MOTOROLA
PROGRAMMING REFERENCE
MMC2001
C-36
REFERENCE MANUAL
C.9.1 UART Receive Register (URX)
This read-only register contains received characters and status. After reset, if the
receiver is enabled (RXEN = 1), the CHARRDY bit is zero until the first character is
received, and the remainder of the register contents are undefined. The RX register is
echoed to 16-word addresses in order to support unloading the FIFO with the load
register quadrant (ldq) instruction.
*Echoes begin at 10009004 and 1000A004 respectively.
X = Undefined
Figure C-36 UART Receive Register
CHARRDY — Character Ready
This read-only bit indicates whether the character in the RX DATA field and associ-
ated flags are valid and ready to be read by the host.
0 =
Character in RX DATA field and associated flags are invalid
1 =
Character in RX DATA field and associated flags valid and ready for reading
At reset, this bit is cleared to zero.
ERR — Error Detect
When set, this read-only bit indicates that the character present in the RX DATA field
has an error status. The error can be an OVRRUN, FRMERR, BRK or PRERR. This
bit is updated and valid for each received character.
0 =
No error status detected
1 =
Error status detected
At reset, this bit is cleared to zero.
OVRRUN — Receiver Overrun
When set, this read-only bit indicates that the receiver ignored data to prevent over-
writing the data in the FIFO. Under normal circumstances, this bit should never be
set. It indicates that the user’s software is not keeping up with the incoming data rate.
This bit is updated and valid for each received character, and when set indicates that
some number of characters were lost
following the character for which the flag is set.
0 =
No FIFO overrun
1 =
A FIFO overrun was detected
At reset, this bit is cleared to zero.
FRMERR — Frame Error
When set, this read-only bit indicates that the current character had a framing error
(missing stop bit). The data is possibly corrupted. This bit is updated for each charac-
ter read from the FIFO.
U0RX — UART0 Receive Register
10009000*
U1RX — UART1 Receive Register
1000A000*
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
R
CHAR
RDY
ERR
OVR
RUN
FRM
ERR
BRK
PR
ERR
0
0
RX DATA
W
RESET:
0
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
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