
CHS0
GO/DONE
ADON
bit 7
bit 0
bit 7
ADFM: A/D Result Formed Select bit
1 = Right justified
0 = Left justified
bit 6
VCFG: Voltage Reference bit
1 = VREF pin
0 = VDD
bit 5-2
CHS<3:0>: Analog Channel Select bits
0000 = Channel 00 (AN0)
0001 = Channel 01 (AN1)
0010 = Channel 02 (AN2)
0011 = Channel 03 (AN3)
0100 = Channel 04 (AN4)
0101 = Channel 05 (AN5)
0110 = Channel 06 (AN6)
0111 = Channel 07 (AN7)
1000 = Channel 08 (AN8)
1001 = Channel 09 (AN9)
1010 = Channel 10 (AN10)
1011 = Channel 11 (AN11)
1100 = CVREF
1101 = VP6
1110 = Reserved. Do not use.
1111 = Reserved. Do not use.
bit 1
GO/DONE: A/D Conversion Status bit
1 = A/D conversion cycle in progress. Setting this bit starts an A/D conversion cycle.
This bit is automatically cleared by hardware when the A/D conversion has completed.
0 = A/D conversion completed/not in progress
bit 0
ADON: A/D Enable bit
1 = A/D converter module is enabled
0 = A/D converter is shut off and consumes no operating current
Legend:
R = Readable bit
W = Writable bit
U = Unimplemented bit, read as ‘0’
- n = Value at POR
‘1’ = Bit is set
‘0’ = Bit is cleared
© 2005 Microchip Technology Inc.
x = Bit is unknown
DS51556A-page 21
Low Pin Count Demo Board User’s Guide
3.2.5
Lesson 5: Variable Speed Rotate
Lesson 5 combines Lessons 3 and 4 by using the Analog-to-Digital Converter (ADC)
to control the speed of rotation.
New Instructions
BTFSS
Bit test, skip if set
BTFSC
Bit test, skip if clear
A conversion is run on every pass through the main loop. The result controls the length
of the outer loop (see Example 3-5).
EXAMPLE 3-5:
VARIABLE SPEED ROTATE EXAMPLE
...
BSF
BTFSS
ADCON0,GO
ADCON0,GO
GOTO
MOVF
ADDLW
MOVWF
$-1
ADRESH,w
1
Delay2
A2DDelayLoop
DECFSZ Delay1,f
;start conversion
;this bit will change to zero when the
;conversion is complete
;Copy the display to the LEDs
;Delay Loop shortened by the ADResult as
;controlled by the Pot.
GOTO
A2DDelayLoop
DECFSZ Delay2,f
GOTO
A2DDelayLoop
FIGURE 3-3:
VARIABLE SPEED ROTATE PROGRAM FLOW
Initialize I/O Port
Initialize ADC
Put Up Display
Get ADC Result
Delay Using ADC
Rotate Display
Did it overflow?
Yes
No
Reset Display
DS51556A-page 22
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.6
Lesson 6: Switch Debouncing
Mechanical switches play an important and extensive role in practically every
computer, microprocessor and microcontroller application. Mechanical switches are
inexpensive, simple and reliable. In addition, switches can be very noisy. The apparent
noise is caused by the closing and opening action that seldom results in a clean
electrical transition. The connection makes and breaks several, perhaps even
hundreds, of times before the final switch state settles.
The problem is known as switch bounce. Some of the intermittent activity is due to the
switch contacts actually bouncing off each other. Imagine slapping two billiard balls
together. The hard non-resilient material doesn’t absorb the kinetic energy of motion.
Instead, the energy dissipates over time and friction in the bouncing action against the
forces push the billiard balls together. Hard metal switch contacts react in much the
same way. Also, switch contacts are not perfectly smooth. As the contacts move
against each other, the imperfections and impurities on the surfaces cause the
electrical connection to be interrupted. The result is switch bounce.
The consequences of uncorrected switch bounce can range from being just annoying
to catastrophic. For example, imagine advancing the TV channel, but instead of getting
the next channel, the selection skips one or two. This is a situation a designer should
strive to avoid.
