AVR065: LCD Driver for the STK502

AVR065: LCD Driver for the STK502
Features
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Software Driver for Alphanumeric Characters
Liquid Crystal Display (LCD) Contrast Control
Interrupt Controlled Updating
Conversion of ASCII to LCD Segment Control Codes (SCC)
Interfacing the STK502 LCD Display
8-bit
Microcontrollers
Application Note
1 Introduction
In applications where user interaction is required it is often useful to be able to
display information to the user. A simple interface could be status LEDs, whereas
more complex interaction can benefit from a display capable of displaying letters,
number, words or even sentences. Liquid Crystal Displays (LCD) are often used for
displaying messages. LCD modules can either be graphical ones, which can be
used to display graphics and text, or alphanumerical ones capable of displaying
10 – 80 characters. The standard alphanumerical LCD modules are easy to
interface but are fairly expensive. These are expensive because they have built-in
drivers/controllers, which handle the generation of the characters/graphics on the
LCD glass.
The LCD glass is the glass plate in which the liquid crystal is contained. To reduce
the cost of an application where a display is required one can choose to use a
MCU that have a built-in LCD driver. The MCU can then drive the LCD glass
directly, eliminating the need for the driver integrated in the LCD module. The cost
of the display can be reduced by as much as a factor of 10, since an LCD glass
has a much lower cost than an LCD module.
The ATmega169(P) Flash microcontroller from Atmel has an integrated LCD driver
capable of controlling up to 100 segments. The highly efficient core and the very
low power consumption of this device make it ideal for battery-powered
applications requiring a human interface.
Figure 1-1. The STK502 LCD.
Rev. 2530E-AVR-07/08
2 Theory of Operation
This section provides a basic overview of common features and an introduction to the
terminology used in relation to LCD glass.
In addition, a summary of the ATmega169(P) display driver, and a description of the
LCD glass used in the application example is discussed. Further, this section contains
a summary of the ATmega169(P) LCD features and a description of the LCD glass
used in the application example.
2.1 LCD Glass Explained
The Liquid Crystal Display is based on a display technology that uses rod-shaped
molecules (liquid crystals) that flow like liquid and bend light. Un-energized, the
crystals direct light through two polarizing filters, allowing a natural background color
to show. When energized, they redirect the light to be absorbed in one of the
polarizers, causing the dark appearance. The more the molecules are twisted, i.e.
higher voltage over the LCD segment, the better the contrast and viewing angle.
The LCD must be driven by alternating current (AC). Direct current (DC) will cause
electrophoresis effects in the liquid crystal and will degrade the display. There are two
AC driving methods: the static driving method and the multiplex driving method.
In the static driving method, the LCD is driven with two square waveforms. The static
driving method is the most basic method by which good display quality can be
obtained. However, it is not suitable for liquid displays with many segments because
one liquid crystal driver circuit is required per segment.
In the multiplex (MUX) driving method, there are a wide variety of drive waveforms
and bias levels (explained below) depending on the driver manufacturers. The MUX
level depends on the number of backplanes (also called common lines or common
terminals). For example, a triple display (1/3 MUX) has three backplanes.
The two most common types of LCDs are: “Twisted Nematic” (TN) and “Super Twist
Nematic” (STN). The TN is used for LCD glass with less than 16 back-planes, while
STN LCDs are capable of having as much as 240 back planes. The limitations are
due to the decrease in transparency as more backplanes are added. More details
about the differences between TN and STN LCDs can be found in
http://en.wikipedia.org/wiki/LCD.
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2.1.1 LCD Frame Rate
The number of times the LCD segments are energized per second is called the LCD
frame rate. The frame rate should be kept above 30 Hz to avoid that the human eye
perceives the segments as flickering.
