
Register Description and Programming
MOTOROLA
UART Modules
15-23
15.4.1.8.3
Receiver Disable
The “receiver disable” command immediately disables the receiver. Any character being received is lost.
The command has no effect on the receiver status bits or any other control register. If the UART module is
programmed to operate in the local loopback mode or multidrop mode, the receiver operates even though
this command is selected. If the receiver is already disabled, this command has no effect.
15.4.1.8.4
Do Not Use
Do not use this bit combination because the result is indeterminate.
15.4.1.9
Receiver Buffer Registers (UBRn)
The receiver buffer (URB) contains three receiver-holding registers and a serial shift register. The RxD pin
is connected to the serial shift register while 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
15.4.1.10
Transmitter Buffer Registers (UTBn)
The transmitter buffer (UTB) consists of two registers: the transmitter-holding register and the transmitter
shift register (see
Figure 15-4). The holding 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 additional
characters until the shift register is ready to accept more data. When the shift register is empty, it checks
the holding register for a valid character to be sent (TxRDY bit cleared). If a valid character is present, the
shift register loads the character and reasserts the TxRDY bit in the USR. Writes to the transmitter buffer
when the channel's UART Status Register (USR) TxRDY bit is clear and when the transmitter is disabled
have no effect on the transmitter buffer.
Table 15-16 Receiver Buffer (URBn)
BITS
7
6
5
4
3
2
1
0
FIELD
RB7
RB6
RB5
RB4
RB3
RB2
RB1
RB0
RESET
111
11
1
R/W
READ ONLY
ADDR
MBAR + $1CC
MBAR + $20C
Table 15-17 Receiver Buffer Bit Descriptions
BIT NAME
DESCRIPTION
RB7–RB0
These bits contain the character in the receiver buffer.
Table 15-18 Transmitter Buffer (UTBn)
BITS
7
6
5
4
3
2
1
0
FIELD
TB7
TB6
TB5
TB4
TB3
TB2
TB1
TB0
RESET
00
000
00
0
R/W
WRITE ONLY
ADDR
MBAR + $1CC
MBAR + $20C
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Freescale Semiconductor, Inc.
For More Information On This Product,
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