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:
- Drive the UPDI line low and hold it low.
- Pull the RESET line low for the minimum reset pulse width and release it.
- Wait for at least 40 ms so the device can reach an initial state.
- Release the UPDI line to its UART IDLE state.
- Issue a negative pulse to the UPDI line.
- Send a CHIP ERASE key to unlock the device and poll for successful completion.
- 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.