ST7MC1K2-Auto datasheet(254/371 Pages) STMICROELECTRONICS

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PWM operating mode

The PWM generator can work in center-aligned or edge-aligned mode depending on the

CMS bit setting in the MPCR register.

Figure 119 shows the corresponding counting sequence.

It offers also an 8-bit mode to get a full 8-bit range with a single compare register write

access by setting the PMS bit in MPCR register.

The comparisons described here are performed between the PWM counter value extended

to 13 bits and the 13-bit compare register. Having a compare range greater than the counter

range is mandatory to get a full PWM range (that is, up to 100% modulation). This principle

is maintained for 8-bit PWM operations.

Center-aligned mode (CMS bit = 1)

In this operating mode, the PWM counter counts up to the value loaded in the 12-bit

compare 0 register then counts down until it reaches zero and restarts counting up.

The PWM signals are set to ‘0’ when the PWM counter reaches, in up-counting, the

corresponding 13-bit compare register value and they are set to ‘1’ when the PWM counter

reaches the 13-bit compare value again in down-counting.

Figure 119. Counting sequence in center-aligned and edge-aligned mode

If the 13-bit compare register value is greater than the extended compare 0 register (the 13th

bit is set to ‘0’), the corresponding PWM output signal is held at ‘1’.

If the 13-bit compare register value is 0, the corresponding PWM output signal is held at ‘0’.

Figure 120 shows some center-aligned PWM waveforms in an example where the

compare 0 register value = 8.

Center-aligned mode

....

Edge-aligned mode

.....

T = PWM period, value of 12-bit compare 0 register = 16

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