If a high frame rate is used, ghosting can occur. Ghosting is when LCD segments are
not properly turned off. This effect also depends on Duty and Bias (explained below),
and it may be necessary to adapt the frame rate to the actual Duty and Bias used. In
general, ghosting can be avoided by using sufficiently low frame rates; A frame rate of
100 Hz prevents ghosting.
2.1.2 Segments Drivers and Common Terminals
Each LCD segment has two terminals. One is connected to a segment driver, and the
other is connected to a common terminal. By applying an alternating current across
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the segment driver and the common terminal, the liquid crystal is polarized
(energized) and becomes visible. The common terminal is, as the term implies,
common for a group of LCD segments.
To be able to activate individual segments through one (common) terminal that is
shared between multiple segments, the driving waveforms must be encoded. If only
one common terminal is used, i.e., if each segment driver only drives one segment,
the segment drivers of the segments that should not be energized should be of
opposite phase of the segment drivers that are energizing segments. This results in a
maximal voltage drop over the segment that should be energized, and no or a small
voltage drop over the segments that should not be energized. Figure 2-1 and Figure
2-2 show the energized and the non-energized segment and their driving waveforms,
respectively, for segments where the LCD drivers are only driving one segment each
(static Duty).
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Figure 2-1: Two LCD Segments Connected to One Common Terminal
Segment Line 1
SEG1
Energized
LCD segment
SEG0
Segment Line 0
Non-energized
LCD segment
Common Terminal 0
Figure 2-2: Driving Waveforms for Two LCD Segments Connected to the Same
Common Terminal
V LCD
V LCD
SEG0
GND
SEG1
GND
V LCD
V LCD
COM0
COM0
GND
GND
V LCD
V LCD
SEG0-COM0
GND
-V LCD
GND
SEG1-COM0
-V LCD
The left side of Figure 2-2 shows the active segment’s driving waveforms and the
right side of Figure 2-2 shows the inactive segments’ driving waveforms.
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The energizing of segments becomes more complex if each LCD driver is driving
more than one segment. This happens when multiple back-planes are present. In this
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case, the so-called Duty Cycle of the driving waveforms is 1/2 or lower. The Duty
Cycle is described in more details below.
2.1.3 Duty Cycle or Duty Ratio
The Duty Cycle or Duty Ratio is a number used to describe for how long each
segment is activated during each frame. When each segment driver is only driving
one segment the Duty Ratio is “static”. If the drivers are driving more than one
segment each the Duty Ratio is given as 1/(segments driven by each LCD driver).
The number of segments driven by each LCD driver is equal to the number of
common terminals. The Duty Ratio is therefore depending on the number of common
terminals in a given LCD glass. Figure 2-3 illustrates the relation between the number
of back-planes and the Duty Ratio used when controlling the LCD.
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Figure 2-3: LCD Segments Controlled by Using One or Three Common Terminals
Duty = Static (1/1)
Duty = 1/3
Segment Line 1
Segment Line 1
SEG2
SEG0
SEG1
SEG0
SEG1
Segment Line 0
Common Terminal 0
Common Terminal 0
Common Terminal 1
Common Terminal 2
2.1.4 Drive Bias
The Drive Bias (or just Bias) is related to the number of voltage levels used when
driving the LCD. The Bias is defined as 1/(number of voltage levels-1). The more
segments driven by each driver, the higher the number of voltage levels is required.
The number of segments driven by a single segment line is depending on the number
of back-planes in the LCD glass. As per the definition of Duty Ratio, there is a direct
relation between the Bias required and the Duty Ratio used.
Table 2-1: Display Duty, Bias and Voltage Levels
Common Terminals
1
2
3
4
6
7
11
12
Duty
Static
1/2
1/3
1/4
1/6
1/7
1/11
1/12
Bias
1
1/2
1/3
1/3
1/3
1/4
1/4
1/5
Voltage Levels
2
3
4
4
4
5
5
6
If three common lines are used in an LCD it has 1/3 Duty Ratio (see Figure 2-3). This
means that each segment driver controls up to three segments. To be able to control
three segments from one segment driver the Bias needs to be 1/3. In other words it
requires four different voltage levels to be able to control three segments using only
one driver. Each of the LCD segments connected to a single segment line has
different common terminals.
