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L OW P IN -C OUNT LCD I N T E R F A C E
1. Introduction
This application note provides an example interface for a C8051F330 device with an example LCD. First, this
application note describes how an LCD works and then describes the two types of LCDs: direct drive and
multiplexed drive. Next, the software interface and structure are explained. Finally, this note describes how to
modify the software example to work with other LCDs.
The code accompanying this application note was originally written for C8051F33x devices. The code can also be
ported to other devices in the Silicon Labs microcontroller range.
2. Key Points
The software provided translates ASCII characters into 7-segment digits, compatible with the printf() standard
library function.
 The LCD used in this example has 19 pins (4 Common and 15 Segment). Seven pins are used on the
microcontroller: four for the Common pins and three that serve as a serial interface to a pair of 74HC595 8-bit,
latched shift registers which are the segment drivers.
 The refresh rate of the LCD is chosen to minimize power consumption as well as minimize flickering.

C8051F330
LCD
COM
Pins
COM PINS
SEGMENT PINS
8
74HC595
7
74HC595
Seg Pin Out
74HC595 Control
2
Figure 1. LCD Interface Block Diagram
Rev. 0.2 7/13
Copyright © 2013 by Silicon Laboratories
Silicon Laboratories Confidential. Information contained herein is covered under non-disclosure agreement (NDA).
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3. LCD
The following sections describe how an LCD works.
3.1. Components of an LCD
An LCD consists of a collection of segments that are individually controlled. When there is no voltage across a
segment, it is turned OFF and assumes the color of the background of the LCD. Applying an ac voltage across a
segment causes it to turn ON and it will look darker than the background of the LCD. The root mean square (rms)
value of the voltage across the segment must be greater than a certain threshold for the segment to turn on. This
threshold is determined by the LCD manufacturer.
Figure 2 shows a diagram of a single segment. Each segment in an LCD has two terminals: a Backplane terminal
and a Segment terminal. Electrically the segment looks like a capacitor. Multiple segments can connect to the
same backplane. The collection of Segment Drivers is called the Frontplane.
Segment
Driver
Frontplane Connection
Capacitor
Segment
Backplane
Backplane Connection
Figure 2. Single Segment in an LCD
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3.2. Direct Drive LCD
In a direct drive LCD, each segment on the LCD is mapped to its own Segment pin. Another pin called the
Common pin (COM) provides the voltage to the backplane. A direct drive LCD with N segments requires a total of
N + 1 pins. Figure 3 below shows a diagram of an direct-drive LCD with seven segments.
S0
S1
S5
S6
S4
S2
S3
COM
Figure 3. Connections for a 7-Digit Segment
Figure 4 shows a sample timing diagram that illustrates how a single segment is turned on and off. The
microcontroller drives all the inputs of a direct drive LCD to either VDD or GND. To turn a segment on, a voltage
difference is applied between the backplane and the segment pin for that specific segment.
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COM
First Half
Second Half
VDD
0
t
S1
VDD
0
t
 S = magnitude (COM – S1)
S
VDD
0
t
ON
OFF
Figure 4. Sample Timing Diagram for a Single Segment
To achieve the best contrast and lowest power consumption, the COM pin should be driven with a 50% duty-cycle
square wave.
In the first half of the timing diagram, the segment pin S1 is driven to a value opposite the value driven on the COM
pin. This leads to a voltage difference across the segment, the magnitude of which is shown by S, and the
segment is turned ON for this length of time. In the second half of the timing diagram, the segment pin S1 is driven
to the same value as the output on the COM pin. This leads to no potential difference across the segment and thus
the segment is turned OFF for this period of time.
In summary, to turn a segment ON in a direct drive LCD, drive the corresponding segment pin to the value opposite
the value of the COM pin. To turn a segment OFF, drive the segment pin to the same value as the COM pin.
Note: An ac excitation waveform is required to turn the segment ON, and the rms value of the voltage across the segment
must be above a certain threshold for the segment to change color.
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3.3. Multiplexed LCD
A multiplexed LCD has more than one backplane, and a corresponding COM pin for each of those backplanes. In
a M-way multiplexed LCD, there are M separate backplanes and M COM pins.
The segments share Segment pins as well as COM pins. The segments are divided equally between the Segment
pins, with each segment possessing a unique combination of Segment and COM pins. If an LCD has M COM pins
and N segment pins, it can support up to M x N segments. For example, and LCD with 4 COM pins and 15
Segment pins can have up to 60 segments.
S1
COM1
COM2
S2
COM3
COM4
Figure 5. Connections for 8 Segments in a 4-way Multiplexed LCD
The LCD discussed in this application note can support up to 60 segments and is 4-way multiplexed. This means it
has 15 groups of 4 pins each. Each group shares a single segment pin. This means that 19 pins are needed to
interface with this LCD (15 for each segment group + 4 COM pins).
If the same LCD was designed as a direct drive LCD, it would require 61 pins (60 pins for each segment and 1
COM pin).
The diagram in Figure 5 shows the connections for a sample 4-way multiplexed LCD.
The COM pins in a multiplexed LCD are driven to one of three voltage levels: VDD, VDD/2, or GND. At any one
moment, only one of the COM pins is driven to either VDD or GND. All other COM pins are driven to VDD/2. The
segment pins are still only driven only to VDD or to GND.
