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Serial Module
8-6
MC68307 USER’S MANUAL
MOTOROLA
8.3.2.1 TRANSMITTER. The transmitter is enabled through its command register (UCR)
located within the serial module. The serial module signals the CPU when it is ready to
accept a character by setting the transmitter-ready bit (TxRDY) in the UART status register
(USR). Functional timing information for the transmitter is shown in
Figure 8-5.The transmitter converts parallel data from the CPU to a serial bit stream on TxD. It auto-
matically sends a start bit followed by the programmed number of data bits, an optional par-
ity bit, and the programmed number of stop bits. The least significant bit is sent first. Data is
shifted from the transmitter output on the falling edge of the clock source.
Following transmission of the stop bits, if a new character is not available in the transmitter
holding register, the TxD output remains high ('mark' condition), and the transmitter empty
bit (TxEMP) in the USR is set. Transmission resumes and the TxEMP bit is cleared when
the CPU loads a new character into the transmitter buffer (UTB). If a disable command is
sent to the transmitter, it continues operating until the character in the transmit shift register,
if any, is completely sent out. If the transmitter is reset through a software command, oper-
mitter is re-enabled through the UCR to resume operation after a disable or software reset.
If clear-to-send operation is enabled, CTS must be asserted for the character to be trans-
mitted. If CTS is negated in the middle of a transmission, the character in the shift register
Figure 8-4. Transmitter and Receiver Functional Diagram
RECEIVER SHIFT REGISTER
COMMAND REGISTER (UCR)
W
STATUS REGISTER (USR)
R
TRANSMIT SHIFT REGISTER
MODE REGISTER 1 (UMR1)
R/W
MODE REGISTER 2 (UMR2)
R/W
TRANSMIT HOLDING REGISTER
W
RECEIVER HOLDING REGISTER 3
RECEIVER HOLDING REGISTER 2
RECEIVER HOLDING REGISTER 1
R
FIFO
RECEIVE
TRANSMIT
BUFFER (TRB)
(2 REGISTERS)
BUFFER (URB)
(4REGISTERS)
CHANNEL A
EXTERNAL
INTERFACE
RXD
TXD