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Figure 2-4 shows the driving waveforms of two LCD segments driven by the same
segment line and connected to two different back-planes. The illustration shows that
the driving waveform has four voltage levels, Bias of 1/3, which is sufficient to drive
up to six back-planes. However since the waveform shows three cycles within one
frame the Duty is 1/3 and indicating that the glass has three back-planes.
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Figure 2-4: Driving Waveforms of Two Different LCD Segments
V LCD
V LCD
2/3V LCD
2/3V LCD
1/3V LCD
SEG0
GND
1/3V LCD
GND
V LCD
V LCD
2/3V LCD
2/3V LCD
1/3V LCD
COM0
GND
1/3V LCD
COM1
GND
V LCD
V LCD
2/3V LCD
2/3V LCD
1/3V LCD
GND
SEG0
1/3V LCD
SEG0-COM0
GND
-1/3V LCD
-1/3V LCD
-2/3V LCD
-2/3V LCD
-V LCD
-V LCD
SEG0-COM1
Figure 2-4 shows driving waveforms of two different LCD segments connected to two
different common terminals. The segment line is shared. The left side figure is
illustrating the active segment and the right side figure is illustrating the inactive
segment. The segment represented by the right side figure is inactive because the
LCD activation voltage threshold is not passed.
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2.1.5 LCD Contrast
LCD contrast is a function of the RMS value of the back-plane minus segment line
waveforms at that segment location. Waveforms can be generated such that, at any
point in the LCD structure, the resulting RMS voltage is either above the saturation
voltage or below the visual threshold voltage. Saturation voltage is the voltage level
where the crystals are fully polarized.
2.2 LCD Features of the ATmega169(P)
The ATmega169(P) is the first AVR with integrated LCD drivers. The ATmega169(P)
can with its four back-plane lines and 25 segment lines drive up to 100 LCD
segments.
To provide high flexibility the LCD driving waveform is selectable. The Duty Ratio and
Bias is programmable making it possible to interface LCD glass with one to four back
planes and between 13 and 25 segment lines. The lines not used for driving the LCD
can be used as general IO. The LCD contrast can be controlled by varying the driver
voltage level between 2.6V and 3.35V – independent of VCC. These voltage levels
only apply to the LCD pins while used by the LCD interface.
To be able to optimize for performance and current consumption the ATmega169(P)
uses programmable frame rates and allow the use of “low power waveforms”. The low
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power waveform ensures that the switching of the segment and common lines are
kept at a minimum frequency. Further, the power saving modes of the ATmega169(P)
allows the MCU to continue driving the LCD glass while reducing its current
consumption to a minimum.
Due to the built-in LCD interrupt source, the software driver can be fully interrupt
driven. This can be used to ensure that the timing related to updating the LCD is
correct. It is therefore possible to avoid that partly updated LCD Data Registers are
latched to the LCD lines.
2.3 The STK502 LCD Glass
The LCD software driver described in this document is made for the STK502, which is
an add-on module for the STK500 development board. A brief description of the LCD
on the STK502 is provided here, further details on the STK500 and STK502 can be
found in their respective User Guide.
The LCD glass mounted on the STK502 is illustrated in Figure 2-5. It consists of
seven alphanumerical symbols and various fixed symbols; Numbers from one to ten,
a bell, a low-battery symbol and navigation arrows.
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Figure 2-5: Layout of the STK502 LCD Glass
Figure 2-6: LCD Segments Connections on the STK502
The LCD glass has in total 120 segments – controlled through four back-planes and
30 segment lines. Since the ATmega169(P) is capable of driving 100 segments some
of the segments of the LCD glass is not connected in the standard configuration that
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the STK502 comes with (see Figure 2-6). The software driver described in this
document assumes that the LCD on the STK502 is connected in accordance with the
standard configuration that the STK502 comes with.
