MMC2107 - Freescale Semiconductor

Freescale Semiconductor, Inc.
Serial Communications Interface Modules (SCI1 and SCI2)
The SCI sets the TDRE flag every time it transfers data from SCIDRH
and SCIDRL to the transmit shift register. TDRE indicates that SCIDRH
and SCIDRL can accept new data. If the TIE bit is set, TDRE generates
an interrupt request.
NOTE:
SCIDRH and SCIDRL transfer data to the transmit shift register and sets
TDRE 9/16ths of a bit time after the previous frame’s stop bit starts to
shift out.
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Hardware supports odd or even parity. When parity is enabled, the most
significant data bit is the parity bit.
When the transmit shift register is not transmitting a frame, the TXD pin
goes to the idle condition, logic 1. Clearing the TE bit while the
transmitter is idle will return control of the TXD pin to the SCI data
direction (SCIDDR) and SCI port (SCIPORT) registers.
If the TE bit is cleared while a transmission is in progress (while TC = 0),
the frame in the transmit shift register continues to shift out. Then the
TXD pin reverts to being a general-purpose I/O pin even if there is data
pending in the SCI data register. To avoid accidentally cutting off a
message, always wait until TDRE is set after the last frame before
clearing TE.
To separate messages with preambles with minimum idle line time, use
this sequence between messages:
1. Write the last byte of the first message to SCIDRH and SCIDRL.
2. Wait until the TDRE flag is set, indicating the transfer of the last
frame to the transmit shift register.
3. Queue a preamble by clearing and then setting the TE bit.
4. Write the first byte of the second message to SCIDRH and
SCIDRL.
When the SCI relinquishes the TXD pin, the SCIPORT and SCIDDR
registers control the TXD pin.
To force TXD high when turning off the transmitter, set bit 1 of the SCI
port register (SCIPORT) and bit 1 of the SCI data direction register
(SCIDDR). The TXD pin goes high as soon as the SCI relinquishes
control of it.
Technical Data MMC2107 – Rev. 2.0
354 Serial Communications Interface Modules (SCI1 and SCI2) MOTOROLA
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