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UARTs
Receive (RX)
Transmit (TX)
196
Serial Interfaces
When the software flow control operation is enabled, the UART compares
incoming data with XOFF1/2 programmed characters (in certain cases,
XOFF1 and XOFF2 must be received sequentially). When the correct XOFF
characters are received, transmission is halted after completing transmission
of the current character. XOFF detection also sets IIR[4] (if enabled via IER[5])
and causes UART_nIRQ to go low.
To resume transmission an XON1/2 character must be received (in certain
cases XON1 and XON2 must be received sequentially). When the correct
XON characters are received, IIR[4] is cleared and the XOFF interrupt
disappears.
If a parity, framing, or break error occurs while receiving a software flow control
character, the character is treated as normal data and is written to the RX
FIFO.
When XON-any and special character detect are disabled and software flow
control is enabled, no valid XON or XOFF characters are written to the RX
FIFO. For example, in EFR[1:0] = 10, if the XON1 and XOFF1 characters are
received they are not written to the RX FIFO.
In the case where pairs of software flow characters are programmed to be
received sequentially (EFR[1:0] = 11), the software flow characters are not
written to the RX FIFO if they are received sequentially. However, received
XON1/XOFF1 characters must be written to the RX FIFO if the subsequent
character is not XON2/XOFF2.
XOFF1: Two characters are transmitted when the RX FIFO passes the
programmed trigger level TCR[3:0].
XON1: Two characters are transmitted when the RX FIFO reaches the
programmed trigger level via TCR[7:4].
If an XOFF character has been sent, software flow control is disabled and the
module transmits XON characters automatically to enable normal
transmission to proceed. The transmission of XOFF/XON (s) follows the same
protocol as transmission of an ordinary byte from the FIFO. This means that
even if the word length is set to be 5, 6, or 7 characters, then the 5, 6, or 7
least-significant bits of XOFF1,2/XON1,2 are transmitted. Note that the
transmission of 5, 6, or 7 bits of a character is seldom done, but this
functionality is included to maintain compatibility with earlier designs.
SPRU760B
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