AN2687

Example of a quadruplex LCD driver with STM8

Doc ID 14296 Rev 1

13/22

same interrupt function) and 2 Overflow events (OVF) in each phase. The latter are

described below:

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segments that have to be turned off. The COM line that corresponds to this phase is set

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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.

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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

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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).

3.2

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).