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The STK502 LCD has six similar segment groups, where each segment group is
capable of displaying an alphanumerical character. One such group of segments
capable of displaying one alphanumerical character is subsequently referred to as a
LCD digit. It consists of 14 separate segments. Figure 2-7 shows an LCD digit and
the letter used to refer to each of the segments within a LCD digit.
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Figure 2-7: Segments and Reference Letters of the LCD Digits
According to the data sheet in the STK502 User Guide, the LCD must be operated at
1/4 Duty and 1/3 Bias. This is to be able to control all four back planes. It is
recommended to supply the LCD with 3V.
3 Implementation
This section contains information about the functions included in this driver, and how
to use them. The firmware can be downloaded from the Atmel website:
http://www.atmel.com/products/AVR/ For compiler info and settings, device settings,
target setup info and comprehensive source documentation please see the
readme.html file included with the source.
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3.1 Connections Between the LCD and the ATmega169(P)
The connections between the LCD display and the ATmega169(P) are described in
the STK502 User Guide. It is important to note that power to the LCD is supplied from
the ATmega169(P) – it does not have separate supply lines.
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3.1.1 LCD Data Registers
The LCD segments are individually controlled through the bits of the 20 LCD Data
Registers (LCDDR19:0). Not all bits in all 20 LCD Data Registers are used, the 7
highest bits in the highest register for each common terminal is not used in the
ATmega169(P). Each segment is uniquely controlled through setting or clearing its
corresponding bit in LDDR19:0. The AVR LCD module handles the encoding of the
physical drive waveforms to the LCD.
3.1.2 Relation Between LCD Data Registers and LCD Segments
To be able to make an efficient LCD software driver, in terms of code density, the
physical connections between the LCD digits and the segments lines and common
lines must be well organized: Each LCD digit must be related in the same/similar way
to the LCD Data Registers. This will simplify the translation from an ASCII character
to LCD segment control codes described below. The Control Codes can be written to
the LCDDR19:0 Registers. The data that is written to the LCDDR19:0 Registers are
thus the direct control of the LCD segments through setting and clearing their
corresponding bits in the LCD Data Registers.
3.1.3 LCD Segment Control Codes
The similarities between connections of the six LCD digits available in the STK502
LCD software driver can be seen from bit mapping tables in the STK502 User Guide.
The even digits are using the low nibbles of the associated LCD Data Registers and
the odd are using the high nibbles. Four different LCD Data Registers are used for
each LCD digit; these are all related to different back planes. The relation between
the LCD Data Registers and the digits are organized so that the address offset
between each LCD Data Register used is fixed; The interspacing between the
addressed are in all cases 0x05. Finally, digit 2 and 3, 4 and 5, and 6 and 7 are in
pairs respectively starting at LCDDR0, LCDDR1, and LCDDR2.
To be able to translate ASCII characters into the LCD segment control codes (SCC)
required to set and clear the bits in the LCD Data Registers, the bit mapping tables in
the STK502 User Guide are used. Since the tables can be reused for all LCD digits
only one bit mapping table needs to be used.
To control all segment lines of a LCD digit 16 bits are required. The LCD SCC is thus
16 bit wide, arranged so that each nibble of the LCD SCC is related to one LCD Data
Register. The SCC can therefore be used as follows:
SCC = {bit 16:0} = {Nibble4:0} ~ {LCDDRx+15, LCDDRx+10, LCDDRx+5, LCDDRx};
The relation between the SCC nibbles and the LCD Data Registers is conditioned by
which LCD digit is accessed. Even LCD digits use the low nibble in the LCD Data
Registers while the odd numbered LCD digits use the high nibble in the LCD Data
Registers.
Consider an example where the letter “A” will be shown in an digit . The letter “A” will
look like described in Figure 3-1.
