Microchip Technology megaAVR 0 Series Manual page 443

This process is repeated for all bits in the frame, including the parity bit and two Stop bits. The baud
generator uses 16 samples, and the majority voting is done on sample 7, 8, and 9.
Figure 29-4. UPDI UART Start Bit and Data/Parity/Stop Bit Sampling
The transmission baud rate must be set up in relation to the selected UPDI clock, which can be adjusted
by UPDICLKSEL in UPDI.ASI_CTRLA. See
rate settings.
Table 29-1. Recommended UART Baud Rate Based on UPDICLKSEL Setting
UPDICLKSEL[1:0]
0x1 (16 MHz)
0x2 (8 MHz)
0x3 (4 MHz) - Default
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 the following table:
Table 29-2. Receiver Baud Rate Error
Data + Parity Bits
9
29.3.1.2 BREAK Character
The BREAK character is used to reset the internal state of the UPDI to the default setting. This is useful if
the UPDI enters an error state due to a communication error, or when the synchronization between the
debugger and the UPDI is lost.
A single BREAK character is enough to reset the UPDI, but in some special cases where the BREAK
character is sent when the UPDI has not yet entered the error state, a double BREAK character might be
needed. A double BREAK is ensured to reset the UPDI from any state. When sending a double BREAK it
is required to have at least one Stop bit between the BREAK characters.
No SYNCH character is required before the BREAK because the BREAK is used to reset the UPDI from
any state. This means that the UPDI will sample the BREAK based on the last stored baud rate setting,
derived from the last received valid SYNCH character. If the communication error was due to an incorrect
sampling of the SYNCH character, the baud rate is unknown to the connected debugger. For this reason,
©
2018 Microchip Technology Inc.
RxD IDLE
Sample
0 0 1 2 3 4 5 6
RxD
Sample
1 2 3 4 5 6
MAX Recommended Baud Rate MIN Recommended Baud Rate
0.9 Mbps
450 kbps
225 kbps
R
slow
96.39
Unified Program and Debug Interface (UPDI)
START
7 8 9 10 11 12 13 14 15 16
BIT n
7 8 9 10 11 12 13 14 15 16
Table 29-1 for recommended maximum and minimum baud
R Max. Total Error
fast
[%]
104.76 +4.76/-3.61
Datasheet Preliminary
®
megaAVR 0-Series
BIT 0
1 2 3
1
0.300 kbps
0.150 kbps
0.075 kbps
Recommended
Max. RX Error [%]
+1.5/-1.5
DS40002015A-page 443