SAM7X128_14 Datasheet, PDF (305/662 Pages) ATMEL Corporation – ARM-based Flash MCU

30.6.3.6 Multidrop Mode
If the PAR field in the Mode Register (US_MR) is programmed to the value 0x6 or 0x07, the USART runs in Multidrop
Mode. This mode differentiates the data characters and the address characters. Data is transmitted with the parity bit at
0 and addresses are transmitted with the parity bit at 1.
If the USART is configured in multidrop mode, the receiver sets the PARE parity error bit when the parity bit is high and
the transmitter is able to send a character with the parity bit high when the Control Register is written with the SENDA bit
at 1.
To handle parity error, the PARE bit is cleared when the Control Register is written with the bit RSTSTA at 1.
The transmitter sends an address byte (parity bit set) when SENDA is written to US_CR. In this case, the next byte
written to US_THR is transmitted as an address. Any character written in US_THR without having written the command
SENDA is transmitted normally with the parity at 0.
30.6.3.7 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 30-13, 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 30-13. Timeguard Operations
Baud Rate
Clock
TG = 4
TG = 4
TXD
Write
US_THR
Start
Bit
D0
D1
D2
D3
D4
D5
D6
D7
Parity Stop
Bit Bit
Start
Bit
D0
D1
D2
D3
D4
D5
D6
D7
Parity Stop
Bit Bit
TXRDY
TXEMPTY
Table 30-7 indicates the maximum length of a timeguard period that the transmitter can handle in relation to the function
of the Baud Rate.
Table 30-7. Maximum Timeguard Length Depending on Baud Rate
Baud Rate
Bit time
Bit/sec
µs
1 200
833
9 600
104
14400
69.4
Timeguard
ms
212.50
26.56
17.71
SAM7X Series [DATASHEET]
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