General-purpose timers (TIM2/TIM3/TIM4/TIM5) RM0440
1266/2126 RM0440 Rev 4
29.4.12 Asymmetric PWM mode
Asymmetric mode allows two center-aligned PWM signals to be generated with a
programmable phase shift. While the frequency is determined by the value of the
TIMx_ARR register, the duty cycle and the phase-shift are determined by a pair of
TIMx_CCRx registers. One register controls the PWM during up-counting, the second
during down counting, so that PWM is adjusted every half PWM cycle:
• tim_oc1refc (or tim_oc2refc) is controlled by TIMx_CCR1 and TIMx_CCR2
• tim_oc3refc (or tim_oc4refc) is controlled by TIMx_CCR3 and TIMx_CCR4
Asymmetric PWM mode can be selected independently on two channels (one tim_ocx
output per pair of CCR registers) by writing ‘1110’ (Asymmetric PWM mode 1) or ‘1111’
(Asymmetric PWM mode 2) in the OCxM bits in the TIMx_CCMRx register.
Note: The OCxM[3:0] bit field is split into two parts for compatibility reasons, the most significant
bit is not contiguous with the 3 least significant ones.
When a given channel is used as asymmetric PWM channel, its secondary channel can also
be used. For instance, if an tim_oc1refc signal is generated on channel 1 (Asymmetric PWM
mode 1), it is possible to output either the tim_oc2ref signal on channel 2, or an tim_oc2refc
signal resulting from asymmetric PWM mode 2.
Table 278. CCR register change dithering pattern in center-aligned PWM mode
LSB
value
PWM period
12345678
Up DnUpDnUpDnUpDnUpDnUpDnUpDnUpDn
0000 - - - - - - - - --------
0001 +1 - - - - - - - --------
0010 +1 - - - - - - - +1 - - - - - - -
0011+1---+1---+1-------
0100 +1 - - - +1 - - - +1 - - - +1 - - -
0101 +1 - +1 - +1 - - - +1 - - - +1 - - -
0110+1-+1-+1---+1-+1-+1---
0111 +1 - +1 - +1 - +1 - +1 - +1 - +1 - - -
1000 +1 - +1 - +1 - +1 - +1 - +1 - +1 - +1 -
1001 +1 +1 +1 - +1 - +1 - +1 - +1 - +1 - +1 -
1010 +1 +1 +1 - +1 - +1 - +1 +1 +1 - +1 - +1 -
1011 +1 +1 +1 - +1 +1 +1 - +1 +1 +1 - +1 - +1 -
1100 +1 +1 +1 - +1 +1 +1 - +1 +1 +1 - +1 +1 +1 -
1101 +1 +1 +1 +1 +1 +1 +1 - +1 +1 +1 - +1 +1 +1 -
1110 +1 +1 +1 +1 +1 +1 +1 - +1 +1 +1 +1 +1 +1 +1 -
1111 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 -
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