Issue with PWM and timer setup

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Issue with PWM and timer setup

(8 posts) (2 voices)

Started 4 years ago by bart_dood
Latest reply from bart_dood

bart_dood
Member

Hi Folks, I have this code in my setup to change the PWM frequency to 10khz. It seems to work fine normally, I use the pins 6,7,8 only for PWM output:

timer1.pause();
timer1.setPrescaleFactor(1);
timer1.setOverflow(7200);
timer1.refresh();
timer1.resume();

However I have to use center or symmetric PWM in my code not edge aligned PWM which is stock for the IDE.
To achieve this I modified the file 'boards.cpp' located at: maple/hardware/leaflabs/cores/maple for version 0.0.12

I changed the line:

regs->CR1 = TIMER_CR1_ARPE;

to

regs->CR1 = TIMER_CR1_ARPE|(1<<5);

Which works to make the PWM center/symmetric and I've confirmed this via a scope. The problem is that it seems to be changing the PWM frequency, so the setup code above now runs at half the speed, so 5khz instead of 10khz.
The other thing I've noticed is that I have a timed interrupt which normally runs every 100microseconds. This works fine normally until I make the change to the boards.cpp file (for center PWM). Then it runs at half speed. I have confirmed all this via the scope.

Does anyone know why this is happening and is there any way around it? I can think of some work arounds but it involves a lower resolution in my PWM output etc. I'd rather try and fix the main problem first.

Thanks for your time!

Posted 4 years ago #
bart_dood
Member

Hi Folks, I've been doing a little searching and found this set of code inside the 'timers.h' file:

/**
* Used to configure the behavior of a timer channel. Note that not
* all timers can be configured in every mode.
*/
/* TODO TIMER_PWM_CENTER_ALIGNED, TIMER_INPUT_CAPTURE, TIMER_ONE_PULSE */
typedef enum timer_mode {
TIMER_DISABLED, /**< In this mode, the timer stops counting,
channel interrupts are detached, and no state
changes are output. */
TIMER_PWM, /**< PWM output mode. This is the default mode for pins
after initialization. */
/* TIMER_PWM_CENTER_ALIGNED, /\**< Center-aligned PWM output mode. *\/ */
TIMER_OUTPUT_COMPARE, /**< In this mode, the timer counts from 0
to its reload value repeatedly; every
time the counter value reaches one of
the channel compare values, the
corresponding interrupt is fired. */
/* TIMER_INPUT_CAPTURE, /\**< In this mode, the timer can measure the */
/* pulse lengths of input signals. *\/ */
/* TIMER_ONE_PULSE /\**< In this mode, the timer can generate a single */
/* pulse on a GPIO pin for a specified amount of */
/* time. *\/ */
} timer_mode;

This seems to have some code for center aligned PWM hidden in it, is this where I should be enabling center/symmetric PWM? Thanks

Posted 4 years ago #
gbulmer
Moderator

bart_dood - centre-aligned/symmetric PWM runs at half the frequency because the timer's counter counts up (one cycle) then counts down (second cycle) to create a single symmetric pulse. So it uses two cycles to make each pulse.
(Think of a triangle wave, and the state of the pin flips on each compare match.)

Double the frequency by halving the prescaler, and you'll be back to where (I think) you want to be.

Posted 4 years ago #
bart_dood
Member

Hi gbulmer, I actually just wrote an e-mail I think you were copied on. I wasn't sure I'd get an answer on the forum because its not a regular question to ask.

My prescalar is already 1 so I can't lower it, so the only thing left to change is the overflow number, its currently 7200 so I can change it to 3600. The bad news is I effectively lose bit resolution which is why I was hoping to find another way around this. Its probably ok though.

I also will have to change my timed interrupt from 100 microseconds to 50 as this is running twice as slow as well.

Thanks for the help.

Posted 4 years ago #
gbulmer
Moderator

bart_dood - sorry I didn't get any email.

Yes, 50microseconds * 72MHz = 3600, and if you want a symmetric (up/down count) PWM, half that :-(

Posted 4 years ago #
bart_dood
Member

Hi, I have made all the changes to the timer setup and interrupt time and everything seems to work as it should.
Having a PWM range of 0-3600 instead of 0-7200 isn't a show stopper after thinking it over some more.

Posted 4 years ago #
gbulmer
Moderator

bart_dood - I am pleased you've got it going. So have you a prototype BDC control close to working?

BTW - I have an STM32F4 evaluation board. Hardware Floating point is so quick, and the timers are capable of running much quicker too, so it seems like a good fit for this sort of thing.

Posted 4 years ago #
bart_dood
Member

hey, I am fairly close to being finished. My timed interrupt works perfectly now, all the math functions for the timed interrupt happen in around 55-60 microseconds so time to spare. My external interrupts only add a small amount of time onto this, around 5 microseconds so perfectly acceptable.
I've checked different cases and the numbers coming out make sense for the PWM duty cycles, I want to double check this though.

It still might take me a few weeks to turn a motor, I'll start doing some temporary wiring from my control box to the rest of the electronics over the new week.

I'll be happy to have everything working by christmas, it would be great to get it running sooner than that though.

The code is surprisingly compact, I don't even use library functions. Library functions don't save any cycles (in fact they seem to increase time lag a little), perhaps once this code is out in the public domain others will tidy it up and maybe break it into libraries.
However the code I wrote does have some specific things in it so for generic use it will need modification.

Posted 4 years ago #

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