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Figure 3-1: The Letter “A” Displayed in a LCD Digit
As seen the letter “A” require activation of the LCD segments {A, B, C, E, F, G, L}. To
describe this in the LCD SCC the bit maps from the STK502 is used. A bit map table
as shown in Table 3-1, represent both even and odd LCD digits.
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Table 3-1. LCD digit segment mapping into the LCD Data Registers
Register Name
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0
LCDDRx
K
–
–
A
K
-
-
A
LCDDRx+5
J
F
H
B
J
F
H
B
LCDDRx+10
L
E
G
C
L
E
G
C
LCDDRx+15
M
P
N
D
M
P
N
D
To determine the LCD SCC for the ASCII character “A” consider the following:
Nibbles
SCC =
{nibble0}
Segments
=
{{K, -, -, A},
Char “A”
=
{{0, 0, 0, 1},
{nibble1}
{J, F, H, B},
{0, 1, 0, 1},
{nibble2}
{L, E, G, C},
{1, 1, 1, 1},
{nibble3}
{M, P, N, D}}
{0, 0, 0, 0}}
The SCC for character “A” can thus be determined to be:
SCC_A = {0x0, 0xF, 0x5, 0x1}=0x0F51
Many standard ASCII characters and symbols are converted to LCD SCC format. The
LCD SCC codes are stored in a flash table, which is used when translating the ASCII
characters to LCD SCC at run-time.
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Several legal ASCII symbols are not used in the Flash table, since some of these are
not possible to display. These locations in the table can e.g., be used to define
customer symbols for the LCD.
3.2 AVR Studio® LCD Plug-in
To evaluate/test the code described in this application note without the STK502, the
LCD plug-in for AVR Studio 4 can be installed. The plug-in is downloaded with the
AVR LCD Visualizer found together with AVR Studio 4.12 on Atmel AVR web
http://www.atmel.com/products/avr/. This makes it possible to see what the LCD on
STK502 looks like by opening the configuration file for the STK502 LCD in AVR
Studio. The file is named Mega169.dis and is on a standard installation found in the
folder: C:\Program Files\Atmel\AVR Tools\AvrLcd\Displays\Mega169\
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The LCD visualizer can be used to design your own LCD display, hook it to any LCD
registers and simulate/emulate the display in AVR Studio.
4 Firmware Description
This section contains detailed information about how to use the LCD driver and how it
is implemented.
In addition to the descriptions found here, a main.c file is provided along with the LCD
driver. The main.c file is an example of how to use the LCD driver and can be studied
to understand the driver fully.
4.1.1 LCD Driver Functions
The functions found in the file LCD_driver.c are listed and commented in Table 4-1.
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Table 4-1. LCD Software Driver Functions
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Function Name
Arguments
Return
Description
LCD_Init (global)
void
void
Initialize the LCD display Data Buffer,
which is used to buffer the LCD Data
Registers. The LCD frame rate and
contrast is selected. The segment lines
and common lines are configured.
LCD_WriteDigit
(global)
unsigned
char c,
unsigned
char digit
void
The ASCII character passed in the “c”
argument is converted to a LCD Segment
Control Code (SCC). The SCC is the data
that maps the ASCII into LCD Segment
Symbols. The SCC is copied into the LCD
Display Data Buffer. (The actual update is
handled by the LCD_SOF_interrupt
routine.)Digit is the number of the digit that
is desired accessed. The value of digit is
in this implementation limited from 2 to 7.
LCD_AllSegments
(global)
unsigned
char input
(used as
bool)
void
Clear or sets all the segments of the LCD
(updating the LCD Display Data Buffer
only – the actual update is handled by the
LCD_SOF_interrupt routine).
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Function Name
Arguments
Return
Description
LCD_SOF_interrupt
(local, interrupt
service routine)
void
void
Latches the LCD Display Data Buffer
to the LCD Data Registers. The
latching is depending on the
LCD_timer variable and the
LCD_status.updateRequired variable.
