This is the implementation of pwm_configure_channel:
static int
stm32f3_pwm_configure_channel(struct pwm_dev *dev, uint8_t cnum, struct
pwm_chan_cfg *cfg)
{
stm32f3_pwm_dev_t *pwm;
assert(dev->pwm_instance_id < PWM_COUNT);
assert(cnum < STM32F3_PWM_CH_COUNT);
pwm = &stm32f3_pwm_dev[dev->pwm_instance_id];
if (STM32F3_PWM_CH_DISABLED != pwm->ch[cnum].config) {
return -1;
}
if (STM32F3_PWM_CH_NOPIN != cfg->pin) {
if (hal_gpio_init_af(cfg->pin,
(uint8_t)(uintptr_t)cfg->data,
HAL_GPIO_PULL_NONE,
0)) {
return -1;
}
}
pwm->ch[cnum].duty = 0;
pwm->ch[cnum].pin = cfg->pin;
pwm->ch[cnum].invert = cfg->inverted;
pwm->ch[cnum].update = 0;
pwm->ch[cnum].enabled = 0;
pwm->ch[cnum].configure = 1;
return 0;
}
On an STM to connect the output pad with the PWM channel I have to configure it
for the alternate function (which is done by hal_gpio_init_af). The AF value,
the result of `(uint8_t)(uintptr_t)cfg->data` is a value between 0 and 15 which
determines what the pin is used for.
Unfortunately there is no universal AF value for pwm channels, on some pins
it's 1, on some it's 6 ... , some pins cannot be used as pwm channel outputs at
all and some pins could be used as the output of multiple channels.
With the construct above the application has to make a call like:
struct pwm_chan_cfg cfg;
cfg.pin = 8;
cfg.inverted = false;
cfg.data = (void*)6;
pwm_chan_config(pwm, 0, &cfg);
But this ONLY works if `pwm` is bound to the mcu's TIM1 and you use channel 0.
For TIM1 and channel 1 the app would have to use `pin=9/data=6`.
It's bad enough that only certain pins work for a given timer/channel but it's
even worse because the driver also needs to know the AF number to use.
As an additional crux, the HW timer to be used (TIM1 above) typically gets
defined by the BSP in hal_bsp_init, whereas pwm_chan_config is application
runtime code. I would prefer to break the coupling, or at least make it less
fragile.
Does this make sense?
Markus
On Thu, 1 Mar 2018 01:58:46 -0300
Miguel Azevedo <[email protected]> wrote:
> Hi markus,
>
> > `struct pwm_chan_config` only specifies the pin number. I've been
> > using the `data` member to put the burden on the application to
> > specify the "correct" number for the alternate function > - but I
> > find this a little unsatisfactory.
> That sounds a lot driver/hw specific. Can you show us the code for
> that?
>
> Thanks,
> Miguel
>
> On Wed, Feb 28, 2018 at 9:57 PM, markus <[email protected]> wrote:
> > While writing a PWM driver for the STM32s mcu family I came across
> > the issue of assigning output pins for a pwm channel.
> >
> > PWM output in STM32s is done by configuring a pin for "alternate
> > functions" - where specific pins have specific (possible) alternate
> > functions. In other words, only a very limited number of
> > timer:channel:pin combinations are possible, each one requiring a
> > very specific alternate function.
> >
> > `struct pwm_chan_config` only specifies the pin number. I've been
> > using the `data` member to put the burden on the application to
> > specify the "correct" number for the alternate function - but I
> > find this a little unsatisfactory.
> >
> > So I was wondering if somebody has a recommendation, better
> > approach to solving this issue?
> >
> > Have fun,
> > Markus
>
>
>
