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6174B–ATARM–07-Nov-05
AT91FR40162S Preliminary
17.6
Break
A break condition is a low signal level which has a duration of at least one character (including
start/stop bits and parity).
17.6.1
Transmit Break
The transmitter generates a break condition on the TXD line when STTBRK is set in US_CR
(Control Register). In this case, the character present in the Transmit Shift Register is completed
before the line is held low.
To cancel a break condition on the TXD line, the STPBRK command in US_CR must be set. The
USART completes a minimum break duration of one character length. The TXD line then returns
to high level (idle state) for at least 12 bit periods to ensure that the end of break is correctly
detected. Then the transmitter resumes normal operation.
The BREAK is managed like a character:
• The STTBRK and the STPBRK commands are performed only if the transmitter is ready (bit
TXRDY = 1 in US_CSR)
• The STTBRK command blocks the transmitter holding register (bit TXRDY is cleared in
US_CSR) until the break has started
• A break is started when the Shift Register is empty (any previous character is fully
transmitted). TXEMPTY is cleared in US_CSR. The break blocks the transmitter shift register
until it is completed (high level for at least 12-bit periods after the STPBRK command is
requested)
In order to avoid unpredictable states:
• STTBRK and STPBRK commands must not be requested at the same time
• Once an STTBRK command is requested, further STTBRK commands are ignored until the
BREAK is ended (high level for at least 12-bit periods)
• 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
(US_CSR.TXRDY = 0) is ignored
• It is not permitted to write new data in the Transmit Holding Register while a break is in
progress (STPBRK has not been requested), even though TXRDY = 1 in US_CSR.
• A new STTBRK command must not be issued until an existing break has ended (TXEMPTY
= 1 in US_CSR)
The standard break transmission sequence is:
1.
Wait for the transmitter ready
(US_CSR.TXRDY = 1)
2.
Send the STTBRK command
(write 0x0200 to US_CR)
3.
Wait for the transmitter ready
(TXRDY = 1 in US_CSR)
4.
Send the STPBRK command
(write 0x0400 to US_CR)
The next byte can then be sent:
5.
Wait for the transmitter ready
(TXRDY = 1 in US_CSR)
Содержание AT91FR40162S
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