In a 4-way multiplexed LCD, each refresh cycle or period is separated into 8 phases. During the first four phases of
the period, each COM pin is alternately driven to VDD, while the other COM pins are held at VDD/2. In the last 4
phases, each COM pin is alternately driven to GND, while the other COM pins are held at VDD/2. The value of the
COM pins during the last 4 phases is an inverse of the values in the first four phases. This is known as “1/4 duty
cycle.”
Determining the segment pin value to turn a segment ON is similar to the method used for a direct drive LCD. Each
segment is connected to one Segment pin and one of the COM pins. When the respective COM driver is high (in
one of the first four phases of the period), the segment pin must be driven low to turn ON the segment and driven
high to turn OFF the segment. The value of the segment pin is not relevant to a segment if its COM pin is not driven
to VDD or GND.
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Driving the pin low when the respective COM pin is high creates a voltage difference across the segment whose
rms value is greater than the threshold necessary to turn on the segment. Whenever a COM pin is set to VDD/2,
and the segment pin is set to VDD or GND, the rms voltage is below the threshold to turn on the segment. As long
as the rms value of the voltage across the segment over the four phases is above a certain threshold, the segment
will remain ON for those four phases. If the rms value is below the threshold, the segment will remain OFF for
those four phases.
Seg4 – OFF
Seg3 – On
Seg2 – OFF
Seg1 – OFF
Seg4 – OFF
Seg3 – On
Seg2 – OFF
Seg1 – OFF
The Segment pin value over the last four phases of the period must be the inverse of the value over the first four
phases. This will cause the segments to remain in the same state (ON/OFF) that they were in the first four phases.
VDD
V_RMS
VDD/2
0
-VDD/2
V_RMS
-VDD
Figure 6. RMS thresholds for LCD segments
Figure 6 is a graph of the voltage difference over time between a specific Segment pin and four COM pins.
Whenever the magnitude of the voltage difference is less than V_RMS, the segment that is between the Segment
pin and the active COM pin is OFF. Whenever the magnitude of the voltage difference is greater than V_RMS, the
segment that is between the Segment pin and the active COM pin is ON.
In Figure 6, the voltage difference is greater than V_RMS only during phase 3 (and the corresponding phase 7)
and thus only segment 3 is ON. The other segments will remain off during this refresh cycle.
To create a rms value greater than the threshold for a certain segment, set the segment pin low whenever the
corresponding COM pin is high, and set the segment pin high whenever the corresponding COM pin is low.
Figure 7 shows the waveforms for the COM signals, a single Segment pin, and the delta values for the four
segments that the Segment pin is connected to. The delta values are the voltages across the segments. Figure 7
also shows which segments are ON and OFF during each period.
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Period 2
Seg1 – Not Active
Seg2 – Active
Seg3 – Not Active
Seg4 – Active
Seg1 – Not Active
Seg2 – Active
Seg3 – Not Active
Seg4 – Active
Seg1 – Active
Seg2 – Not Active
Seg3 – Active
Seg4 – Not Active
Seg1 – Active
Seg2 – Not Active
Seg3 – Active
Seg4 – Not Active
Period 1
Segment Connections
VDD
COM 1
t
0
COM 1
seg 1
COM 2
seg 2
COM 3
seg 3
COM 4
seg 4
VDD
S
During Period 1
COM 2
t
0
COM 1
seg 1
COM 2
VDD
COM 3
t
0
COM 3
seg 3
S
COM 4
VDD
COM 4
During Period 2
t
0
COM 1
COM 2
VDD
S
t
0
 S1
VDD
ON
t
0
VDD
OFF
COM 3
COM 4
S
seg 4
OFF
-VDD
 S2
seg 2
ON
 S1 = COM 1 – S
 S2 = COM 2 – S
 S3 = COM 3 – S
 S4 = COM 4 – S
t
0
-VDD
 S3
VDD
ON
OFF
t
0
-VDD
 S4
VDD
OFF
ON
t
0
-VDD
Figure 7. Timing Diagram for a Sample 4-Way Multiplexed LCD
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4. How to Generate Segment Values for a Particular Digit
Figure 8 shows a single, 7-segment digit from a 4-way multiplexed LCD. Part A of Figure 8 shows the connections
between the segment and the Segment pins S1 and S2. Part B of Figure 8 shows the connections between the
segment and backplane COM pins COM1, COM2, COM3, and COM4.
This example shows how to generate the digit “5”. From Figure 8, segments 0, 2, 3, 5, and 6 need to be turned ON.
Segments 1 and 4 need to be turned OFF. Figure 9 shows the timing diagram for one refresh cycle necessary to
generate the digit “5”.
The Sn show whether a segment is ON or OFF. The delta values for segments 0, 2, 3, 5, and 6 all have an rms value
greater than VDD/2. This means that the segments are on for as long as this S pattern is presented to the LCD.
