UART/IrDA Functional Description
12-99
UART Devices
12.9.11.2
TX
XOFF1: Two characters are transmitted when the RX FIFO has passed the
programmed trigger level TCR(3:0).
XON1: Two characters are transmitted when the RX FIFO reaches the trigger
level programmed via TCR(7:4).
If, after an XOFF character has been sent, software flow control is disabled,
the module transmits XON characters automatically to enable normal trans-
mission to proceed.
The transmission of XOFF/XON(s) follows the exact same protocol as trans-
mission of an ordinary byte from the FIFO. This means that even if the word
length is set to be 5, 6, or 7 characters, the 5, 6, or 7 least significant bits of
XOFF1,2/XON1,2 are transmitted. The transmission of 5, 6, or 7 bits of a char-
acter is seldom done, but this functionality is included to maintain compatibility
with earlier designs.
It is assumed that software flow control and hardware flow control are never
enabled simultaneously.
12.9.12
Frame Closing
There are two methods by which a transmission-frame can be properly
terminated.
1) The frame-length method is selected when MDR1[7] = 0. The host (MPU
or DSP) writes the frame-length value to TXFLH and TXFLL registers. The
device automatically attaches end flags to the frame once the number of
bytes transmitted becomes equal to the frame-length value.
2) The set-EOT bit method is selected when MDR1[7] = 1. The host writes
1 to ACREG[0] (EOT bit) just before it writes the last byte to the TX FIFO.
When the host writes the last byte to the TX FIFO, the device internally sets
the tag bit for that particular character in the TX FIFO. As the TX state
machine reads data from the TX FIFO, it uses this tag-bit information to
attach end flags and properly terminate the frame.