Generating a PWM that starts high, but ends low after N pulses have been completed.

[Pepijn]

I have an Arduino Giga (STM32H7) and I'm using the timers to generate PWM's. Everything's working fine, programming is no problem, but I cannot come up with a way to implement what I want. I included a simple visual to describe what I want the output to look like.

 

TIM1 is the master timer, it generates a PWM, set as active high (for CNT<CCR). I have a slave TIM8, which should do the following: when TIM1 starts over again/goes high, TIM8 should generate N pulses. The time it takes for N pulses to complete is shorter than the period of TIM1, I want them to retrigger each time TIM1 goes high again. Here's the catch: The pulses should immediately start high, however, once all the pulses have fired, the output should stay low.

If it weren't for the last part, the solution would be simple: TIM1 uses UEV as output trigger, TIM8 takes an internal trigger input from TIM1, set TIM8 to One-Pulse Mode, set the repetition counter to N, and voila, you get bursts of N pulses on each TIM1 high (yes technically the first time it won't trigger, doesn't matter).

However, this will cause the pulses of TIM8 to either start 'delayed', since for CNT<CCR, the output will be low in OPM, or if we flip the polarity, it will start high, but after having finished N pulses, TIM8->CNT=0 (because of OPM), which causes the output to be high...

Can anyone think of a good solution/setup? Again, programming is no issue, it's just finding a way to get this behaviour, if that's even possible... Also, I would prefer not to use interrupts handlers for reasons related to the rest of my program.

[TDK]

Very doable with a number of methods

Perhaps the easiest is to use the scheme that almost works except use PWM mode 2 and set it so output is low at CNT=0, high at CNT=1... X, and then low again until it reaches ARR.

 

If you need them to exactly match, you could use TIM8 in gated slave mode to get exactly 4 pulses at the start of each TIM1 period. Can describe this more if you need.

 

[Pepijn]

Ah the combined mode, I hadn't considered that but I'm sure that will work. The single count offset is no problem. Thanks a lot!

[waclawek.jan]

// Basler: The Basler camera is triggered by a combined PWM mode, to ensure a low level at the end of the frame triggers,
 // while still (almost, at CNT=1) starting high.
 // 12: Combined PWM mode 1 - OC1REF has the same behavior as in PWM mode 1. OC1REFC is the logical OR between OC1REF and OC2REF.
 // 13: Combined PWM mode 2 - OC1REF has the same behavior as in PWM mode 2. OC1REFC is the logical AND between OC1REF and OC2REF.

 TIM8->CCMR1 &= ~TIM_CCMR1_OC1M_Msk; // for TIM8_CH1 - AND
 TIM8->CCMR1 |= (0x13UL << TIM_CCMR1_OC1M_Pos); // 

 TIM8->CCMR1 &= ~TIM_CCMR1_OC2M_Msk; // for TIM8_CH2 - OR
 TIM8->CCMR1 |= (0x12UL << TIM_CCMR1_OC2M_Pos); // 

1. 0x12/0x13 != 12/13

2. TIMx_CCMRx.OCxM bits don't form a continuous bitfield (for historical reasons)

JW