S2
0
0
1
5
6
4
5
S1
COM2
6
4
2
COM4
3
Part A
COM1
1
COM3
2
3
Part B
Figure 8. Sample Connections for a 7-Segment, 4-way Multiplexed LCD
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VDD
VDD
COM1
0
 S0
0
ON
-VDD
VDD
VDD
COM2
0
S1
0
OFF
-VDD
VDD
COM3
0
VDD
S2
0
ON
-VDD
VDD
COM4
0
VDD
S3
0
ON
-VDD
VDD
VDD
S1
S4
0
0
OFF
-VDD
S2
VDD
VDD
0
S5
0
ON
-VDD
S0 = COM1 –
S1 = COM2 –
S2 = COM3 –
S3 = COM4 –
S4 = COM4 –
S5 = COM2 –
S6 = COM3 –
S2
S1
S1
S1
S2
S2
S2
VDD
S6
0
ON
-VDD
Figure 9. Timing Diagram Showing How to Display the Digit “5”
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5. Software Example
This section describes how the user can interface to the LCD using the putchar() function.
5.1. Software Interface
The software provided in "10. Software Example Source Code" on page 16 provides an interface for a C8051F330
to the LCD. This LCD has six 7-segment digits. The four backplanes and 15 segment pins allow for 60 segments,
but this LCD only makes 42 segments visible.
The example LCD library overloads the standard library putchar() function. The flow chart for the putchar() function
is shown below in Figure 10.
START
Yes
Is the input
== ‘/n’?
No
Clear all LCD
digits
Does the input
need to be
translated?
No
Yes
Do translation
table lookup
Disable Interrupts
Shift all digits to
the left
Insert the input to
the rightmost digit
Enable Interrupts
Return printed
character to calling
function
Figure 10. Flowchart for putchar()
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The software overloads the standard putchar() function to print its output to the LCD. The function putchar() will
insert the character in the rightmost digit on the LCD. Multiple calls to putchar() will shift the text to the left and
insert the newest character in the rightmost digit's place. Interrupts are disabled when the display is updated to
avoid flickering.
The most significant bit (MSB) of the 8-bit character passed to putchar() determines if putchar() uses a translation
table or directly displays the value. If an 8-bit character whose ASCII value is between 0 and 127 (MSB is 0) is
passed to putchar(), a translation will be made using the translation table. If a value from 128 to 255 (MSB is 1) is
passed to putchar(), the digit will be directly displayed on the LCD.
If the bit is directly displayed, the 7 lower bits are translated as follows: if the bit is “0”, the corresponding segment
is ON; if the bit is “1”, the corresponding segment is OFF. Figure 11 shows which segments are mapped to which
bits. This allows the user to easily create every combination possible with seven segments. The function putchar()
also clears the six LCD digits when it is passed the newline character, “\n”, whose ASCII value is 10. The
translation table is described in detail in "7. LCD driver" on page 13.
Figure 11. Bit Mapping Between Segments in a Digit and the putchar() Input Value
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Table 1. Pin Map for the Example LCD
Pin#
1
2
3
4
5
6
7
8
A2
9
10 11
A3
12
A4
13 14 15
A5
16
COM1
A1
COM2
F1 B1 F2 B2
COM3
G1 C1 G2 C2 G3 C3 G4 C4 G5 C5 G6 C6
COM4 COM4
E1 D1 E2 D2 E3 D3 E4 D4 E5 D5 E6
F2 B3 F4 B4
17
18
A6
F5 B5 F6
19
COM1
B6
COM2
COM3
D6
6. Interpreting the LCD Data Sheet
The LCD data sheet provides the mapping between the segment pins and Segment pins and the COM pins.
Table 1 shows the mapping for the example LCD.
Pins 1, 17, 18, and 19 are the COM pins. Pins 2–16 are the Segment pins. The intersection between the segment
pin number and the COM pin is the segment connected between those pins. The blank spaces in columns 2-16
indicate that there is no segment between that Segment pin and COM pin. There are blank spaces in columns 1,
17, 18, and 19 because are the COM pins.
Table 1 indicates which bits should be shifted out to the shift register in each state of the LCDrefresh_ISR(). For
example, when COM2 is active, the segment status bits for segments F1 through B6 (row 1) need to be shifted to
the shift register. Any value can be shifted to 2, 3, and 4 on the LCD while COM2 is high because there is no
segment that can be activated on those pins during the COM2 phase.
Table 2. Example LCD Specifications
Electrical / Optical
Characteristics
Value
Units
Operating Temperature Range
–10°–60°
°C
Operational Voltage, RMS
2.5–3.5
V-RMS
Drive Frequency
60–300
Hz
Current Consumption
15
nA/mm2
Turn On Time
80
ms
Turn Off Time
80
ms
The specification relevant to the firmware design is the drive frequency. The drive frequency determines how many
times the LCDrefresh_ISR() should be triggered.
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7. LCD driver
There are two main components to the LCD driver: the LCD refresh state machine and the ASCII translation table.
The LCD refresh state machine is located in LCDrefresh_ISR(). This function is responsible for driving the 4 COM
signals and the 15 segment pins. This function is executed every time Timer2 overflows which is scheduled to
happen 2000 times a second. This leads to a refresh rate of 250 Hz, which is in the ideal range for the LCD.
Each state in the 8-state state machine represents which COM driver is active at the time. Because there are eight
phases in each period, this function is called eight times each refresh cycle. Each of the 42 segments has a bit in
memory that holds its state. These bits are stored in the variable LCD_digits. During each run of the function, the
ISR shifts the 15 bits that hold the state for the segments that are active this phase to the shift registers. These
values are then passed in parallel to the LCD Segment pins. After four such phases, four more phases follow with
all the output values inverted to match the associated polarity reversal of the COM signals. The flowchart for the
ISR is shown in Figure 12.
