
MC68336/376
QUEUED SERIAL MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
9-26
9.4.1.2 Status Register
SCSR contains flags that show SCI operating conditions. These flags are cleared
either by SCI hardware or by reading SCSR, then reading or writing SCDR. A long-
word read can consecutively access both SCSR and SCDR. This action clears
receiver status flag bits that were set at the time of the read, but does not clear TDRE
or TC flags.
If an internal SCI signal for setting a status bit comes after reading the asserted status
bits, but before reading or writing SCDR, the newly set status bit is not cleared. SCSR
must be read again with the bit set, and SCDR must be read or written before the sta-
tus bit is cleared.
Reading either byte of SCSR causes all 16 bits to be accessed, and any status bit
already set in either byte is cleared on a subsequent read or write of SCDR.
9.4.1.3 Data Register
SCDR contains two data registers at the same address. The receive data register
(RDR) is a read-only register that contains data received by the SCI serial interface.
Data enters the receive serial shifter and is transferred to RDR. The transmit data reg-
ister (TDR) is a write-only register that contains data to be transmitted. Data is first
written to TDR, then transferred to the transmit serial shifter, where additional format
bits are added before transmission. R[7:0]/T[7:0] contain either the first eight data bits
received when SCDR is read, or the first eight data bits to be transmitted when SCDR
is written. R8/T8 are used when the SCI is configured for 9-bit operation. When it is
configured for 8-bit operation, they have no meaning or effect.
9.4.2 SCI Pins
Two unidirectional pins, TXD (transmit data) and RXD (receive data), are associated
with the SCI. TXD can be used by the SCI or for general-purpose I/O. Function is
assigned by the port QS pin assignment register (PQSPAR). The receive data (RXD)
pin is dedicated to the SCI. Table 9-4 shows SCI pin function.
9.4.3 SCI Operation
SCI operation can be polled by means of status flags in SCSR, or interrupt-driven
operation can be employed by the interrupt enable bits in SCCR1.
9.4.3.1 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.
Table 9-4 SCI Pins
Pin Names
Mnemonics
Mode
Function
Receive Data
RXD
Receiver disabled
Receiver enabled
Not used
Serial data input to SCI
Transmit Data
TXD
Transmitter disabled
Transmitter enabled
General-purpose I/O
Serial data output from SCI