Switch bounce has been a problem even before the earliest computers. The classic
solution involved filtering, such as through a resistor-capacitor circuit, or through
re-settable shift registers (see Figures 3-4 and 3-5). These methods are still effective
but they involve additional cost in material, installation and board real estate. Why
suffer the additional expense when software is free and program memory is abundant.
FIGURE 3-4:
FILTERING DEBOUNCE SOLUTION
+V
R2
R1
Filtered
Switch
Output
C1
SW
FIGURE 3-5:
SHIFT REGISTER DEBOUNCE SOLUTION
+V
R1
D
SW
CLK
Qn
Filtered
Switch
Output
CLR
Debounce
Clock
© 2005 Microchip Technology Inc.
DS51556A-page 23
Low Pin Count Demo Board User’s Guide
One of the simplest ways to switch debounce is to sample the switch until the signal is
stable or continue to sample the signal until no more bounces are detected. How long
to continue sampling requires some investigation. However, 5 mS is usually plenty
long, while still reacting fast enough that the user won’t notice it.
Lesson 6 shows how to sample the line at a 1 mS rate waiting for a number of
sequential state changes, which is a simple matter of counting to 5, then resetting the
counter every time it’s still in the original unchanged state.
The Switch on the LPC Demo Board doesn’t bounce much, but it is good practice to
debounce all switches in the system.
FIGURE 3-6:
SIMPLE SWITCH DEBOUNCE PROGRAM FLOW
Yes
Switch has
changed states?
Increment Counter
No
Reset Counter
No
Is Counter = 5?
Yes
Change State
Reset Counter
Delay 1 mS
DS51556A-page 24
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.7
Lesson 7: Reversible Variable Speed Rotate
Lesson 7 combines Lessons 5 and 6 using the button to reverse the direction of rotation
when the button is pressed and adjusting the potentiometer to control the speed of
rotation.
The program needs to keep track of rotation direction and new code needs to be added
to rotate in the other direction.
Lesson 5 rotates right and checks for a ‘1’ in the Carry bit to determine when to restart
the sequence. In Lesson 7, we’ll also need to rotate left and check for a ‘1’ in bit 4 of
the display. When the ‘1’ shows up in bit 4 of the display, re-insert it into the bit 0
position.
EXAMPLE 3-6:
REVERSIBLE VARIABLE SPEED ROTATE EXAMPLE
Original Version:
Rotate
RRF
BTFSC
BSF
Display,f
STATUS,C
Display,3
;Did the bit rotate into the carry?
;yes, put it into bit 3.
Bidirectional Version:
Rotate
BCF
BTFSS
GOTO
STATUS,C
Direction,0
RotateLeft
RotateRight
RRF
Display,f
BTFSC STATUS,C
BSF
Display,3
GOTO
MainLoop
RotateLeft
RLF
Display,f
BTFSC Display,4
BSF
Display,0
GOTO
MainLoop
© 2005 Microchip Technology Inc.
;ensure the carry bit is clear
;Did the bit rotate into the carry?
;yes, put it into bit 3.
;did it rotate out of the display
;yes, put it into bit 0
DS51556A-page 25
Low Pin Count Demo Board User’s Guide
3.2.8
Lesson 8: Function Calls
Lesson 8 shows the reversible LEDs but with the Delay Loop rewritten as a function.
New Instructions
CALL
Invokes functions or subroutines
RETURN
Terminates functions or subroutines
RETLW
Terminates functions or subroutines
Functions or Subroutines are invoked with the CALL instruction and terminated with a
RETURN or RETLW instruction. RETURN jumps back to the original program at the
location following the CALL. RETLW also returns to the calling program, but loads Wreg
with a constant.
The mid-range PICmicro MCU device’s CALL stack can hold up to 8 return addresses.
If a ninth CALL is made, it will overwrite the first one and then the program will not be
able to RETURN all the way back.
Passing Arguments
Arguments to the subroutine may be passed in a number of ways. Wreg is a convenient
place to pass one byte and the FSR may be used to pass another byte, if not otherwise
used. If more data must be passed, a buffer must be allocated.
