RM0312 datasheet(135/275 Pages) STMICROELECTRONICS

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RM0312

16-bit general purpose timer (TIM2/TIM3)

170

OCi polarity is software programmable using the CCiP bit in the TIMx_CCERi register. It can

be programmed as active high or active low. OCi output is enabled by a combination of

CCiE, MOE, OISi, OSSR and OSSI bits (TIMx_CCERi and TIMx_BKR registers). Refer to

the TIMx_CCERi register description for more details.

In PWM mode (1 or 2), TIMx_CNT and TIMx_CCRi are always compared to determine

whether TIMx_CCRi

≤TIMx_CNT or TIMx_CNT≤TIMx_CCRi (depending on the direction of

the counter).

The timer is able to generate PWM in edge-aligned mode or center-aligned mode

depending on the CMS bits in the TIMx_CR1 register.

PWM edge-aligned mode

Up-counting configuration

Up-counting is active when the DIR bit in the TIMx_CR1 register is low.

In the following example, we consider the PWM mode 1. The reference PWM signal

OCiREF is high as long as TIMx_CNT <TIMx_CCRi else it becomes low. If the compare

value in TIMx_CCRi is greater than the auto-reload value (in TIMx_ARR) then OCiREF will

be held at ‘1’. If the compare value is 0 then OCiREF will be held at ‘0’. Figure 57 shows

some edge-aligned PWM waveforms in an example where TIMx_ARR=8.

Figure 57. Edge-aligned counting mode PWM mode 1 waveforms (ARR=8)

Down-counting configuration

Down-counting is active when DIR bit in TIMx_CR1 register is high. Refer to Down-counting

mode on page 111

In PWM mode 1, the reference signal OCiREF is low as long as TIMx_CNT> TIMx_CCRi

else it becomes high. If the compare value in TIMx_CCRi is greater than the auto-reload

value in TIMx_ARR, then OCiREF will be held at ‘1’. 0% PWM is not possible in this mode.

PWM center-aligned mode

Center-aligned mode is active when the CMS bits in TIMx_CR1 register are different from

‘00’ (all the remaining configurations having the same effect on the OCiREF/OCi signals).

COUNTER REGISTER

‘1’

‘0’

OCiREF

CCiIF

OCiREF

CCiIF

OCiREF

CCiIF

OCiREF

CCiIF

CCRx = 4

CCRx = 8

CCRx > 8

CCRx = 0

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