36.3.2.2.1.1 Time-Out Disable
When the start-up negative edge detector releases the pin after the UPDI has received its clock, or when the regulator is stable and the system has power in a multi-voltage system, the default pull-up drives the UPDI pin high. If the programmer does not detect that the pin is high and does not initiate a transmission of the SYNCH character within 16.4 ms at 4 MHz UPDI clock after the UPDI has released the pin, the UPDI will disable itself.
Note: Start-up oscillator frequency is
device-dependent. The UPDI will count for 65536 cycles on the UPDI clock before issuing
the time-out.