When the Delay function is pulled out to a subroutine, the ADC result is moved into
Wreg, then the CALL transfers control to the Delay subroutine. The RETURN transfers
control to the MOVLW following the CALL.
EXAMPLE 3-7:
FUNCTION CALL EXAMPLE
MOVF
CALL
ADRESH,w
Delay
...
GOTO
xxx
;call delay function
;returns here when done
; Delay function.
; Delay time is Wreg value * 771 uS
Delay
MOVWF Delay2
DelayLoop
DECFSZ Delay1,f
GOTO
DelayLoop
DECFSZ Delay2,f
GOTO
DelayLoop
RETURN
DS51556A-page 26
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.9
Lesson 9: Timer0
Timer0 is a counter implemented in the processor. It may be used to count processor
clock cycles or external events. Lesson 9 configures it to count instruction cycles and
set a flag when it rolls over. This frees up the processor to do meaningful work rather
than just wasting cycles.
Timer0 is an 8-bit counter with an optional prescaler, which is configured to divide by
256 before reaching the Timer0 counter.
FIGURE 3-7:
TIMER0 SIMPLIFIED
Clock/4 or
Prescaler
T0CKI pin
TMR0
T0IF
Prescaler may be configured
to divide by 2, 4, 8, 16, 32, 64,
128 or 256.
Note:
Flag set when
TMR0 overflows.
Must be cleared
in software.
See PIC16F690 Timer0 section for more details.
TMR0 is a Special Function Register (SFR) and may be read or modified by the
program. The Prescaler is not a SFR and thus cannot be read or modified by the
program. However, writing to TMR0 clears the Prescaler.
The timer may be fed either by the same clock that drives the processor or by an
external event. Driven by the processor clock, it increments once for every instruction
cycle. This is a convenient method of marking time, better than delay loops, as it allows
the processor to work on the problem rather than waste cycles in delay loops.
The prescaler is configured through the OPTION_REG, see Figure 3-8.
FIGURE 3-8:
X
PRESCALER CONFIGURATION THROUGH OPTION_REG
X
T0CS
T0SE
PSA
PS2
PS1
bit 7
PS0
bit 0
Legend:
X:
Don’t cares – not Timer0 related.
T0CS: Timer0 Clock Source 0 for Instruction Clock.
T0SE: Timer0 Source Edge – Don’t care when connected to instruction clock.
PSA:
Prescaler Assignment 0 assigns to Timer0.
PS:
Prescaler rate select ‘111’ – full prescaler, divide by 256.
Lesson 9 configures Timer0 with the Prescaler for a maximum delay on Timer0. The
prescaler will divide the processor clock by 256 and Timer0 will divide that by 256
again. Thus, Timer0 Flag will be set every 65536 μs (0.0000001 second * 256 * 256),
or about 15 times a second. The main program sits in a loop waiting for the rollover and
when it does, it increments the display and then loops back.
© 2005 Microchip Technology Inc.
DS51556A-page 27
Low Pin Count Demo Board User’s Guide
EXAMPLE 3-8:
TIMER0 EXAMPLE
ORG 0
BSF
MOVLW
STATUS,RP0
b'00000111'
MOVWF
CLRF
CLRF
BCF
OPTION_REG
TRISC
Display
STATUS,RP0
ForeverLoop
BTFSS INTCON,T0IF
GOTO
ForeverLoop
BCF
INTCON,T0IF
INCF
Display,f
MOVF
Display,w
MOVWF PORTC
GOTO
ForeverLoop
;configure Timer0. Sourced from the
;Processor clock
;Maximum Prescaler
;Make PORTC all output
;wait here until Timer0 rolls over
;flag must be cleared in software
;increment display variable
;send to the LEDs
END
DS51556A-page 28
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.10
Lesson 10: Interrupts
New Instructions
RETFIE
Return from Interrupt
SWAPF
Swap nibbles in file register
Interrupt Sources
Most of the peripherals can generate an interrupt; also some of the I/O pins may be
configured to generate an interrupt when they change state.
