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AT91SAM9X25-EK
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SAM9X25 [DATASHEET]
11054E–ATARM–10-Mar-2014
Figure 39-25.Framing Error Status
39.7.3.13 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
to 0. However, the transmitter holds the TXD line at least during one character until the user requests the break condition
to be removed.
low during at least one complete character. It appears the same as a 0x00 character sent with the parity and the stop bits
to 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 to 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.
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 to 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.
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 to 1 and the start of the break condition clears the TXRDY and TXEMPTY
bits as if a character is processed.
account only if the TXRDY bit in US_CSR is to 1 and the start of the break condition clears the TXRDY and TXEMPTY
bits as if a character is processed.
Writing US_CR with both STTBRK and STPBRK bits to 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.
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 transmitter
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.
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.
illustrates the effect of both the Start Break (STTBRK) and Stop Break (STPBRK) commands on the TXD
line.
D0
D1
D2
D3
D4
D5
D6
D7
RXD
Start
Bit
Parity
Bit
Stop
Bit
Baud Rate
Clock
Write
US_CR
FRAME
RXRDY
RSTSTA = 1