`HardwareTimer` PWM resolution stuck at 12-bit?

[jeffstaley]

If I run a one-line Arduino program on a STM32F7 of

analogWrite(FAN1_PIN, 255)

I get a PWM signal with a very low duty cycle instead of the expect 100%.

Digging into the code I see pwm_start() setting up a HardwareTimer with a hard coded 12-bit resoultion:

  HT->setCaptureCompare(channel, value, RESOLUTION_12B_COMPARE_FORMAT);

[fpistm]

FAN1_PIN is equal to?

[jeffstaley]

Any of the PinMap_PWM pins. But in this case 30.

[fpistm]

The default PWM resolution is 12 bits.
So you should do:
analogWrite(FAN1_PIN, 4095);
To get a 100% duty cycle.
I have to update the wiki
https://github.com/stm32duino/wiki/wiki/API#example-1
The default resolution moved from 8 to 12.

[jeffstaley]

But if

#define PWM_RESOLUTION              12

is redefined in variant.h to

#define PWM_RESOLUTION              8

analogWrite() is still in 12-bit mode.

[fpistm]

The 8 bits value is converted to 12 bits thanks the map function.
Which board you used?

[jeffstaley]

I'm not seeing that. If PWM_RESOLUTION is set to 8 the map function does nothing (since we're in 8-bit mode). But the HWTimer is still hard coded to 12-bit mode.

It appears that the map function is used when you dynamically change the resoultion via analogWriteResolution()

Board is RemRam v1

[fpistm]

@jeffstaley
You're right there is an issue.
There is a misalignment when the PWM_RESOLUTION is redefined.
This was introduced when HardwareTimer was released. We will fix this.
As a workaround simply comment the PWM_RESOLUTION in the variant.h.

[jeffstaley]

Commenting out PWM_RESOLUTION in variant.h doesn't work for me since all the code I'm using (Marlin) assumes 8-bit resolution. Presumably I could do that and then dynamically change the resolution to 8-bit with analogWriteResolution() but I haven't tested that. Instead I pass in RESOLUTION_8B_COMPARE_FORMAT to the HardwareTimer module when compiled in 8-bit PWM mode. That my not be a general enough solution for you if accept additional resolutions.

[fpistm]

By default analogWriteResolution() is 8 bits. So the range you can used is 0-255. At a lower layer it is converted as 12 bits value to match Timer config at 12 bits. So you should have the proper duty cycle if you comment the PWM_RESOLUTION in variant.h.

[jeffstaley]

OK I see that now and that should be cleaner. I'll give that a try. Thanks.