F.A.Q.

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How come I can't find the Trinket Serial (COM) Port? Why is no Serial port found when the Trinket is plugged in?

Trinket (and Pro Trinket) do not have a USB-serial converter chip on board, this is how we can make them so small! Instead of a serial console, the Trinket is programmed directly over USB. No COM/Serial port is used at all!

I can't seem to upload to my Trinket when it's plugged into a USB 3.0 port (newer Macbooks have USB 3 ports)

Trinket's bootloader is finicky about USB 3 ports, and might not work on them. Try connecting to any USB 2 ports you have or go through a USB 1 or USB 2 hub (nearly all hubs are v2 or v1 instead of v3)

I'm trying to use Software Serial on the Pro Trinket 3V and getting compiler errors

If you're running an older version of the IDE, there may not be support for 12MHz software serial. Find your Arduino installation folder (if you're using a Mac you may need to explore the package) and locate the libraries/SoftwareSerial folder and open up SoftwareSerial.h inside. Add the following lines  before

#elif F_CPU == 8000000

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#elif F_CPU == 12000000
static const DELAY_TABLE PROGMEM table[] =
{
// baud rxcenter rxintra rxstop tx
  { 115200, 1, 11, 11, 8, },
  { 57600, 6, 26, 26, 23, },
  { 38400, 14, 41, 42, 39, },
  { 31250, 19, 51, 52, 49, },
  { 28800, 23, 56, 56, 53, },
  { 19200, 37, 86, 86, 83, },
  { 14400, 52, 116, 116, 113, },
  { 9600, 82, 175, 175, 173, },
  { 4800, 172, 354, 354, 351, },
  { 2400, 350, 711, 711, 708, },
  { 1200, 707, 1425, 1425, 1422, },
  { 300, 2850, 5711, 5711, 5708, },
};

const int XMIT_START_ADJUSTMENT = 5;

Once added, save and restart the IDE

My Pro Trinket sometimes doesn't "boot" or "run code" on power up

There's a slight bug in the Optiboot bootloader where it has a busy-wait loop while reading commands. If there is some noise on the RX pin, it can start the bootloader which then waits forever for more data

You can solve this in one of two ways

  1. Connect a pullup resistor of anywhere from 2K - 47K from the RX pin to the 5V pin (Pro Trinket 5V) or 3V pin (Pro Trinket 3V)
  2. Re-burn the bootloader with our patched version that seems to fix this bug by timing out on noisy RX reads 
I've tried everything, but I still get the "Could not find USBtiny device" error

Try a different USB cable.  Some cables are "charge only" and don't have data lines.

And even with data cables, some work better than others.

Can I use an EZ-Link to program a Pro Trinket?

We haven't personally tried it but some customers report that - Yes you can use an EZ-Link with a 3V or 5V Pro Trinket in FTDI-programming-mode

 Re-programming Bootloader Downloads 

This guide was first published on Aug 20, 2014. It was last updated on Aug 20, 2014.

This page (F.A.Q.) was last updated on May 15, 2021.

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