33.3.2.1 UPDI Enable with Fuse Override of RESET Pin
When the RESET Pin Configuration (RSTPINCFG) bits in FUSE.SYSCFG0 are 0x1, the RESET pin will be overridden, and the UPDI will take control of the pin and configure it as input with pull-up. When the pull-up is detected by a connected debugger, the UPDI enable sequence, as depicted below, is started.
When the pull-up is detected, the debugger initiates the enable sequence by driving the line low for a duration of TDeb0 to ensure that the line is released from the debugger before the UPDI enable sequence is done.
The negative edge is detected by the UPDI, which requests the UPDI clock. The UPDI will continue to drive the line low until the clock is stable and ready for the UPDI to use. The duration of this TUPDI will vary, depending on the status of the oscillator when the UPDI is enabled. After this duration, the data line will be released by the UPDI and pulled high.
When the debugger detects that the line is high, the initial SYNCH character (0x55) must be sent to properly enable the UPDI for communication. If the Start bit of the SYNCH character is not sent well within maximum TDebZ, the UPDI will disable itself, and the enable sequence must be repeated. This time is based on counted cycles on the 4 MHz UPDI clock, which is the default when enabling the UPDI. The disable is performed to avoid the UPDI being enabled unintentionally.
After successful SYNCH character transmission, the first instruction frame can be transmitted.