Hi all, I have a weird issue, when I set the PWM duty cycle on pins 2 or 3 to 100%, my code works great. When I switch one pin to be pin 4 or 5 I experience really odd behavior. My code gradually increases the duty cycle to 100% and when it hits 100 it drops to 0, on pins 4 and 5, but not pins 2 or 3. I've confirmed it's not my code as the only thing I changed was which pin I'm using. I've also confirmed the duty cycle is indeed dropping to 0% using my oscilloscope (very cheap DIY one, but it works for my needs). It's not entirely a huge deal as I only need 2 pins, I'm more curious if anyone has any insight into why this happens.
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Hi Pert, it's not an error.
Hi. Very sorry about that. I copy/pasted the wrong reply from my collection of stock replies. Here's what I meant to write:
Please post your full sketch.
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No problem, I appreciate the replies. So my sketch is over 1300 lines long. I'm not sure if it's worth going through it. Certainly if you disagree then I'll gladly post it. Thing is if I switch the pin (via a variable so it's just one line) to pin 2 from 4 (or 5) it works. So I really don't think it's code related, it seems hardware related.
Edit: nm I'll post the sketch when I get a chance. You can decide if it's worth looking over 
You're welcome write a minimal, complete, verifiable example (MCVE) sketch that contains the minimum amount of code required to demonstrate the problem and then post it here. Often I find that by the time I'm done producing my MCVE, I've found the problem myself and no longer need to ask for help.
An MCVE I can do! And as I suspected it happens even with this minimal piece of code. Try it yourself, hook up your arduino to an oscilloscope (or PWM a fan, motor, LED), you'll see that the duty cycle drops to 0% when 4096 is written to pins 4 or 5. Pins 2 and 3 work correctly, 4096 is 100% duty cycle. Note I haven't tried any other pin besides these four.
const byte pin = 2; //works correctly
//const byte pin = 3; //works correctly
//const byte pin = 4; //doesn't work correctly
//const byte pin = 5; //doesn't work correctly
const byte pwmResolution = 12; //12 bits for 4096 levels of resolution
void setup() {
analogWriteResolution(pwmResolution);
pinMode(pin, OUTPUT);
//When 4096 is "written" to the PWM pin that's 100% duty cycle, but on pins 4 and 5,
//upon 100%, the voltage drops back down to 0, or 0% duty cycle.
for(int i = 0; i <= 4096; i++)
{
analogWrite(pin, i);
delay(5);
}
}
void loop() {
// put your main code here, to run repeatedly:
}
I figured out the issue. It IS my code. I'm using values between 0 and 4096 (including 4096), instead of 4095. For some odd reason for pins 2 and 3 when exceeding the highest allowed value it just stays at 100% duty cycle, but for pins 4 and 5, it drops it down to 0 instead.
Nice work finding the bug! Thanks for taking the time to post an update.
I must admit I don't have an explanation for the different behavior between the pins. Maybe someone else here will have an answer. Otherwise, we can just chalk it up to "undefined behavior".
pins 2 and 3 use TCC type of timer on SAMD and 4 and 5 use TC type timer, so the timer setup for pwm is different.
Pert, welcome! Glad I was able to figure out more info.
Juraj, do you have a link that explains the difference between timers and why they act different? My curiosity is getting the best of me.
mnl1121:
Pert, welcome! Glad I was able to figure out more info.Juraj, do you have a link that explains the difference between timers and why they act different? My curiosity is getting the best of me.
chapter 30. and 31.
Atmel's timers behave in ways one would not expect when the compare match value or TOP is set to 0.
I spent many hours this weekend trying to figure out why TCB1 in periodic interrupt mode on the attiny3216 (it's a megaAVR - a pretty baller chip too, now fully supported by megaTinyCore GitHub - SpenceKonde/megaTinyCore: Arduino core for the tinyAVR 0/1/2-series - this is any ATtiny with 2, 4, 6, or 7 in the 1's place, 0, 1, or 2 in the tens, and the flash size in the remaining digits. LIBRARY AUTHORS - Does your library have issues with this core? Please touch base w/me so we can ensure a solution that works on all parts and won't be broken by future changes ) wasn't firing while trying to make Servo work. Turned out that I hadn't set TCB1.CCMP (due to an < instead of <= in the initialization code), so it was 0 - and in that configuration, it never generates the interrupt (I'd have expected it to either run until it overflowed and then fire, or constantly fire - the former would have not even been noticed, while the latter would have taken minutes to figure out instead of hours)