Programming Considerations
Certain precautions must be taken when programming the LCD. One of these is to ensure that the
LCD memory is properly initialized after the microcontroller is powered on. Like the General Pur-
pose Data Memory, the contents of the LCD memory are in an unknown condition after power-on.
As the contents of the LCD memory will be mapped into the actual LCD, it is important to initialize
this memory area into a known condition soon after applying power to obtain a proper display pat-
Consideration must also be given to the capacitive load of the actual LCD used in the application.
As the load presented to the microcontroller by LCD pixels can be generally modeled as mainly ca-
pacitive in nature, it is important that this is not excessive, a point that is particularly true in the
case of the COM lines which may be connected to many LCD pixels. The accompanying diagram
depicts the equivalent circuit of the LCD.
S E G 0
C O M 0
S E G 1
S E G 2
S E G n
C O M 1
C O M 2
C O M 3
LCD Panel Equivalent Circuit
Setting the correct frequency of the LCD clock is another factor which must be taken into account
in user applications. To have the LCDs operate at their best frame frequency, which is normally be-
tween 25Hz and 250Hz, it is important to select an appropriate LCD clock frequency configuration
option. The correct option should be chosen to ensure that an LCD clock frequency as close to
4kHz as possible is achieved. With such a frequency chosen, the microcontroller internal LCD
driver circuits will ensure that the appropriate LCD driving signals are generated to obtain a suit-
able LCD frame frequency.
One additional consideration that must be taken into account is what happens when the
microcontroller enters a HALT condition. A configuration option permits the LCD to be powered off
when in the Power Down Mode to reduce power consumption. If this option is selected, after a
instruction is executed, the driving signals to the LCD will cease, producing a blank display
pattern but reducing any power consumption associated with the LCD. As the LCD memory re-
mains unaffected by the execution of a
instruction, when the microcontroller wakes-up
and the LCD driving signals resume, the original display pattern will be restored. If the configura-
tion option selects the LCD display to remain on when in the Power Down Mode, the LCD driving
signals will continue to be generated, therefore the LCD pattern will remain undisturbed, however,
it should be noted that such action will result in power being consumed.
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