Datasheet

SAM9G45 [DATASHEET]
Atmel-6438O-ATARM-SAM9G45-Datasheet_08-Dec-15
592
32.7.3.11 Transmitter Timeguard
The timeguard feature enables the USART interface with slow remote devices.
The timeguard function enables the transmitter to insert an idle state on the TXD line between two characters. This
idle state actually acts as a long stop bit.
The duration of the idle state is programmed in the TG field of the Transmitter Timeguard Register (US_TTGR).
When this field is programmed at zero no timeguard is generated. Otherwise, the transmitter holds a high level on
TXD after each transmitted byte during the number of bit periods programmed in TG in addition to the number of
stop bits.
As illustrated in Figure 32-23, the behavior of TXRDY and TXEMPTY status bits is modified by the programming of
a timeguard. TXRDY rises only when the start bit of the next character is sent, and thus remains at 0 during the
timeguard transmission if a character has been written in US_THR. TXEMPTY remains low until the timeguard
transmission is completed as the timeguard is part of the current character being transmitted.
Figure 32-23. Timeguard Operations
Table 32-10 indicates the maximum length of a timeguard period that the transmitter can handle in relation to the
function of the baud rate.
D0 D1 D2 D3 D4 D5 D6 D7
TXD
Start
Bit
Parity
Bit
Stop
Bit
Baud Rate
Clock
Start
Bit
TG = 4
Write
US_THR
D0 D1 D2 D3 D4 D5 D6 D7
Parity
Bit
Stop
Bit
TXRDY
TXEMPTY
TG = 4
Table 32-10. Maximum Timeguard Length Depending on Baud Rate
Baud Rate (bit/s) Bit Time (µs) Timeguard (ms)
1 200 833 212.50
9 600 104 26.56
14400 69.4 17.71
19200 52.1 13.28
28800 34.7 8.85
33400 29.9 7.63
56000 17.9 4.55
57600 17.4 4.43
115200 8.7 2.21