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 a ‘1’ to US_CR.STTBRK. 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 a ‘1’ to US_CR.STPBRK. 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 processed only if US_CSR. TXRDY = 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 US_THR 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.
The following figure illustrates the effect of both the Start Break (STTBRK) and Stop Break (STPBRK) commands on the TXD line.
