XR17D154 Exar Corporation, XR17D154 Datasheet - Page 24

• Manufacturer Part Number: XR17D154
• Description: Four-channel Pci-bus Uart
• Manufacturer: Exar Corporation

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PRELIMINARY
Each UART has its own Baud Rate Generator (BRG) with a prescaler for the transmitter and receiver. The
prescaler is controlled by a software bit in the MCR 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 divisor between 1 and (2
serial data rate. The sampling 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.
Programming the Baud Rate Generator Registers DLM and DLL provides the capability for selecting the
operating data
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).
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5.1
divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 16),
divisor (decimal) = (XTAL1 clock frequency / prescaler / (serial data rate x 8),
Programmable Baud Rate Generator
F
IGURE
rate.Table 9
XTAL1
XTAL2
10. B
AUD
shows the standard data rates available with a 14.7456 MHz crystal or external
R
C rystal
Buffer
O sc/
ATE
G
C hannels
To O ther
ENERATOR
D ivide by 4
D ivide by 1
Prescaler
Prescaler
24
UNIVERSAL (3.3V AND 5V) PCI BUS QUAD UART
M C R Bit-7=0
M C R Bit-7=1
(default)
16
-1) to obtain a 16X or 8X sampling clock of the
D LL and D LM
Baud R ate
G enerator
R egisters
Logic
WITH
WITH
8XMODE [3:0]
8XMODE [3:0]
R ate C lock to
and R eceiver
Transm itter
16X or 8X
Sam pling
IS
IS
1
0
XR17D154
REV. P1.0.0