START
Select the next
active COM signal
and tri-state all other
COM signals
Shift next segment
pin value to shift
register
No
Shifted all
segment pins?
Yes
Latch shift
registers
Set current COM pin
to push-pull and
drive
Figure 12. Flowchart for the LCDrefresh_ISR
The ASCII translation table is used whenever putchar() is called with an ASCII character as its parameter. The
ASCII table is used to translate the ASCII character to a 7-segment digit. The table indicates which segments
should be OFF and ON to best represent that character. If the character cannot be translated properly, the
translation table displays a space, which is represented by 0xFF in the table. Figure 13 shows the digit mapping for
each ASCII character. The first 32 characters generate spaces, so they are not part of the table. The top left
number in each box is the value stored in the translation table. The bottom left number is the ASCII value. Both of
these numbers are shown in hexadecimal notation. The bottom right character is the character being translated. If
the box is empty, there is no translation available for that ASCII character and the LCD will display a space instead.
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Figure 13. Mapping for the Basic Set of ASCII Characters (0–127)
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8. Implementation Notes
To generate the VDD/2 necessary for the backplane, a voltage divider is created using two equal sized resistors.
Whenever the COM output pin on the microcontroller is set to “analog in” (high impedance), the voltage divider will
provide the necessary VDD/2 voltage to the LCD. Whenever the COM output pin is set to digital output, 1 (VDD) or
0 (GND) will be sent to the LCD. See "11. Schematic" on page 28 for further details.
It is also important to note that increasing the refresh rate of the LCD to remove flickering also increases the power
consumption. The refresh rate should be set to the minimum amount necessary to prevent flickering. This ideal
refresh rate will vary for each manufacturer's LCD.
The shift registers (74HC595) are used to reduce the number of pins required on the microcontroller. It is important
to choose shift registers that also provide a latching capability. In each phase, all the segment pin values should be
shifted to the shift registers before latching those values to the LCD. This will prevent flickering on the LCD.
9. How to Customize the Software Example for a Different Multiplexing LCD
There are two parts of the code that need to change to accommodate different LCDs.
The Port I/O configuration has to change if the number of backplanes changes. For each backplane, a COM port
pin must be allocated. If the number of Segment pins changes, more pins need not be allocated because the bits
are shifted out serially. However, the number of shift registers depends directly on how many Segments pins are on
the LCD.
The LCDrefresh_ISR() must be changed to accommodate the number of backplanes as well as the number of
segment pins. This involves changing the number of states if the number of backplanes is different. The structures
that store the segment state information must be modified to match the segments on the new LCD. The mapping
between which segments are connected to which backplanes will determine which bits are sent to the LCD during
each phase.
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10. Software Example Source Code
This section contains the source code for the software example.
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//----------------------------------------------------------------------------// LCDInterface.c
//----------------------------------------------------------------------------// Copyright 2004 Silicon Laboratories, Inc.
//
// AUTH: GP
// DATE: 19 NOV 04
//
// This program interfaces a C8051F330 device with an example LCD.
//
// Target: C8051F33x
// Tool Chain : Keil
//
//----------------------------------------------------------------------------// Includes
//----------------------------------------------------------------------------#include <c8051F330.h>
#include <stdio.h>
//----------------------------------------------------------------------------// 16-bit SFR Definitions for 'F3xx
//----------------------------------------------------------------------------sfr16 TMR2RL
sfr16 TMR2
= 0xca;
= 0xcc;
// Timer2 reload value
// Timer2 counter
sfr16 TMR3RL
sfr16 TMR3
= 0x92;
= 0x94;
// Timer3 reload value
// Timer3 counter
//----------------------------------------------------------------------------// Structures, Unions, Enumerations, and Type Definitions
//----------------------------------------------------------------------------// The translation table provides the mapping between ASCII characters
// and the segment pin values
//
//
//
//
The first 32 characters (except 10) just produce a space;
Character 10 (newline) clears the LCD digits
