
XR16C854/XR16C854D
3.3V AND 5V QUAD UART WITH 128-BYTE FIFO
REV. 2.0
á
14
2.10 T
RANSMITTER
The transmitter section comprises of an 8-bit Transmit
Shift Register (TSR) and 128 bytes of FIFO which in-
cludes a byte-wide Transmit Holding Register (THR).
TSR shifts out every data bit with the 16X internal
clock. A bit time is 16 clock periods. The transmitter
sends the start-bit followed by the number of data
bits, inserts the proper parity-bit if enabled, and adds
the stop-bit(s). The status of the FIFO and TSR are
reported in the Line Status Register (LSR bit-5 and
bit-6).
2.10.1 Transmit Holding Register (THR) - Write
Only
The transmit holding register is an 8-bit register pro-
viding a data interface to the host processor. The host
writes transmit data byte to the THR to be converted
into a serial data stream including start-bit, data bits,
parity-bit and stop-bit(s). The least-significant-bit (Bit-
0) becomes first data bit to go out. The THR is the in-
put register to the transmit FIFO of 128 bytes when
FIFO operation is enabled by FCR bit-0. Every time a
write operation is made to the THR, the FIFO data
pointer is automatically bumped to the next sequential
data location.
2.10.2 Transmitter Operation in non-FIFO Mode
The host loads transmit data to THR one character at
a time. The THR empty flag (LSR bit-5) is set when
the data byte is transferred to TSR. THR flag can
generate a transmit empty interrupt (ISR bit-1) when
it is enabled by IER bit-1. The TSR flag (LSR bit-6) is
set when TSR becomes completely empty.
2.10.3 Transmitter Operation in FIFO Mode
The host may fill the transmit FIFO with up to 128
bytes of transmit data. The THR empty flag (LSR bit-
5) is set whenever the FIFO is empty. The THR empty
flag can generate a transmit empty interrupt (ISR bit-
T
ABLE
6: T
YPICAL
DATA
RATES
WITH
A
14.7456 MH
Z
CRYSTAL
OR
EXTERNAL
CLOCK
O
UTPUT
Data Rate
MCR Bit-7=1
O
UTPUT
Data Rate
MCR Bit-7=0
(D
EFAULT
)
D
IVISOR
FOR
16x
Clock (Decimal)
D
IVISOR
FOR
16x
Clock (HEX)
DLM
P
ROGRAM
V
ALUE
(HEX)
DLL
P
ROGRAM
V
ALUE
(HEX)
D
ATA
R
ATE
E
RROR
(%)
100
600
1200
2400
4800
9600
19.2k
38.4k
57.6k
115.2k
230.4k
400
2400
4800
9600
19.2k
38.4k
76.8k
153.6k
230.4k
460.8k
921.6k
2304
384
192
96
48
24
12
6
4
2
1
900
180
C0
60
30
18
0C
06
04
02
01
09
01
00
00
00
00
00
00
00
00
00
00
80
C0
60
30
18
0C
06
04
02
01
0
0
0
0
0
0
0
0
0
0
0
F
IGURE
7. T
RANSMITTER
O
PERATION
IN
NON
-FIFO M
ODE
Transmit
Holding
Register
(THR)
Transmit Shift Register (TSR)
Data
Byte
L
S
B
M
S
B
THR Interrupt (ISR bit-1)
Enabled by IER bit-1
TXNOFIFO1
16X
Clock