Serial Module
MOTOROLA
MC68307 USER’S MANUAL
8-25
into the search-for-start-bit state. If the receiver is already enabled, this command has no
effect.
Receiver Disable—The receiver disable command disables the receiver immediately. Any
character being received is lost. The command has no effect on the receiver status bits or
any other control register. If the serial module is programmed to operate in the local loop-
back mode or multidrop mode, the receiver operates even though this command is select-
ed. If the receiver is already disabled, this command has no effect.
Do Not Use—Do not use this bit combination because the result is indeterminate.
8.4.1.6 RECEIVER BUFFER (URB). The receiver buffer contains three receiver holding
registers and a serial shift register. The RxD pin is connected to the serial shift register. The
holding registers act as a FIFO. The CPU reads from the top of the stack while the receiver
shifts and updates from the bottom of the stack when the shift register has been filled (see
RB7–RB0—These bits contain the character in the receiver buffer.
8.4.1.7 TRANSMITTER BUFFER (UTB). The transmitter buffer consists of two registers,
the transmitter holding register and the transmitter shift register (see
Figure 8-3). The hold-
ing register accepts characters from the bus master if the TxRDY bit in the channel's USR
is set. A write to the transmitter buffer clears the TxRDY bit, inhibiting any more characters
until the shift register is ready to accept more data. When the shift register is empty, it checks
to see if the holding register has a valid character to be sent (TxRDY bit cleared). If there is
a valid character, the shift register loads the character and reasserts the TxRDY bit in the
USR. Writes to the transmitter buffer when the channel's USR TxRDY bit is clear and when
the transmitter is disabled have no effect on the transmitter buffer.
TB7–TB0—These bits contain the character in the transmitter buffer.
URB
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7
6
5
4
3
2
1
0
RB7
RB6
RB5
RB4
RB3
RB2
RB1
RB0
RESET:
0
Read Only
Supervisor or User
UTB
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7
6
5
4
3
2
1
0
TB7
TB6
TB5
TB4
TB3
TB2
TB1
TB0
RESET:
0
Write Only
Supervisor or User