Characters that can't be translated produce a space
The MSB in the byte is meaningless because there are only 7 segments
// If the bit is low, the corresponding bar in the digit is active
// The 'diagram' below shows which bit corresponds to which bar in the LCD digit
//
//
//
//
//
//
//
0
_
5 | | 1
4 | | 2
3
the middle inner bar is bit 6
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unsigned char code translation_table[128] = {
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
0xFF,
//
//
//
//
0 - 7
8 - 15
16 - 23
24 - 31
0xFF,
0xC6,
0x40,
0x00,
0xCF,
0xF0,
0xF9,
0x10,
0xDD,
0xFF,
0xA4,
0xF9,
0xFF,
0xFF,
0xB0,
0xF9,
0x92,
0xDF,
0x99,
0x9E,
0x9B,
0xBF,
0x92,
0xB7,
0xFF,
0xEF,
0x02,
0xBC,
0xDF,
0xFF,
0xF8,
0x18,
//
//
//
//
32
40
48
56
-
39
47
55
63
0xFF,
0x09,
0x0C,
0x09,
0x08,
0xF9,
0x18,
0x11,
0x00,
0xE1,
0xAF,
0xA4,
0xC6,
0x07,
0x92,
0x06,
0x21,
0xC7,
0xF8,
0xFF,
0x06,
0xFF,
0x41,
0xF0,
0x0E,
0x48,
0xE3,
0xDC,
0x02,
0x40,
0xFF,
0xF7,
//
//
//
//
64
72
80
88
-
71
79
87
95
0xDF,
0x0B,
0x0C,
0x09,
};
0x20,
0xFB,
0x18,
0x11,
0x03,
0x61,
0xAF,
0xA4,
0xA7,
0x07,
0x92,
0xC6,
0x21,
0xF9,
0x07,
0xF9,
0x04,
0xFF,
0xE3,
0xF0,
0x0E,
0xAB,
0xE3,
0xBF,
0x10,
0x23,
0xFF,
0xFF,
//
//
//
//
96 - 103
104 - 111
112 - 119
120 - 127
//----------------------------------------------------------------------------// Global Constants
//----------------------------------------------------------------------------#define
#define
#define
#define
SYSCLK
TIMER2_RATE
TIMER3_RATE
PULSE_LENGTH
24500000
1000
2000
25
// SYSCLK frequency in Hz
// Timer 2 overflow rate in Hz
// Timer 3 overflow rate in Hz
//----------------------------------------------------------------------------// Port names
//----------------------------------------------------------------------------sbit SRCLK = P1^1;
sbit RCLK = P1^2;
sbit SER
= P1^3;
// shift register clock
// shift register latch
// shift register serial in
sbit
sbit
sbit
sbit
//
//
//
//
COM1
COM2
COM3
COM4
=
=
=
=
P1^4;
P1^5;
P1^6;
P1^7;
COM1
COM2
COM3
COM4
pin
pin
pin
pin
on
on
on
on
LCD
LCD
LCD
LCD
//----------------------------------------------------------------------------// Global LCD Variables
//----------------------------------------------------------------------------unsigned char com_cycle = 1;
unsigned char com_invert = 0;
18
// start at COM 1
// start with positive cycle
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// Below are the bit maps for each of the bars on the LCD; If the bit is low
// then the bar is opaque (ON). If the bit is high, the bar is transparent (OFF).
// one char per digit on the LCD; initialized to OFF
unsigned char bdata LCD_digits[6] = {0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
// The naming scheme: D1A means the A segment of digit 1
// Digit
sbit D1A
sbit D1B
sbit D1C
sbit D1D
sbit D1E
sbit D1F
sbit D1G
1
=
=
=
=
=
=
=
(D1) is the leftmost digit on the LCD
LCD_digits[0] ^ 0;
LCD_digits[0] ^ 1;
// D1 is controlled by S1 and S2
LCD_digits[0] ^ 2;
LCD_digits[0] ^ 3;
LCD_digits[0] ^ 4;
LCD_digits[0] ^ 5;
LCD_digits[0] ^ 6;
sbit
sbit
sbit
sbit
sbit
sbit
sbit
D2A
D2B
D2C
D2D
D2E
D2F
D2G
=
=
=
=
=
=
=
LCD_digits[1]
LCD_digits[1]
LCD_digits[1]
LCD_digits[1]
LCD_digits[1]
LCD_digits[1]
LCD_digits[1]
^
^
^
^
^
^
^
0;
1;
2;
3;
4;
5;
6;
// D2 is controlled by S3 and S4
sbit
sbit
sbit
sbit
sbit
sbit
sbit
D3A
D3B
D3C
D3D
D3E
D3F
D3G
=
=
=
=
=
=
=
LCD_digits[2]
LCD_digits[2]
LCD_digits[2]
LCD_digits[2]
LCD_digits[2]
LCD_digits[2]
LCD_digits[2]
^
^
^
^
^
^
^
0;
1;
2;
3;
4;
5;
6;
// D3 is controlled by S5 and S6
sbit
sbit
sbit
sbit
sbit
sbit
sbit
D4A
D4B
D4C
D4D
D4E
D4F
D4G
=
=
=
=
=
=
=
LCD_digits[3]
LCD_digits[3]
LCD_digits[3]
LCD_digits[3]
LCD_digits[3]
LCD_digits[3]
LCD_digits[3]
^
^
^
^
^
^
^
0;
1;
2;
3;
4;
5;
6;
// D4 is controlled by S7 and S8
sbit
sbit
sbit
sbit
sbit
sbit
sbit
D5A
D5B
D5C
D5D
D5E
D5F
D5G
=
=
=
=
=
=
=
LCD_digits[4]
LCD_digits[4]
LCD_digits[4]
LCD_digits[4]
LCD_digits[4]
LCD_digits[4]
LCD_digits[4]
^
^
^
^
^
^
^
0;
1;
2;
3;
4;
5;
6;
// D5 is controlled by S9 and S10
sbit
sbit
sbit
sbit
sbit
D6A
D6B
D6C
D6D
D6E
=
=
=
=
=
LCD_digits[5]
LCD_digits[5]
LCD_digits[5]
LCD_digits[5]
LCD_digits[5]
^
^
^
^
^
0;
1;
2;
3;
4;
// D6 is controlled by S11 and S12
Rev. 0.2
19
AN202
sbit D6F = LCD_digits[5] ^ 5;
sbit D6G = LCD_digits[5] ^ 6;
//----------------------------------------------------------------------------// Function Prototypes
//----------------------------------------------------------------------------void
void
void
void
char
void
void
SYSCLK_Init (void);
Port_IO_Init();
Timer2_Init (int);
Timer3_Init (int);
putchar(char);
Strobe();
wait_one_ms(unsigned int);
//----------------------------------------------------------------------------// MAIN Routine
//----------------------------------------------------------------------------void main(void)
{
PCA0MD &= ~0x40;
SYSCLK_Init();
Port_IO_Init();
Timer2_Init (SYSCLK / TIMER2_RATE);
Timer3_Init (SYSCLK / TIMER3_RATE);
//
//
//
//
//
//
