44.3.1.4 Special Considerations for Programming

Any reset source will also reset the UPDI on a locked device. The reset may be triggered during boot of the device, which results in a repeating reset loop. Use a UPDI handshake mechanism to access a device in this state.

Important: Both the UPDI pin and the RESET pin must be connected to the programmer/debugger to use the UPDI handshake mechanism.

The UPDI handshake procedure:

  1. Drive the UPDI line low and hold it low.
  2. Pull the RESET line low for the minimum reset pulse width and release it.
  3. Wait for at least 40 ms so the device can reach an initial state.
  4. Release the UPDI line to its UART IDLE state.
  5. Issue a negative pulse to the UPDI line.
  6. Send a CHIP ERASE key to unlock the device and poll for successful completion.
  7. Toggle power on the device.
Important: Steps 4 to 6 must be completed within a 64 ms time window.
: After performing the chip erase, the lock bits on the device will be cleared. However, any reset source will continue to reset the UPDI until the next Power-On Reset happens.