When a peripheral needs service, it sets its interrupt flag. Each interrupt flag is ANDed
with its enable bit and then these are ORed together to form a Master Interrupt. This
master interrupt is ANDed with the Global Interrupt Enable (GIE). See the Interrupt
Logic Figure in the PIC16F685/687/689/690 Data Sheet (DS41262) for a complete
drawing of the interrupt logic. The enable bits allow the PICmicro to limit the interrupt
sources to certain peripherals.
FIGURE 3-9:
INTERRUPT LOGIC SIMPLIFIED
Interrupt Flag
Interrupt Enable
Master Interrupt
Global Interrupt Enable
Other Interrupt Sources
When the master interrupt line is asserted, the PICmicro finishes the current
instruction, stores the next address on the CALL stack then jumps to the Interrupt
Service Routine (ISR). It also clears the GIE bit, preventing another interrupt from
occurring while servicing the current one.
Save Current Context
The first thing the ISR must do is to save the current context of the processor so it can
be restored before returning to the main program. Any SFR that may be changed in the
ISR must be saved, which means the Wreg and Status registers at the very least. The
last 16 bytes of each PIC16F690 file register page are unbanked and are good places
to save the context, as they may be accessed from any register page without regard to
the RP0 and RP1 bits in the Status register. The location of unbanked registers may
vary from part to part. Check the register map to find the unbanked region for a specific
part.
Identify Triggering Event
Next, the ISR has to figure out what triggered the interrupt. It has to check the interrupt
flags to determine what caused the interrupt. When it finds the source, then it can
service the peripheral.
© 2005 Microchip Technology Inc.
DS51556A-page 29
Low Pin Count Demo Board User’s Guide
Restore Context
Once the peripheral is serviced, it needs to restore the context and resume the main
program. Restoring the context is a little harder than it might seem at first. The obvious
method doesn’t work because the MOVF W_Temp,w may affect the Z flag, which was
restored in the previous instruction. Instead, a pair of SWAPF instructions can restore
Wreg without affecting the flags in the Status register. SWAPF exchanges the high and
low nibbles. The first SWAPF switches the nibbles in the file register and the second one
switches them back and puts the result in Wreg.
EXAMPLE 3-9:
CONTEXT RESTORE
;incorrect context restore
MOVF
STATUS_Temp,w
MOVWF STATUS
MOVF
W_Temp
;this may change the Z bit
;in the Status register
;good context restore
MOVF
STATUS_Temp,w
MOVWF STATUS
SWAPF W_Temp,f
SWAPF W_Temp,w
;swap in place
;swap with Wreg destination
Finally, RETFIE transfers control back to the original program and sets the GIE bit,
re-enabling interrupts.
FIGURE 3-10:
DS51556A-page 30
SWAPF INSTRUCTION
Before
1 0 1 0
0 0 1 1
After
0 0 1 1
1 0 1 0
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.11
Lesson 11: Indirect Data Addressing
The FSR (File Select Register) allows the specifying of a file register address. A
subsequent read or write to the INDF (Indirect File register) refers to the file register
addressed by the FSR.
This may be used to implement a moving average filter. The moving average keeps a
list of the last n values and averages them together. The Filter needs two parts: A
circular queue and a function to calculate the average.
FIGURE 3-11:
MOVING AVERAGES
Conceptual View
Average
Time
n
105
102
101
104
99
103
105
107
103
n+1
106
105
102
101
104
99
103
105
103
n+2
110
106
105
102
101
104
99
103
104
The rest move down one
Newest value inserted here
Implementation View
Average
Time
n
107
105
101
104
99
101
102
105
103
99
101
102
Pointer to oldest value
n+1
106
105
102
101
105
103
n+2
106
110
Older value overwritten, pointer advanced
103
99
99
101
102
105
104
Pointer advanced
Calculating averages in a mid-range PICmicro is best accomplished by using the FSR
to keep track of where the next value will be inserted. This ensures the oldest value is
always overwritten with the newest and doesn’t waste time moving values within the
memory.
