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MC68F375
QUEUED SERIAL MULTI-CHANNEL MODULE
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
6-50
6.8.6 SCI Pins
The RXD1 and RXD2 pins are the receive data pins for the SCI1 and SCI2, respec-
tively. TXD1 and TXD2 are the transmit data pins for the two SCI modules. An external
clock pin, ECK, is common to both SCIs. The pins and their functions are listed in
6.8.7 SCI Operation
The SCI can operate in polled or interrupt-driven mode. Status flags in SCxSR reflect
SCI conditions regardless of the operating mode chosen. The TIE, TCIE, RIE, and ILIE
bits in SCCxR1 enable interrupts for the conditions indicated by the TDRE, TC, RDRF,
and IDLE bits in SCxSR, respectively.
6.8.7.1 Definition of Terms
Bit-time — The time required to transmit or receive one bit of data, which is equal
to one cycle of the baud frequency.
Start bit — One bit-time of logic zero that indicates the beginning of a data frame.
A start bit must begin with a one-to-zero transition and be preceded by at least
three receive time samples of logic one.
Stop bit— One bit-time of logic one that indicates the end of a data frame.
Frame — A complete unit of serial information. The SCI can use 10-bit or 11-bit
frames.
Data frame — A start bit, a specified number of data or information bits, and at
least one stop bit.
Idle frame — A frame that consists of consecutive ones. An idle frame has no start
Table 6-26 SCxSR Bit Settings
Bit(s)
Name
Description
15:9
—
Reserved
8:0
R[8:0]/
T[8:0]
R[7:0]/T[7:0] contain either the eight data bits received when SCxDR is read, or the eight data
bits to be transmitted when SCxDR is written. R8/T8 are used when the SCI is configured for
nine-bit operation (M = 1). When the SCI is configured for 8-bit operation, R8/T8 have no mean-
ing or effect.
Accesses to the lower byte of SCxDR triggers the mechanism for clearing the status bits or for
initiating transmissions whether byte, half-word, or word accesses are used.
Table 6-27 SCI Pin Functions
Pin Names
Mnemonic
Mode
Function
Receive Data
RXD1, RXD2
Receiver disabled
Receiver enabled
General purpose input
Serial data input to SCI
Transmit Data
TXD1, TXD2
Transmitter disabled
Transmitter enabled
General purpose output
Serial data output from SCI
External Clock
ECK
Receiver disabled
Receiver enabled
Transmitter disabled
Transmitter enabled
Not used
Alternate input source to baud
Not used
Alternate input source to baud
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