Intel
®
81341 and 81342—UARTs
Intel
®
81341 and 81342 I/O Processors
Developer’s Manual
December 2007
890
Order Number: 315037-002US
15.4.1
UART x Receive Buffer Register
In non-FIFO mode, this register holds the character(s) received by the UART Receive
Shift register. When it receives fewer than eight bits, the bits are right-justified and the
leading bits are zeroed. Reading the register empties the register and resets the data
ready (DR) bit in the Line Status register to 0. Other (error) bits in the Line Status
register are not cleared. In FIFO mode, this register latches the value of the data
byte(s) at the bottom of the FIFO.
When the UART is in eight-bit Peripheral Bus mode, the 24 most significant bits must be
ignored and not used. Reading these bits returns unpredictable results.
.
15.4.2
UART x Transmit Holding Register
This register holds the next data byte(s) to be transmitted. When the Transmit Shift
register becomes empty, the contents of the Transmit Holding register are loaded into
the Shift register and the Transmit Data Request (TDRQ) bit in the Line Status register
is set to one (see
Table 568, “Interrupt Identification Register Decode” on page 893
In FIFO mode, writing to THR puts data to the top of the FIFO. The data at the bottom
of the FIFO is loaded to the Shift register when it is empty. In eight-bit Peripheral
mode, the 24 most significant bits are ignored and is not transmitted.
Table 564. UART x Receive Buffer Register - (UxRBR)
Bit
Default
Description
31:8
00h
Reserved
7:0
00h
Data Byte
P
C
I
IO
P
A
tt
ri
b
u
te
s
A
tt
ri
b
u
te
s
28
24
20
16
12
8
4
0
31
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
ro
na
ro
na
ro
na
ro
na
ro
na
ro
na
ro
na
ro
na
Unit #
01
Intel XScale
®
Core internal bus address
+2300H (DLAB=0)
+2340H (DLAB=0)
Attribute Legend:
RV = Reserved
PR = Preserved
RS = Read/Set
RW = Read/Write
RC = Read Clear
RO = Read Only
NA = Not Accessible
Table 565. UART x Transmit Holding Register - (UxTHR)
Bit
Default
Description
31:8
00h
Reserved
7:0
00h
Data Byte
PC
I
IO
P
A
tt
ri
b
u
te
s
A
tt
ri
b
u
te
s
28
24
20
16
12
8
4
0
31
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
rv
na
wo
na
wo
na
wo
na
wo
na
wo
na
wo
na
wo
na
wo
na
Unit #
01
Intel XScale
®
Core internal bus address
+2300H (DLAB=0)
+2340H (DLAB=0)
Attribute Legend:
RV = Reserved
PR = Preserved
RS = Read/Set
RW = Read/Write
RC = Read Clear
RO = Read Only
WO = Write Only
NA = Not Accessible