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AN2687 Datasheet(PDF) 13 Page - STMicroelectronics
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Example of a quadruplex LCD driver with STM8
Doc ID 14296 Rev 1
same interrupt function) and 2 Overflow events (OVF) in each phase. The latter are
During CCR1_1, V
is applied to the segments that have to be turned on and 0, to the
segments that have to be turned off. The COM line that corresponds to this phase is set
to low level. Other COM lines are set to the V
During OVF1, all COM lines are inactive (set to low level), then if Vrms has to be
decreased (see Figure 6) all segments are set low or, if it has to be increased (see
Figure 7), all segments are set high.
During CCR1_2, the segment lines are supplied with voltage levels that are inverted
compared to those applied during CCR1_1. The COM line that corresponds to this
phase is set to high level. Other COM lines are set to the V
During OVF2, if Vrms has to be decreased then all COM lines and segments are
inactive (set to low level) and if Vrms has to be increased (see Figure 7), the COM lines
are set high and the segments are set low.
In order to reduce consumption, the MCU is placed in WFI (wait for interrupt) mode in the
main routine. The MCU is then woken up by the Timer 2 interrupts and the external
interrupts (PA4/PA5 connected to push-buttons, refer to Section 6 for more information).
Second method: Auto-wakeup
In this method, the LCD timing is generated by the Auto-wakeup time base. The MCU is
placed in Auto-wakeup mode, meaning that it is in Halt mode but woken up periodically as
the LSI source clock (embedded low-power RC around 128 kHz) remains active. When
woken up, the MCU toggles the COM and SEG lines in the AWU interrupt routine, exactly as
it was doing in the previous method in the Timer 2 interrupt routine.
The software contrast control is not as easy to implement with this method as AWU timeouts
are fixed. It could be less fine-tuned.
Using this method, consumption can be very low (a total consumption of 1.08 mA at 3.3 V
with HSI as the master clock).
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