Hi,
I am currently working on a project where I want to adjust the duty cycle and trigger an ADC measurement on every counter rollover.
I managed to get the PWM running in center-aligned mode, but I’m having trouble triggering the counter rollover at the correct moment.
To verify whether the trigger is correct, I routed the output compare signal to an external pin and observed it on an oscilloscope.
The basic idea is to run an additional counter alongside the PWM. This counter should count up to ARR and then generate a trigger that starts the ADC conversion. Becuase in the center aligned mode two update event occurs both on CNT=0 and CNT=ARR.
On the oscilloscope, it looks like the trigger—presumably when CNT = 0 (at least that’s my assumption)—produces an edge.
Im using NUCLEO-G491RE.
What could I be missing here? What might be causing this behavior?
Yellow (PWM TIM8 Channel3), Blue (PWM TIM8 Channel1), Orange (Capture Compare TIM8 Channel 4)
Thank you in advance!
Simon
1 ACCEPTED SOLUTION
Accepted Solutions
> I implemented this using a capture/compare configuration.
There is no "capture/compare" configuration. The individual channels in a timer can be set to either Output Compare (TIMx_CCMRx.CCxS = 0b00), or to Input Capture (all 3 other settings of TIMx_CCMRx.CCxS). You've set TIM8_CH4 (i.e. not "an additional counter alongside the PWM" as you've written in your initial post, but an additional channel in the same TIM8 where you've set CH1 and CH3 to PWM), to Output Compare; and, what's important in this context, to Toggle mode (by setting TIM8_CCMR2.OC4M=0b0011, even if it was Cube/HAL which set it for you). You've set - consistently to your initial description - TIM8_CCR4=TIM8_ARR (=1699), and thus the CH4 CCx signal - i.e. which you probably want to propagate to ADC via TRGO2 - should indeed provide a pulse when TIM8_CNT reaches TIM8_ARR.
Neither of the channels is inverted by setting TIMx_CCER.CCxP, and CH1 and CH3 are set to PWM1 mode (i.e. TIMx_CCMRx.OCxM=0b0110), for which:
which means, that:
so, as you see, TIM8_CH4 toggles exactly as you've set it, at TIM8_CNT=TIM8_ARR=1699.
The reason why it's different than your drawing of expected waveform is, that the polarity of PWM channels is opposite to what you expect. You can either change that or change to PWM2 mode; but note, that the pulse-high-length is then not given directly by TIM8_CCRx, but by TIM8_ARR - TIM8_CCRx.
JW
> I implemented this using a capture/compare configuration.
There is no "capture/compare" configuration. The individual channels in a timer can be set to either Output Compare (TIMx_CCMRx.CCxS = 0b00), or to Input Capture (all 3 other settings of TIMx_CCMRx.CCxS). You've set TIM8_CH4 (i.e. not "an additional counter alongside the PWM" as you've written in your initial post, but an additional channel in the same TIM8 where you've set CH1 and CH3 to PWM), to Output Compare; and, what's important in this context, to Toggle mode (by setting TIM8_CCMR2.OC4M=0b0011, even if it was Cube/HAL which set it for you). You've set - consistently to your initial description - TIM8_CCR4=TIM8_ARR (=1699), and thus the CH4 CCx signal - i.e. which you probably want to propagate to ADC via TRGO2 - should indeed provide a pulse when TIM8_CNT reaches TIM8_ARR.
Neither of the channels is inverted by setting TIMx_CCER.CCxP, and CH1 and CH3 are set to PWM1 mode (i.e. TIMx_CCMRx.OCxM=0b0110), for which:
which means, that:
so, as you see, TIM8_CH4 toggles exactly as you've set it, at TIM8_CNT=TIM8_ARR=1699.
The reason why it's different than your drawing of expected waveform is, that the polarity of PWM channels is opposite to what you expect. You can either change that or change to PWM2 mode; but note, that the pulse-high-length is then not given directly by TIM8_CCRx, but by TIM8_ARR - TIM8_CCRx.
JW
