XR17C158 Exar Corporation, XR17C158 Datasheet - Page 22

• Manufacturer Part Number: XR17C158
• Description: Eight-channel Pci-based (UART)
• Manufacturer: Exar Corporation

PRELIMINARY
register. The MCR register bit-7 sets the prescaler to
divide the input crystal or external clock by 1 or 4.
The output of the prescaler clocks to the BRG. The
BRG further divides this clock by a programmable di-
visor between 1 and (2
sampling clock of the serial data rate. The sampling
Programming the Baud Rate Generator Registers
DLM and DLL provides the capability for selecting the
operating data rate.
rates available with a 14.7456 MHz crystal or external
O
divisor (decimal) = (XTAL1 clock frequency / prescaler) / ( serial data rate x 16 ),
divisor (decimal) = (XTAL1 clock frequency / prescaler / (serial data rate x 8),
UTPUT
MCR Bit-7=1
T
115.2k
230.4k
ABLE
19.2k
38.4k
57.6k
1200
2400
4800
9600
100
600
Data Rate
9: T
F
IGURE
YPICAL DATA RATES WITH A
XTAL1
XTAL2
O
UTPUT
Table 9
8. B
MCR Bit-7=0
16
153.6k
230.4k
460.8k
921.6k
19.2k
38.4k
76.8k
2400
4800
9600
400
AUD
Data Rate
-1) to obtain a 16X or 8X
shows the standard data
R
Crystal
Buffer
ATE
Osc/
G
Channels
Clock (Decimal)
To Other
D
ENERATOR
IVISOR FOR
2304
14.7456 MH
384
192
96
48
24
12
6
4
2
1
Divide by 4
Divide by 1
Prescaler
Prescaler
16x
D
22
IVISOR FOR
Z CRYSTAL OR EXTERNAL CLOCK AT
Clock (HEX)
clock is used by the transmitter for data bit shifting
and receiver for data sampling. The BRG divisor (DLL
and DLM registers) defaults to a random value upon
power up. Therefore, the BRG must be programmed
during initialization to the operating data rate.
clock at 16X clock rate. At 8X sampling rate, these
data rates would double. When using a non-standard
data rate crystal or external clock, the divisor value
can be calculated with the following equation(s).
MCR Bit-7=0
MCR Bit-7=1
900
180
C0
0C
60
30
18
06
04
02
01
(default)
16x
DLL and DLM
Baud Rate
Generator
PCI BUS OCTAL UART
Registers
Logic
DLM P
V
WITH
ALUE
WITH
09
01
00
00
00
00
00
00
00
00
00
ROGRAM
8XMODE [7:0]
(HEX)
8XMODE [7:0]
Rate Clock to
and Receiver
Transmitter
16X or 8X
Sampling
DLL P
V
ALUE
C0
0C
00
80
60
30
18
06
04
02
01
16X S
ROGRAM
IS
(HEX)
1
IS
0
XR17C158
AMPLING
D
E
ATA
REV. 1.0.0
RROR
0
0
0
0
0
0
0
0
0
0
0
R
ATE
(%)