// WDTE = 0 (clear watchdog timer enable)
//
//
//
//
initialize the oscillator
initialize the ports
enable timer to interrupt at some Hz
enable timer to overflow at some Hz
We first configure the COM ports to analog inputs. This allows us
to set them to high impedance if we write a 1 to the COM port. Along with
some external resistors, we can then create a VDD/2 voltage. When it is
time for the corresponding COM cycle, we can set the pin to push-pull and
drive the output to VDD or GND. These 3 levels (VDD, VDD/2, GND) are
necessary only for the backplane (Common) pins on the LCD
COM1
COM2
COM3
COM4
RCLK
SRCLK
=
=
=
=
=
=
1;
1;
1;
1;
0;
0;
EA = 1;
//
//
//
//
//
//
high impedance
high impedance
high impedance
high impedance
don't output anything to LCD
don't shift anything to registers yet
// enable global interrupts
while (1)
{
printf ("Hello");
wait_one_ms (1000);
printf ("\n");
wait_one_ms (1000);
}
20
Rev. 0.2
AN202
}
//----------------------------------------------------------------------------// Init Functions
//----------------------------------------------------------------------------//----------------------------------------------------------------------------// SYSCLK_Init
//----------------------------------------------------------------------------//
// This routine initializes the system clock to use the internal 24.5MHz
// oscillator as its clock source. Also enables missing clock detector reset.
//
void SYSCLK_Init (void)
{
OSCICN |= 0x03;
// Configure internal osc to max freq
RSTSRC = 0x04;
// Enable missing clock detector
}
//----------------------------------------------------------------------------// Port_IO_init
//----------------------------------------------------------------------------//
// This routine initializes the ports and enables the crossbar
//
void Port_IO_Init(void)
{
// P0.0 - Unassigned, Open-Drain, Digital
// P0.1 - Unassigned, Open-Drain, Digital
// P0.2 - Unassigned, Open-Drain, Digital
// P0.3 - Unassigned, Open-Drain, Digital
// P0.4 - Unassigned, Open-Drain, Digital
// P0.5 - Unassigned, Open-Drain, Digital
// P0.6 - Unassigned, Open-Drain, Digital
// P0.7 - Unassigned, Open-Drain, Digital
//
//
//
//
//
//
//
//
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
P0MDOUT
P1MDOUT
P1MDIN
P1SKIP
XBR1
-
Unassigned,
Skipped,
Skipped,
Skipped,
Skipped,
Skipped,
Skipped,
Skipped,
=
=
=
=
=
0x80;
0x0E;
0x0F;
0xFE;
0x40;
Open-Drain,
Push-Pull,
Push-Pull,
Push-Pull,
Open-Drain,
Open-Drain,
Open-Drain,
Open-Drain,
Digital
Digital
Digital
Digital
Digital
Digital
Digital
Digital
//
//
//
//
SRCLK for 74HC595
RCLK for 74HC595
SER for 74HC595
COM1 for LCD
COM2 for LCD
COM3 for LCD
COM4 for LCD
configure above pins to Push-Pull
configure Pins 1.4 - 1.7 to analog in
skip pins 1.1 to 1.7
enable crossbar
}
Rev. 0.2
21
AN202
//----------------------------------------------------------------------------// Timer2_Init
//----------------------------------------------------------------------------//
// The timer overflows at a rate of TIMER2_RATE times a second
// The interrupt generated in handled by the LCD_refresh ISR
//
void Timer2_Init (int counts)
{
TMR2CN = 0x00;
// STOP Timer2; Clear TF2H and TF2L;
// disable low-byte interrupt; disable
// split mode; select internal timebase
CKCON |= 0x10;
// Timer2 uses SYSCLK as its timebase
TMR2RL = -counts;
TMR2
= TMR2RL;
ET2 = 1;
TR2 = 1;
//
//
//
//
Init reload values
Init Timer2 with reload value
enable Timer2 interrupts
start Timer2
}
//-----------------------------------------------------------------------------------// Timer3_Init
//-----------------------------------------------------------------------------------//
// Configure the Timer to overflow without interrupts
// The overflow will be used in the wait function
//
void Timer3_Init (int count)
{
TMR3CN = 0x00;
// STOP Timer3; Clear TF3H and TF3L;
// disable low-byte interrupt; disable
// split mode; select internal timebase
CKCON |= 0x40;
// Timer3 uses SYSCLK as its timebase
TMR3RL
TMR3
EIE1
= -count;
= TMR3RL;
&= 0x7F;
// Init reload values
// Init Timer3 with reload value
// disable Timer3 interrupts
TMR3CN
|= 0x01;
// start Timer3
}
//----------------------------------------------------------------------------// Interrupt Service Routines
//----------------------------------------------------------------------------// LCDrefresh is triggered on a Timer2 Overflow
//
//
//
//
22
Takes what is in the LCD bar bits and shift them into the two 74HC595
shift registers depending on the COM cycle; The most signficant
LCD pin (pin 16) gets shifted out first; Only 15 bits get shifted each
COM cycle;
Rev. 0.2
AN202
void LCDrefresh_ISR (void) interrupt 5
{
int i = 0;
if (com_cycle
{
SER = 1
SER = D6A
SER = 1
SER = D5A
SER = 1
SER = D4A
SER = 1
SER = D3A
SER = 1
SER = D2A
SER = 1
SER = D1A
SER = 1
SER = 1
SER = 1
== 1)
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
// non-existent segment
// non-existent segment
// non-existent segment
// non-existent segment
