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Hardware Description
Block Diagram
Supported Baud Rates
When using the standard 14.7456 MHz oscillator, the CM17320HR supports the standard baud
rates of PC serial port (e.g. 2400, 9600, 19.2k, etc). The CM17320HR can also support non-
standard baud rates via a User Oscillator.
The baud rate of a serial port is set by dividing the input clock (typically 14.7546MHz). For the
Exar XR17D158, there are two dividers to specify. First is the Prescaler (MCR Bit 7), divides the
input clock by either 1 or 4. Second is the Baud Rate Generator, which further divides the
frequency further to achieve the desired baud rate. The Baud Rate Generator can be set to a
value between 1 and (2
16
-1).
+3.3V
+5V
U3 VCC
UART Input
Clock
PCI
Bus
P
C
/1
04
-
P
lu
s
C
on
ne
ct
or
(C
N
3)
Exar
XR17D158
PCI UART
COM a
COM b
COM c
COM d
COM e
COM f
COM g
COM h
CN4
CN5
CN6
CN7
CN8
CN9
CN10
CN11
RS-232/422/485
RS-232/422/485
RS-232/422/485
RS-232/422/485
RS-232/422/485
RS-232/422/485
RS-232/422/485
RS-232/422/485
Signal Conditioning
(Jumper-Controlled)
14.7456 MHz
Oscillator
User Oscillator
(U3)
Oscillator Select
Jumper (JP2)
User Oscillator Power
Jumper (JP27)
Figure 1 CM17320 Block Diagram