EXAMPLE 3-10:
FILE SELECT REGISTER EXAMPLE
;insert new value into a queue, enter with new value in
;Wreg
MOVF
MOVF
MOVWF
MOVF
MOVWF
© 2005 Microchip Technology Inc.
temp
QueuePointer,w
FSR
temp,w
INDF
;save the latest value
;load FSR with the queue pointer
;Write the latest value to the queue
DS51556A-page 31
Low Pin Count Demo Board User’s Guide
Lesson 11 adds a Moving Average Filter to the Analog-to-Digital code in Lesson 4.
Twisting the potentiometer changes the value read by the Analog-to-Digital. The filtered
value is then sent to the LED display. The filter only runs every 0.2 seconds to slow
down the display changes and make it visible. The display appears to count from the
old potentiometer position to the new position.
The filter averages the last 8 readings. Choosing a power of two for the number of
samples allows division by simple rotates instead of longhand.
Rather than summing the array every time, it’s faster to keep a running sum, then
subtract out the oldest value in the queue and adding in the new value.
DS51556A-page 32
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
3.2.12
Lesson 12: Look-up Table (ROM Array)
Lesson 8 introduced function calls. Lesson 12 shows how function calls and calculated
modification of the Program Counter may be used to implement a Look-up Table (see
Example 3-11).
It is sometimes useful to implement a table to convert from one value to another.
Expressed in a high-level language it might look like this:
y = function(x);
That is for every value of x, it returns the corresponding y value.
Look-up tables are a fast way to convert an input to meaningful data because the
transfer function is pre-calculated and “looked up” rather than calculated on the fly.
PICmicro MCUs implement these by directly modifying the Program Counter. For
example, a function that converts hexadecimal numbers to the ASCII equivalent. We
can strip out the individual nibble and call the Look-up Table. The index advances the
program counter to the appropriate RETLW instruction to load Wreg with the constant
and returns to the calling program.
EXAMPLE 3-11:
LOOK-UP TABLE
;Enter with index in Wreg
Look-upTable
ADDWF
RETLW
RETLW
...
RETLW
PCL,f
'0'
'1'
;jump to
;index 0
;index 1
'F'
;index 15
Calling the Look-up Table works most of the time. However, if the table falls across a
256 byte page boundary, or if somehow the Look-up Table is called with an out of
bounds index value, it will jump to a location out of the table.
Good programming practices dictate a few additional instructions. First, since the table
is only sixteen entries, make sure a number no larger than 16 is passed in. The simplest
way to do this is to logically AND the contents of Wreg before modifying PCL: ANDLW
0x0F. More complex error recovery schemes may be appropriate, depending on the
application.
In addition, there are some nuances to be aware of should the table cross a 256 word
boundary. The Program Counter is 13 bits wide, but only the lower 8 bits are
represented in PCL (see Figure 3-12). The remaining 5 bits are stored in PCLATH.
However, an overflow of the lower 8 bits is not automatically carried over into PCLATH.
Instead, be sure to check for and handle that case in our code. See the PCL and
PCLATH Section in the PIC16F685/687/689/690 Data Sheet (DS41262) for more
details of how PCLATH is used.
FIGURE 3-12:
PC LOADING AS DESTINATION OF INSTRUCTION
PCH
12
PCL
8
7
0
PC
5
PCLATH<4:0>
Instruction with
PCL as
Destination
8
ALU Result
PCLATH
© 2005 Microchip Technology Inc.
DS51556A-page 33
Low Pin Count Demo Board User’s Guide
This lesson uses the Look-up Table to implement a binary to Gray code converter. Gray
code is a binary code in which only a single bit changes from one sequence to the next.
They are frequently used in encoder applications to avoid wild jumps between states.
Binary encoders are typically implemented an opaque disk sensed by light sensors.
Due to different threshold levels on different bits, bits may change at slightly differently
times yielding momentary invalid results. Gray code prevents this because only one bit
changes from one sequence to the next. The current code is correct until it transitions
to the next.
The algorithm to convert between binary and Gray code is fairly complex. For a small
number of bits, the table look-up is smaller and faster.
This lesson also takes the Analog-to-Digital value and converts it to Gray code
displayed on the LEDs. The code changes one bit at a time as the potentiometer
rotates across its range (see Example 3-12).