// non-existent segment
// non-existent segment
// non-existent segment
// non-existent segment
// non-existent segment
RCLK = 1;
for (i=0; i<PULSE_LENGTH; i++);
RCLK = 0;
// put shifted data to LCD - rising edge
// keep clock high for a while
// turn off clock
P1MDIN
P1MDIN
// configure COM4 to ANALOG_IN;
// and COM1 to digital
&= ~0x80;
|= 0x10;
P1MDOUT &= ~0x80;
P1MDOUT |= 0x10;
COM4
= 1;
COM1
= 1 ^ com_invert;
com_cycle = 2;
//
//
//
//
//
make COM4 an open-drain
make COM1 a push-pull
set COM4 to high impedance
start the COM1 cycle
next state
}
else if (com_cycle == 2)
{
SER = D6B ^ com_invert;
SER = D6F ^ com_invert;
SER = D5B ^ com_invert;
SER = D5F ^ com_invert;
SER = D4B ^ com_invert;
SER = D4F ^ com_invert;
SER = D3B ^ com_invert;
SER = D3F ^ com_invert;
SER = D2B ^ com_invert;
SER = D2F ^ com_invert;
SER = D1B ^ com_invert;
SER = D1F ^ com_invert;
SER = 1
^ com_invert;
SER = 1
^ com_invert;
SER = 1
^ com_invert;
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Rev. 0.2
// non-existent segment
// non-existent segment
// non-existent segment
23
AN202
RCLK = 1;
for (i=0; i<PULSE_LENGTH; i++);
RCLK = 0;
// put shifted data to LCD - rising edge
// keep clock high for a while
// turn off clock
P1MDIN
P1MDIN
// configure COM1 to ANALOG_IN;
// and COM2 to digital
&= ~0x10;
|= 0x20;
P1MDOUT &= ~0x10;
P1MDOUT |= 0x20;
COM1
= 1;
COM2
= 1 ^ com_invert;
com_cycle = 3;
//
//
//
//
//
make COM1 an open-drain
make COM2 a push-pull
set COM1 to high impedance
start the COM2 cycle
next state
}
else if (com_cycle == 3)
{
SER = D6C ^ com_invert;
SER = D6G ^ com_invert;
SER = D5C ^ com_invert;
SER = D5G ^ com_invert;
SER = D4C ^ com_invert;
SER = D4G ^ com_invert;
SER = D3C ^ com_invert;
SER = D3G ^ com_invert;
SER = D2C ^ com_invert;
SER = D2G ^ com_invert;
SER = D1C ^ com_invert;
SER = D1G ^ com_invert;
SER = 1
^ com_invert;
SER = 1
^ com_invert;
SER = 1
^ com_invert;
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
// non-existent segment
// non-existent segment
// non-existent segment
RCLK = 1;
for (i=0; i<PULSE_LENGTH; i++);
RCLK = 0;
// put shifted data to LCD - rising edge
// keep clock high for a while
// turn off clock
P1MDIN
P1MDIN
// configure COM2 to ANALOG_IN;
// and COM3 to digital
&= ~0x20;
|= 0x40;
P1MDOUT &= ~0x20;
P1MDOUT |= 0x40;
COM2
= 1;
COM3
= 1 ^ com_invert;
com_cycle = 4;
//
//
//
//
//
make COM2 an open-drain
make COM3 a push-pull
set COM2 to high impedance
start the COM3 cycle
next state
}
else if (com_cycle == 4)
{
SER = D6D ^ com_invert;
SER = D6E ^ com_invert;
SER = D5D ^ com_invert;
SER = D5E ^ com_invert;
SER = D4D ^ com_invert;
SER = D4E ^ com_invert;
24
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Rev. 0.2
AN202
SER
SER
SER
SER
SER
SER
SER
SER
SER
=
=
=
=
=
=
=
=
=
D3D
D3E
D2D
D2E
D1D
D1E
1
1
1
^
^
^
^
^
^
^
^
^
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
com_invert;
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
Strobe();
// non-existent segment
// non-existent segment
// non-existent segment
RCLK = 1;
for (i=0; i<PULSE_LENGTH; i++);
RCLK = 0;
// put shifted data to LCD - rising edge
// keep clock high for a while
// turn off clock
P1MDIN
P1MDIN
// configure COM3 to ANALOG_IN;
// and COM4 to digital
&= ~0x40;
|= 0x80;
P1MDOUT &= ~0x40;
P1MDOUT |= 0x80;
COM3
= 1;
COM4
= 1 ^ com_invert;
com_cycle = 1;
//
//
//
//
//
make COM3 an open-drain
make COM4 a push-pull
set COM3 to high impedance
start the COM4 cycle
next state
com_invert = com_invert ^ 1;
// toggle com_invert
}
TF2H = 0;
// clear TF2
} // end LCDrefresh_ISR
//----------------------------------------------------------------------------// Strobe
//----------------------------------------------------------------------------//
// Strobe is used to clock the data into the 74HC595 shift registers
//
void Strobe()
{
int i = 0;
SRCLK = 1;
for (i = 0; i < PULSE_LENGTH; i++);
SRCLK = 0;
for (i = 0; i < PULSE_LENGTH; i++);
// wait a few cycles
// wait a few cycles
}
//----------------------------------------------------------------------------// wait_one_msec
//----------------------------------------------------------------------------//
// Assumes Timer3 overflows once every 500 usec
//
void wait_one_ms(unsigned int count)
Rev. 0.2
25
AN202
{
count = count * 2;
// overflows once every 500 usec
// so double that is 1 ms
TMR3CN &= ~0x80;
TMR3 = TMR3RL;
TMR3CN = 0x04;
// Clear Timer3 overflow flag
while (count--)
{
while (!(TMR3CN & 0x80)) {}
TMR3CN &= ~0x80;
}
TMR3CN &= ~0x04;
// Start Timer3
// wait for overflow
// clear overflow indicator
// Stop Timer3
}
//----------------------------------------------------------------------------// LCD functions
//----------------------------------------------------------------------------//----------------------------------------------------------------------------// putchar
//----------------------------------------------------------------------------//
// putchar only handles the digit components on the LCD screen.