4.1.2 Macros
The macros relevant when using the LCD software driver are listed in Table 4-2.
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Table 4-2. Macros used with the LCD Software Driver
Macro Name
Arguments
Description
LCD_CONTRAST_LEVEL
level
Adjust the LCD contrast. Ranges from 0 to
15, where 15 gives highest contrast level.
LCD_SET_COLON
active
Display or hide the colons in the LCD.
Valid input is [TRUE/FALSE]
4.1.3 Global Variables
The variables listed in are the global variables that are required to get control of the
communication between the main LCD driver functions and the LCD interrupt
function. Details on the functionality of the variables are provided in.
Table 4-3. Global Variables Required When Using the LCD Software Driver
Variable Name
Description
LCD_status.updateRequired
(bitfield, bool)
If TRUE the LCD_SOF_interrupt routine will be allowed to
latch the LCD Display Data Buffer to the LCD Data
Registers. If FALSE the interrupt routine will not latch the
LCD display data.This variable can thus be used to
request or block the LCD Display Data Buffer latching:
While updating the LCD display data buffer the variable
should be set to FALSE, so that no latching is performed
until the LCD display data buffer is fully updated.
LCD_status.updateComplete
(bitfield, bool)
The variable is set to TRUE when the LCD Display Data
Buffer has been latched. This is done in the
LCD_SOF_interrupt routine.The variable can thus be used
to test if the data written to the LCD display buffer has
been latched after accessing the LCD display data buffer.
It can be used to handle update timing since the LCD
interrupt is occurring with fixed intervals.The variable can
be used to control calls to the LCD_WriteDigit function.
LCD_timer (Unsigned char)
Variable is decremented in the LCD_SOF_interrupt
routine. When the value becomes 0, the next latching of
the LCD display data buffer will occur. The default timer
seed is also reloaded at this time.The variable controls the
duration of the interval between LCD updates. The LCD
update also depend on the LCD_status.updateRequired
variable. If the update cannot be performed because
LCD_status.updateRequired is FALSE, the LCD update
will be attempted during the next LCD SFO interrupt.The
variable can be set to one from the main application if
immediate updating of the LCD is desired.
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4.2 Function Flowchart
The driver consists of the functions described in text and flowchart below.
4.2.1 LCD_Init
The function used to initialize the LCD software driver is called LCD_Init.
The LCD_Init function first clears the LCDdisplayData buffer. This approach is chosen
since it is more code size efficient than initializing it when the LCD_displayData buffer
is defined. The AVR LCD module is enabled and configured to match the LCD
mounted on the STK502; The contrast level is set, the clock source is set to the
external 32 kHz asynchronous Oscillator and the LCD clock prescaler and divider are
set to generate a frame rate of 64 Hz. Finally, the LCD Start Of Frame (SOF) interrupt
is enabled. The global interrupt is not enabled – this will be most often handled in the
main function.
Figure 4-1. Flowchart of the LCD_Init function
LCD_Init
Initialize the
LCD_displayData buffer
Enable LCD interface
Set LCD contrast level
Setup clock source,
prescaler and divider
(32 kHz async oscillator frame rate 64Hz)
Set up Segment and
Common lines
Enable LCD interrupt
Return
4.2.2 LCD_WriteDigit
The purpose of the LCD_WriteDigit function is to convert and copy an ASCII
character into the LCD_displayData buffer. Note that the latching of the buffer into the
LCD Data Registers is handled in the LCD SOF interrupt.
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The character passed as a function argument is tested to verify that it is within the
character range in the ASCII table. If the character is a lower case character, it is
converted to upper case. The character minus the offset from NULL is used as a look
up in a Flash table. The data retrieved from the table is the LCD Segment Control
Code, which is a two byte value. Each of the four nibbles in the LCD SCC is then
merged into the corresponding LCD Display Data Buffer elements related to the
specific LCD digit that is being updated. Recall that the LCD Display Data Buffer is
the information that is latched directly into the LCD Data Registers. The four nibbles
of the LCD SCC are therefore merged into the corresponding LCD Data Registers
through the buffer.
