Input capture of PWM signal 40KHz

[Madril]

On pyboard D with STM32F722 I want to implement input capture for PWM signal with frequency aroound 40KHz curent implementation of my code is below:

Code: Select all

import pyb


class IC_Channel:
    def __init__(self, pin_name, timer, channel):
        self.channel = channel
        self.rising = 0
        self.falling = 0
        self.high_time = 0
        self.low_time = 0
        self.period = 0
        self.icpin = pyb.Pin(pin_name)
        self.timer_channel = timer.channel(channel, pyb.Timer.IC, pin=self.icpin, polarity=pyb.Timer.BOTH,
                                           callback=self.callback)
        self.tick = (timer.prescaler() + 1) / timer.source_freq()

        if timer in (2,5):
            self.overflow = 0x0fffffff
        else:
            self.overflow = 0x0fff

    def callback(self, tim):
        '''
        Callback function for the input capture channel
        :param tim: timer of the callback source
        :return:
        '''
        # Read the GPIO pin to figure out if this was a rising or falling edge
        if self.icpin.value():
            # Rising edge - start of the pulse
            self.rising = self.timer_channel.capture()
            self.low_time = (self.rising - self.falling) & self.overflow
        else:
            # Falling edge - end of the pulse
            self.falling = self.timer_channel.capture()
            self.high_time = (self.falling - self.rising) & self.overflow

        self.period = self.high_time + self.low_time

    def get_freq(self):
        '''
        Get frequency of the input capture channel
        :return:
        '''
        if(self.period > 0):
            freq = 1 / (self.period * self.tick)
        else:
            freq = 0
        return (freq)

    def get_period(self):
        '''
        Get period of the input capture channel
        :return:
        '''
        return (self.period * self.tick)

    def get_high_time(self):
        '''
        Get time (high state) of the input capture channel
        :return:
        '''
        return (self.high_time * self.tick)

    def get_low_time(self):
        '''
        Get time (low state) of the input capture channel
        :return:
        '''
        return (self.low_time * self.tick)

For now it always returns 0 for frequencys that is higher than 9,6KHz is there way to do this measurments for higher frequencies do I need to use specific pin and timer for example?

[jimmo]

What frequency is the timer running at?

[Madril]

Timer is 84MHz and prescaler is set to 83

[dhylands]

I have an example that runs on the original pyboard:
https://github.com/dhylands/upy-example ... ic_test.py

It should work on the STM32F7.

I was only testing a 50 Hz signal at the time.

This was part of an example that is documented more over here:
http://wiki.micropython.org/platforms/b ... r-Examples

[dhylands]

On the original pyboard, I don't think its fast enough to capture 40kHz using straight python. That's only 25 usec per cycle.

If the F7 is running at a faster MHz, and you use viper or asm mode, then you might be able to do it, but I doubt you can do anything useful on a continuous basis (i.e. capturing every cycle at 40 kHz).

I would try at a lower frequency to get things working and then ramp up the frequency to see where your limits are.

[pythoncoder]

It might help if you explained what is the overall purpose. At risk of stating the obvious, one option might be to apply the PWM signal to an ADC via a CR network...