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Problems with trinket PWM on pin 4

by andygrove on Tue Sep 24, 2013 10:22 pm


I've created a simple RGB LED "blinky" project using an Adafruit Arduino Trinket (the 5V version) and it is working pretty well ( but I haven't been able to get PWM working on pin #4 and could use some help. PWM works fine on pins #0 and #1 though.

I'm using common annode RGB LED from Adafruit also ( The annode pin is connected to the trinket's 5V out and the other three pins are connected to digital pins #0, #1, and #4 (each one via a 220 ohm resistor).

PIns #0 and #1 give the expected PWM glowing LED effect but pin #4 is just switching between ON and OFF. I have tried changing which color is connected to pin #4 and the same thing happens so the issue is specific to this pin. I am powering the trinket from a 9V battery when running the code and do not have USB connected (I know pin #4 is shared with USB).

I'd appreciate any pointers on where I am going wrong with this. Thanks in advance.

Here is the sketch that demonstrates the issue:

Code: Select all | TOGGLE FULL SIZE
int pinR = PB0; // Digital pin #0
int pinG = PB1; // Digital pin #1
int pinB = PB4; // Digital pin #4
void setup() {
  pinMode(pinR, OUTPUT);
  pinMode(pinG, OUTPUT);
  pinMode(pinB, OUTPUT);

void set(int rgb[]) {
  set(rgb[0], rgb[1], rgb[2]);

void set(int r, int g, int b) {
  // using common annode LED so low means ON!
  analogWrite(pinR, 255-r);
  analogWrite(pinG, 255-g);
  analogWrite(pinB, 255-b);

int index = 0;
void loop() {
  int rgb[3];
  int d = 20;
  rgb[0] = 0;
  rgb[1] = 0;
  rgb[2] = 0;
  for (int i=0; i<255; i+=5) {
    rgb[index] = i;
  for (int i=255; i>=0; i-=5) {
    rgb[index] = i;
  if (++index == 3) {
    index = 0;
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Re: Problems with trinket PWM on pin 4

by franklin97355 on Tue Sep 24, 2013 11:37 pm

You really should post Trinket questions in the Trinket forum, that's what it was created for.
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Re: Problems with trinket PWM on pin 4

by adafruit_support_mike on Wed Sep 25, 2013 12:51 am


Actually we have another thread about exactly the same issue currently running: viewtopic.php?f=52&t=43572

The short version is that this is a known glitch. The hardware supports PWM on pin4 but we're not sure if the Arduino core has the necessary code. It's on our "things to look at" list.
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Re: Problems with trinket PWM on pin 4

by mrburnette on Wed Sep 25, 2013 11:25 am


I use several tiny cores and I compiled the Op's code under the following two cores as noted and then used my scope to double-check the PWM on the three pins. As shown, both compiles when uploaded to the Trinket were successful.
Screen cut&paste of actual command line in bold:
With Digispark 16MHz core:
avrdude -c usbtiny -p attiny85 -U flash:w:sketch_sep25a.cpp.hex
Results: PWM on 0, 1, 4 verified

Source: ...
ATtiny85 @ 16 MHz (internal PLL; 4.3 V BOD):
avrdude -c usbtiny -p attiny85 -U flash:w:sketch_sep25a.cpp.hex
Results: PWM on 0, 1, 4 verified
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