BREAK in One-Wire Mode

In One-Wire mode, the programmer/debugger and UPDI can be totally out of synch, requiring a worst-case length for the BREAK character to be sure that the UPDI can detect it. Assuming the slowest UPDI clock speed of 4 MHz (250 ns), the maximum length of the 8-bit SYNCH pattern value that can be contained in 16 bits is

65535×250ns=16.4ms/byte=16.4ms/8bits=2.05ms/bit.

This gives a worst-case BREAK frame duration of 2.05ms×12bits24.6ms for the slowest prescaler setting. When the prescaler setting is known, the time of the BREAK frame can be relaxed according to the values from Table 1.
Table 1. Recommended BREAK Character Duration
UPDICLKSEL[1:0] Recommended BREAK Character Duration
0x1 (16 MHz) 6.15 ms
0x2 (8 MHz) 12.30 ms
0x3 (4 MHz) 24.60 ms