Implications of underclocking HRTIM

[Gazmundo]

Hi, 

I've got a bit of an odd question but I'm hoping someone will be able to help or at least point me in the direction of some further information.

I am currently looking at a project where we need around 10 PWM outputs so we were looking at using the HRTIM on a STM32G47x to simplify the setup and control as it supports up to 12 PWM output channels.

The issue I have is that we want to use PWM frequencies down to around 100Hz, which I realise is generally the opposite end of the spectrum of the HRTIMs targeted use.

The documentation states that input clock to the HRTIM has to be within 100 MHz to 170 MHz but even with 100 MHz input frequency, the slowest PWM output that can be generated is 381 Hz.

I have run a quick test setting the HRTIM up with a 100 MHz input clock and configuring a couple of output PWM signals at 400hz. I then used the APB2 prescaler to reduce the HRTIM input frequency down to 25 MHz which does appear to successfully generate a 100 Hz PWM output.

My question is...

Is there any documentation that details the reasoning behind the 100-170 MHz input range, or can anyone provide any information on what potential implications there may be with this approach of clocking the HRTIM slower than the specified minimum.

Could running the peripheral slower than the specified range affect the DLLs ability to reliably synchronise/lock/calibrate or are there other aspects that I need to investigate before I go any further with this approach?

Any info appreciated!

Thanks.

[waclawek.jan · Accepted solution]

> Could running the peripheral slower than the specified range affect the DLLs ability to reliably synchronise/lock/calibrate

Yes, the delay line is analog and there are limits to its usability.

The rest of HRTIM appears to be OK at whatever frequency, so if you don't specifically need the sub-system-clock-period resolution, you should be probably OK. The HRTIM 'H7 does not have the delay line implemented, you may want to check if there's any lower bound to its frequency range but I don't think there is.

JW