Datasheet

SAM9G20
DS60001516A-page 464  2017 Microchip Technology Inc.
Figure 31-25: Framing Error Status
31.6.3.14 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 with-
out 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.
After holding the TXD line for this period, the transmitter resumes normal operations.
Figure 31-26 illustrates the effect of both the Start Break (STTBRK) and Stop Break (STPBRK) commands on the TXD line.
Figure 31-26: Break Transmission
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
D0 D1 D2 D3 D4 D5 D6 D7
TXD
Start
Bit
Parity
Bit
Stop
Bit
Baud Rate
Clock
Write
US_CR
TXRDY
TXEMPTY
STPBRK = 1
STTBRK = 1
Break Transmission End of Break