// This functions shifts the digit values to the left, shifting out the
// left-most digit. This function has 3 potential actions based on the input:
//
// 1. Any input whose ASCII code is between 0 and 127 gets translated
//
according to the translation table above
//
// 2. Any input whose ASCII code is between 128 and 255 is directly sent to
//
the LCD. The lower 7 bits indicate which of the seven segments are lit.
//
// 3. Passing a newline char '\n' to this function clears all 6 digits
//
// This function, unlike standard putchar, does not have any error return msgs.
//
// This function will not cause an interrupt to force output. The input char
// will be displayed on the screen on the next refresh cycle
char putchar(char charIN)
{
unsigned char iter = 0;
if (charIN != '\n')
{
if ((charIN & 0x80) == 0) {
charIN = translation_table [charIN]; }
26
// not a new line
// translation necesssary
// quick lookup
EA = 0;
// prevent partial display
for (iter = 0; iter < 5; iter++) {
// shift the digits left
Rev. 0.2
AN202
LCD_digits[iter] = LCD_digits[iter+1]; }
LCD_digits[5] = charIN;
// new digit is rightmost
EA = 1;
// enable interrupts again
}
else
{
EA = 0;
for (iter = 0; iter < 6; iter++) {
LCD_digits[iter] = 0xFF; }
EA = 1;
}
// input is a newline
if (charIN == 0xFF) {
charIN = ' '; }
// couldn't interpret OR space
// return space
return charIN;
// just like putchar
// disable interrupts
// clear all digits
// enable interrupts
}
Rev. 0.2
27
C8051F330
28
GND
VDD
17
1
P1.7
Rev. 0.2
3
4
5
6
7
8
9
SRCLRb
74HC595
74HC595
RCLK
OEb
RCLK
P1.2
SER
SRCLK
QH’
11
12
13
14
15
16
OEb
SRCLRb
QH’
QA QB QC QD QE QF QG QH
10
SRCLK
SER
QA QB QC QD QE QF QG QH
2
P1.1
P1.3
18
P1.6
19
KTD522 LCD
P1.5
P1.4
All resistors are 500K
VDD
VDD
AN202
11. Schematic
AN202
CONTACT INFORMATION
Silicon Laboratories Inc.
400 West Cesar Chavez
Austin, TX 78701
Tel: 1+(512) 416-8500
Fax: 1+(512) 416-9669
Toll Free: 1+(877) 444-3032
Please visit the Silicon Labs Technical Support web page:
https://www.silabs.com/support/pages/contacttechnicalsupport.aspx
and register to submit a technical support request.
Patent Notice
Silicon Labs invests in research and development to help our customers differentiate in the market with innovative low-power, small size, analogintensive mixed-signal solutions. Silicon Labs' extensive patent portfolio is a testament to our unique approach and world-class engineering team.
The information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice.
Silicon Laboratories assumes no responsibility for errors and omissions, and disclaims responsibility for any consequences resulting from
the use of information included herein. Additionally, Silicon Laboratories assumes no responsibility for the functioning of undescribed features or parameters. Silicon Laboratories reserves the right to make changes without further notice. Silicon Laboratories makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Silicon Laboratories assume any
liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation
consequential or incidental damages. Silicon Laboratories products are not designed, intended, or authorized for use in applications intended to support or sustain life, or for any other application in which the failure of the Silicon Laboratories product could create a situation where
personal injury or death may occur. Should Buyer purchase or use Silicon Laboratories products for any such unintended or unauthorized
application, Buyer shall indemnify and hold Silicon Laboratories harmless against all claims and damages.
Silicon Laboratories and Silicon Labs are trademarks of Silicon Laboratories Inc.
Other products or brandnames mentioned herein are trademarks or registered trademarks of their respective holders.
Rev. 0.2
29
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