RM0090: Reference manual - STMicroelectronics

RM0090 Advanced-control timers (TIM1&TIM8)
PWM edge-aligned mode
● Upcounting configuration
Upcounting is active when the DIR bit in the TIMx_CR1 register is low. Refer to
Section : Upcounting mode on page 353.
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 TIMx_CCRx is greater than the auto-reload value (in TIMx_ARR)
then OCxREF is held at ‘1’. If the compare value is 0 then OCxRef is held at ‘0’.
Figure 103 shows some edge-aligned PWM waveforms in an example where
TIMx_ARR=8.
Figure 103. Edge-aligned PWM waveforms (ARR=8)
● Downcounting configuration
Downcounting is active when DIR bit in TIMx_CR1 register is high. Refer to Section :
Downcounting mode on page 356
In PWM mode 1, the reference signal OCxRef is low as long as
TIMx_CNT > TIMx_CCRx else it becomes high. If the compare value in TIMx_CCRx is
greater than the auto-reload value in TIMx_ARR, then OCxREF is held at ‘1’. 0% PWM
is not possible in this mode.
PWM center-aligned mode
Counter register
OCXREF
CCRx=4
CCxIF
OCXREF
CCRx=8
CCxIF
OCXREF
CCRx>8
CCxIF
OCXREF
CCRx=0
CCxIF
‘1’
‘0’
0 1 2 3 4 5 6 7 8 0 1
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 OCxRef/OCx signals).
The compare flag is set when the counter counts up, when it counts down or both when it
counts up and down depending on the CMS bits configuration. The direction bit (DIR) in the
TIMx_CR1 register is updated by hardware and must not be changed by software. Refer to
Section : Center-aligned mode (up/down counting) on page 358.
Figure 104 shows some center-aligned PWM waveforms in an example where:
● TIMx_ARR=8,
● PWM mode is the PWM mode 1,
● The flag is set when the counter counts down corresponding to the center-aligned
mode 1 selected for CMS=01 in TIMx_CR1 register.
Doc ID 018909 Rev 3 372/1416