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Figure 4-2. Flowchart of the LCD_WriteDigit function
LCD_WriteDigit
Yes
Input char
within range
No
Return
(error)
Yes
Input char is
lower case
No
Point the first relevant LCD
Data Register
Convert to uppercase
Copy low nibble of the SCC
Lookup LCD segment
control code (SCC) in LCD
character table
Shift the low nibble out of
the SCC
No
Is LCD digit
even?
Is LCD digit
even?
Decoding mask is 0x0F
Yes
Yes
Swap high and low nibble of
SCC copy
No
Decoding mask is 0xF0
Store SCC copy in LCDDR
pointed to; increment
LCDDR pointer by 5
LCD digit
within
range?
Yes
No
All four
LCDDR
written?
No
Yes
Return
(error)
Return
4.2.3 LCD_AllSegments
Depending on the calling argument the LCD_AllSegments clear or set the entire
LCD_displayData buffer. This will eventually clear or set the LCD segments.
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Figure 4-3. Flowchart of the LCD_AllSegments function
LCD_AllSegments
Yes
Input is
TRUE?
Store 0xFF in all LCD
display data elements
No
Store 0x00 in all LCD
display data elements
Return
4.2.4 LCD_SOF_interrupt
Updating of the LCD is handled by the LCD_SOF_interrupt; It latches the data from
the LCD_displayData Buffer to the LCD Data Registers when the interrupt occurs.
Two variables are influencing the LCD update – the LCD_status.updateRequired Flag
and the LCD_timer variable.
Every time the LCD_SOF_interrupt is executed the LCD_timer is decremented. Once
it reaches zero the LCD_status.updateRequested Flag is tested. If it tests TRUE the
LCD_timer is reloaded with the default timer seed and then the LCD_displayData is
latched into the LCD Data Registers. When an LCD update is complete the
LCD_status.updateComplete Flag is set to indicate that the LCD_displayData has
been latched.
If however the LCD_timer has been decremented to zero, but the LCD update is
blocked by the LCD_status.updateRequested being FALSE, the LCD_timer is loaded
with the smallest possible timer seed (one) and the LCD_status.updateComplete Flag
is set to FALSE. This will ensure that a new LCD update is attempted in the first
consecutive LCD SOF interrupt. The reason that the LCD_status.updateComplete
Flag is cleared is that this should be used in the main routine to test if the
LCD_displayData Buffer can be updated. If the LCD_status.update Complete Flag is
cleared the main routine should not initiate further updating of the LCD_displayData
Buffer. Still, ongoing access to the LCD_displayData buffer should not be terminated
while the LCD_status.updateComplete Flag is cleared.
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Figure 4-4. Flowchart of the LCD Start Of Frame Interrupt Service Routine (the
LCD_SOF_interrupt function)
LCD_SOF_interrupt
Decrement
LCD_timingControl
No
LCD_timer
== 0
Yes
Yes
Replant LCD_timer Seed
LCD_update
Required ==
TRUE
No
Plant smallest possible
LCD_timer seed
LCD_updateComplete =
TRUE
Block further access to the
LCD display data buffer
Copy LCD data struct to
LCD IO registers
Return
4.3 Literature List
1. STK502 User Guide found at Atmel AVR web: http://www.atmel.com/products/avr/
2. ATmega169(P) Data Sheet found at Atmel AVR web site under application notes:
http://www.atmel.com/dyn/products/datasheets.asp?family_id=607
3. LCD, H4042-DL DE5156/L data sheet, included in the STK502 User Guide.
4. LCD technology, http://en.wikipedia.org/wiki/LCD
3U
4U
5U
16
AVR065
2530E-AVR-07/08
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