
Register Description and Programming
MOTOROLA
UART Modules
15-19
15.4.1.4
Clock-Select Registers (USCRn)
The UCSR registers select the internal clock (timer mode) or the external clock in synchronous or
asynchronous mode. To use the timer mode for either the receiver and transmitter channel, program the
UCSR registers to the value $DD.
The transmitter and receiver can be programmed to different clock sources.
TxEMP
Transmitter Empty
1 = The transmitter has underrun (both the transmitter holding register and transmitter shift
registers are empty). This bit is set after transmission of the last stop bit of a character if
there are no characters in the transmitter-holding register awaiting transmission.
0 = The transmitter buffer is not empty. Either a character is currently being shifted out or
the transmitter is disabled. Users can enable/disable the transmitter by programming the
TCx bits in the UCR.
TxRDY
Transmitter Ready
1 = The transmitter-holding register is empty and ready to be loaded with a character. This
bit is set when the character is transferred to the transmitter shift register. This bit is also set
when the transmitter is first enabled. Characters loaded into the transmitter holding register
while the transmitter is disabled are not transmitted.
0 = The CPU has loaded the transmitter-holding register or the transmitter is disabled.
FFULL
FIFO Full
1 = Three characters have been received and are waiting in the receiver buffer FIFO.
0 = The FIFO is not full but can contain as many as two unread characters.
RxRDY
Receiver Ready
1 = One or more characters have been received and are waiting in the receiver buffer FIFO.
0 = The CPU has read the receiver buffer and no characters remain in the FIFO after this
read.
Table 15-10 Clock Select Register (UCSRn)
BITS
7
6
5
4
3
2
1
0
FIELD
RCS3
RCS2
RCS1
RCS0
TCS3
TCS2
TCS1
TCS0
RESET
110
11
10
1
R/W
WRITE ONLY
ADDR
MBAR + $1C4
MBAR + $204
Table 15-9 Status Bit Descriptions (Continued)
BIT NAME
DESCRIPTION
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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