SAM7S256_14 Datasheet, PDF (377/775 Pages) ATMEL Corporation – ARM-based Flash MCU

Figure 31-17. Framing Error Status
Baud Rate
Clock
RXD
Write
US_CR
FRAME
Start
Bit
D0
D1
D2
D3
D4
D5
D6
D7
Parity Stop
Bit Bit
RXRDY
RSTSTA = 1
31.6.3.10 Transmit Break
The user can request the transmitter to generate a break condition on the TXD line. A break condition drives the
TXD line low during at least one complete character. It appears the same as a 0x00 character sent with the parity
and the stop bits at 0. However, the transmitter holds the TXD line at least during one character until the user
requests the break condition to be removed.
A break is transmitted by writing the Control Register (US_CR) with the STTBRK bit at 1. This can be performed at
any time, either while the transmitter is empty (no character in either the Shift Register or in US_THR) or when a
character is being transmitted. If a break is requested while a character is being shifted out, the character is first
completed before the TXD line is held low.
Once STTBRK command is requested further STTBRK commands are ignored until the end of the break is
completed.
The break condition is removed by writing US_CR with the STPBRK bit at 1. If the STPBRK is requested before the
end of the minimum break duration (one character, including start, data, parity and stop bits), the transmitter
ensures that the break condition completes.
The transmitter considers the break as though it is a character, i.e. the STTBRK and STPBRK commands are
taken into account only if the TXRDY bit in US_CSR is at 1 and the start of the break condition clears the TXRDY
and TXEMPTY bits as if a character is processed.
Writing US_CR with the both STTBRK and STPBRK bits at 1 can lead to an unpredictable result. All STPBRK
commands requested without a previous STTBRK command are ignored. A byte written into the Transmit Holding
Register while a break is pending, but not started, is ignored.
After the break condition, the transmitter returns the TXD line to 1 for a minimum of 12 bit times. Thus, the transmit-
ter ensures that the remote receiver detects correctly the end of break and the start of the next character. If the
timeguard is programmed with a value higher than 12, the TXD line is held high for the timeguard period.
After holding the TXD line for this period, the transmitter resumes normal operations.
Figure 31-18 illustrates the effect of both the Start Break (STTBRK) and Stop Break (STPBRK) commands on the
TXD line.
SAM7S Series [DATASHEET]
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