General-purpose timers (TIM9 to TIM14) RM0033
452/1381 RM0033 Rev 9
The TIMx_CCRx register can be updated at any time by software to control the output
waveform, provided that the preload register is not enabled (OCxPE=’0’, else TIMx_CCRx
shadow register is updated only at the next update event UEV). An example is given in
Figure 176.
Figure 176. Output compare mode, toggle on OC1.
15.3.9 PWM mode
Pulse Width Modulation mode allows the user to generate a signal with a frequency
determined by the value of the TIMx_ARR register and a duty cycle determined by the value
of the TIMx_CCRx register.
The PWM mode can be selected independently on each channel (one PWM per OCx
output) by writing ‘110’ (PWM mode 1) or ‘111’ (PWM mode 2) in the OCxM bits in the
TIMx_CCMRx register. Enable the corresponding preload register by setting the OCxPE bit
in the TIMx_CCMRx register, and eventually the auto-reload preload register by setting the
ARPE bit in the TIMx_CR1 register.
As the preload registers are transferred to the shadow registers only when an update event
occurs, before starting the counter, the user has to initialize all the registers by setting the
UG bit in the TIMx_EGR register.
The OCx polarity is software programmable using the CCxP bit in the TIMx_CCER register.
It can be programmed as active high or active low. The OCx output is enabled by the CCxE
bit in the TIMx_CCER register. Refer to the TIMx_CCERx register description for more
details.
In PWM mode (1 or 2), TIMx_CNT and TIMx_CCRx are always compared to determine
whether TIMx_CNT ≤ TIMx_CCRx.
The timer is able to generate PWM in edge-aligned mode only since the counter is
upcounting.
PWM edge-aligned mode
In the following example, we consider PWM mode 1. The reference PWM signal OCxREF is
high as long as TIMx_CNT < TIMx_CCRx else it becomes low. If the compare value in
MS31092V2
OC1REF= OC1
TIM1_CNT B200 B2010039
TIM1_CCR1 003A
Write B201h in the CC1R register
Match detected on CCR1
Interrupt generated if enabled
003B
B201
003A
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