The communication is initiated from the debugger/programmer side. Every transmission must start with a SYNCH character, which the UPDI can use to recover the transmission baud rate and store this setting for the incoming data. The baud rate set by the SYNCH character will be used for both reception and transmission of the subsequent instruction and data bytes. See the UPDI Instruction Set section for details on when the next SYNCH character is expected in the instruction stream.

There is no writable Baud Rate register in the UPDI, so the baud rate sampled from the SYNCH character is used for data recovery when sampling the data byte.

The transmission baud rate of the PHY layer is related to the selected UPDI clock, which can be adjusted by writing to the UPDI Clock Divider Select (UPDICLKSEL) bit field in the ASI Control A (UPDI.ASI_CTRLA) register. The receive and transmit baud rates are always the same within the accuracy of the auto-baud. It is recommended that the clock frequency does not run faster than the required frequency for the desired baud rate. The default UPDICLKSEL setting after Reset and enable is 4 MHz. Any other clock output selection is only recommended when the BOD is at the highest level. For all other BOD settings, the default 4 MHz selection is recommended.

Table 1. Recommended UART Baud Rate Based on UPDICLKSEL Setting
UPDICLKSEL[1:0] Max. Recommended Baud Rate Min. Recommended Baud Rate
0x0 (32 MHz) 1.6 Mbps 0.600 kbps
0x1 (16 MHz) 0.9 Mbps 0.300 kbps
0x2 (8 MHz) 450 kbps 0.150 kbps
0x3 (4 MHz) - Default 225 kbps 0.075 kbps
The UPDI Baud Rate Generator utilizes fractional baud counting to minimize the transmission error. With the fixed frame format used by the UPDI, the maximum and recommended receiver transmission error limits can be seen in Table 2.
Table 2. Receiver Baud Rate Error
Data + Parity Bits Rslow Rfast Max. Total Error [%] Recommended Max. RX Error [%]
9 96.39 104.76 +4.76/-3.61 +1.5/-1.5