Arduino Zero / Adafruit Feather M0 - PWM Outputs

Hi wallarug,

This can be confusing.  A good place to start is the datasheet which Adafruit has linked to at the top of the Downloads section of your link.  Note that Adafruit is using the G variant (SAM D21G).  In the configuration summary on page 3 the datasheet states that there are 14 ADC channels.  Paging down to the pinout description on Page 15 the 14 ADC channels are shown next to their physical pins on the microcontroller along with the port / pin designation.  For example, physical pin 3 is connected to PA02.

The pins can be multiplexed, i.e. assigned to different peripheral functions.  These capabilities are described in Section 10 which starts on Page 21.  Using physical pin 3 again as an example, we see it can be configured among other things as ADC AIN(0) using the Atmel nomenclature.  Note that not all of the ADC configurations are available for the G variant.

So, now the capabilities are known, but it is necessary to look at what pins Adafruit (or Arduino) has brought out on their board for use and how they mapped it to the Arduino IDE.  Returning to the pinout map in your link.  Look for physical pin 3 again in the dark grey box nearest the representation of the board.  Note that it is labeled A0 on the board.  Next to the physical pin 3 box is an olive colored one that has PA02 in it - the same as in the datasheet.  We also see the other configurations from the datasheet.  In the furthest most box we see '14' and 'A0'.  The legend for the pinout tells us that these are the Arduino IDE designations.

Adafruit has somewhat arbitrarily decided to bring PA02, located on physical pin 3 of the SAMD21G, and call it A0.  I count 7 pins which Adafruit has brought out and labeled as PWM on the pinout.  You can go through the same exercise with the Arduino Zero. 

Frank

Hi Frank,

Thank you for your very informative and helpful response.

Just to confirm one final point... from the datasheet, the function to look for to indicate that the pin is PWM capable is ADC.

From the datasheet it appears that the following pins on the SAM D21G have this capability (if you had access to all pins):

Thank you very much for guiding me through this process. 

Good catch.  What I stated above is factually correct but I read your question wrong / had a brain lapse and have misled you.  I wish they wouldn't call PWM analogWrite.  PWM is really about setting up the output from timers and is not analog.  It is possible of course to have PWM without any ADC capability.  Arduino uses the tilde (~) to designate PWM.  I am not really an Arduino user, have only just started using the Arduino MKR boards, and have not used the Adafruit.  Look on your board for the tilde and that is where you have PWM.  Those are the pins that they decided to map to the timers.  It is still a good idea to get familiar with the datasheet.  The timers are very flexible and powerful if used outside the Arduino IDE.

Also, the timer counters start with TC in the table and the discussion starts in section 29 on page 610!  I need to get more familiar with this microcontroller.

Also, the timer counters start with TC in the table and the discussion starts in section 29 on page 610!  I need to get more familiar with this microcontroller.

I believe your new information invalidates some of the pins in this list above.  Do does this mean that more of the pins are capable of PWM than my list above?

I guess an easier way of putting it is this... is the diagram on this website correct based on your knowledge of the datasheet?

I'm still a bit bamboozled.  

wallarug  wrote:

 

I guess an easier way of putting it is this... is the diagram on this website correct based on your knowledge of the datasheet?

 

I'm still a bit bamboozled.  

I am still learning this microcontroller myself and don't have the Adafruit board.  The honest answer is I don't know whether it is right or not for the Adafruit board.  One way to find out is to write a short sketch that tries to apply PWM to each pin you want to test and put a LED on it and see if it dims.

It is worth remembering that Arduino purposely abstracts all this detail away to make it simple to use.  Once we need greater access to the capability that is hidden by the abstractions it is sometimes easier to use a more professional IDE from a clean slate and access the registers directly.

Looking at the datasheet again, we find this snippet in the features that covers timers:

I take this to mean that there are three 24-bit timer/counters set up for explicit control of PWM which Atmel calls Timer/Counters for Control (TCC).  They are labeled TCC in column F of the multiplexing table.  Looking at this table and what pins are brought out would tell us where Adafruit could have chosen to use these timer/counters to control PWM.  You might try looking for the variant.cpp file for your board.

PWM is pretty simple, if maybe somewhat tedious, to set using registers.  Basically choose a pin connected to a timer/counter and set up a clock to the timer/counter.  Set the registers for how long the pin is high and how long it is low.  Twiddle other bits as needed.  This is all done in hardware without interruption to the microcontroller.  But we could do it without that hardware support.  We could use any timer and set it to interrupt after say a certain interval high.  In the interrupt flip the pin low and set the time for the next interval.  Then flip the pin again and reset the interval.  Repeat over and over.  In this way, any GPIO could be used for PWM.  It may be possible to do either of these approaches within the Arduino IDE but I don't know what conflicts it might introduce.

I know this can be confusing but hopefully helps.  If you test the pins as described above I would be interested in what you find.