36.3.1.2.1 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 × 250 ns = 16.4 ms/byte = 16.4 ms / 8 bits = 2.05 ms/bit.

This gives a worst-case BREAK frame duration of 2.05 m s × 12 b i t s 24.6 ms 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 36-3.
Table 36-3. 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