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參數(shù)資料
| 型號(hào)： | MC68HC708LN56 |
| 廠商： | Motorola, Inc. |
| 英文描述： | 8-Bit Microcontroller Units (MCU).(8位微控制器) |
| 中文描述： | 8位微控制器單元（MCU）。（8位微控制器） |
| 文件頁(yè)數(shù)： | 7/20頁(yè) |
| 文件大小： | 103K |
| 代理商： | MC68HC708LN56 |
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Application Note
Sample Application
AN1762
MOTOROLA
7
Sample Application
In a typical battery application, the power supply gradually decays over
time. Since the top LCD voltage is dependent on V
DD
, to maintain a
constant contrast on the LCD display, the contrast register must be
updated with a new value that will result in the same top LCD voltage at
all times. This can be done by using the A/D converter to periodically
measure V
DD
and then to determine the correct setting for the contrast
control register. By doing this continuously in software, automatic
contrast control is achieved.
In this sample application, the contrast on the LCD display is maintained
at a constant value for a battery-powered supply ranging from 4.2 volts
down to 3.2 volts.Manual contrastcontrol also is implemented by up and
down contrast buttons which allows the user to manually increment or
decrement the contrast setting.
A schematic diagram of the circuit for this application is shown in
Figure 4
. PB0 is used as the input channel for the A/D. A MAXIM 6125
voltage reference chip generates V
REFH
for the A/D. This reference of
2.5-volt must be greater than the maximum A/D input voltage. A simple
resistor divider is used to divide V
DD
in half, which allows a maximum
A/D input voltage of 2.1 volts (max V
DD
= 4.2 volts). The LCD display
used in this circuit is a custom display with 32 backplanes and 40
frontplanes. Displays of this size can be ordered readily from many
manufacturers and can be optimized to the user’s specifications.
The software uses the timebase module to create a periodic interrupt
every set interval of time. At this time, the A/D samples V
DD
. An offset is
generated to a lookup table of contrast control values from the A/D
value. The contrast control register is written with this value which will
keep V
LL7
at the original value.
The two buttons used for manual contrast control are connected to
PA1/KBD1 and PA0/KBD0 and when pushed, generate a keyboard
interrupt. These keyboard interrupts will either increase or decrease the
conversion factor for the lookup table by a constant amount. In this
application, the manual adjustments were designed to change V
LL7
by
~100-mv steps. All automatic contrast control adjustments are then
relative to the latest manual adjustment.
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