Gray Code Converter
DS51556A-page 34
Decimal
Binary
0
0000
1
0001
2
0011
3
0010
4
0110
5
0111
6
0101
7
0100
8
1100
9
1101
10
1111
11
1110
12
1010
13
1011
14
1001
15
1000
© 2005 Microchip Technology Inc.
LPC Demo Board Lessons
EXAMPLE 3-12:
CONVERT BINARY TO GRAY CODE
; Convert 4 bit binary to 4 bit Gray code
;
BinaryToGrayCode
ANDLW 0x0F
;mask off invalid entries
MOVWF temp
MOVLW high TableStart ;get high order part of the beginning of
;the table
MOVWF PCLATH
MOVLW low TableStart
;load starting address of table
ADDWF temp,w
;add offset
BTFSC STATUS,C
;did it overflow?
INCF
PCLATH,f
;yes: increment PCLATH
MOVWF PCL
;modify PCL
TableStart
RETLW b'0000'
;0
RETLW b'0001'
;1
RETLW b'0011'
;2
RETLW b'0010'
;3
RETLW b'0110'
;4
RETLW b'0111'
;5
RETLW b'0101'
;6
RETLW b'0100'
;7
RETLW b'1100'
;8
RETLW b'1101'
;9
RETLW b'1111'
;10
RETLW b'1110'
;11
RETLW b'1010'
;12
RETLW b'1011'
;13
RETLW b'1001'
;14
RETLW b'1000'
;15
© 2005 Microchip Technology Inc.
DS51556A-page 35
Low Pin Count Demo Board User’s Guide
NOTES:
DS51556A-page 36
© 2005 Microchip Technology Inc.
© 2005 Microchip Technology Inc.
1
2
+V
+V
+V
RA5
RA4
RA3
RC5
RC4
RC3
RA0
RA1
RA2
RC0
RC1
RC2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
J1
RA5
RA4
RA3
RC5
RC4
RC3
RA0
RA1
RA2
RC0
RC1
RC2
+5V
GND
VDD
VSS
RA5/T1CKI/OSC1/CLKIN
RA0/AN0/C1IN+/ICSPDAT/ULPWU
RA4/AN3/T1G/OSC2/CLKOUT RA1/AN1/C12IN-/VREF/ICSPCLK
RA3/MCLR/VPP
RA2/AN2/T0CLKI/INT/C1OUT
RC5/CCP1/P1A
RC0/AN4/C2IN+
RC4/C2OUT/P1B
RC1/AN5/C12INRC3/AN7/P1C
RC2/AN6/P1D
RC6/AN8/SS
RB4/AN10/SDI/SDA
RC7/AN9/SDO
RB5/AN11/RX/DT
RB6/SCK/SCK
RB7/RX/CK
U1
PIC16F690/P
20
19
18
17
16
15
14
13
12
11
RA0
RA1
RA2
RC0
RC1
RC2
RB4
RB5
RB6
RC3
RC2
RC1
RC0
RA0
+V
JP4
JP3
JP2
JP1
R7
1KΩ
JP5
R2
1 KΩ
DS4
DS3
DS2
R6
470Ω
R5
470Ω
R4
470Ω
R3
470Ω
C2
0.1 μF
DS1
R1
10 KΩ
SW1
10 KΩ
RP1
FIGURE A-1:
P2
PWR
P1 TM
ICSP
1
2
3
4
5
6
VPP
VDD
GND
ICSPDAT
ICSPCLK
T1G
1
2
3
4
5
6
7
8
9
10
RA3
A.1
RA5
RA4
RA3
RC5
RC4
RC3
RC6
RC7
RB7
C1
0.1 μF
+V
PICKITTM 2 USER’S GUIDE
Appendix A. Hardware Schematics
INTRODUCTION
This appendix contains the Low Pin Count Demo Board Diagrams.
LOW PIN COUNT DEMO BOARD SCHEMATIC DIAGRAM
DS51556A-page 37
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DS51556A-page 38
© 2005 Microchip Technology Inc.