xr17v258 Exar Corporation, xr17v258 Datasheet - Page 31

• Manufacturer Part Number: xr17v258
• Description: 66mhz Pci Bus Octal Uart With Power Management Support
• Manufacturer: Exar Corporation

xr
REV. 1.0.0
There are 8 UARTs channel [7:0] in the V258. Each has its own 64-byte of transmit and receive FIFO, a set of
16550 compatible control and status registers, and a baud rate generator for individual channel data rate
setting. Eight additional registers per UART were added for the EXAR enhanced features.
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
16) to obtain a 16X or 8X sampling clock of the serial data rate. The sampling clock is used by the transmitter
for data bit shifting and receiver for data sampling. The BRG divisor (DLL, DLM and DLD registers) defaults to
a random value upon power up. Therefore, the BRG must be programmed during initialization to the operating
data rate. The DLL and DLM registers provide the integer part of the divisor and the DLD register provides the
fractional part of the divisor. Only the four lower bits of the DLD are implemented and they are used to select a
value from 0 (for setting 0000) to 0.9375 or 15/16 (for setting 1111). Programming the Baud Rate Generator
Registers DLL, DLM and DLD provides the capability for selecting the operating data rate.
standard data rates available with a 14.7456 MHz crystal or external clock at 16X clock rate. If the pre-scaler is
used (MCR bit [7] = 1), the output data rate will be 4 times less than that shown in
rate, these data rates would double. Also, when using 8X sampling mode, please note that the bit-time will
have a jitter (+/- 1/16) whenever the DLD is an odd number. When using a non-standard data rate crystal or
external clock, the divisor value can be calculated with the following equation(s):
4.0 UART
4.1
Required Divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 16),
Required Divisor (decimal) = (XTAL1 clock frequency / prescaler / (serial data rate x 8),
Programmable Baud Rate Generator with Fractional Divisor
T
ABLE
11: T
CH0 0x000 Read RHR
CH2 0x400 Read RHR
CH3 0x600 Read RHR
CH4 0x800 Read RHR
CH3 0x600 Write THR
CH5 0xA00 Write THR
CH0 0x000 Write THR
CH1 0x200 Write THR
CH1 0x200 Read RHR
CH2 0x400 Write THR
CH4 0x800 Write THR
CH5 0xA00 Read RHR
CH6 0xC00 Read RHR
CH7 0xE00 Read RHR
CH6 0xC00 Write THR
CH7 0xE00 Write THR
RANSMIT AND
THR and RHR Address Locations For CH0 to CH7 (16C550 Compatible)
66MHZ PCI BUS OCTAL UART WITH POWER MANAGEMENT SUPPORT
R
ECEIVE
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
Bit-7
D
ATA
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
Bit-6
R
EGISTER IN
31
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-5
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
Bit-4
B
YTE FORMAT
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
Bit-3
16
- 0.0625) in increments of 0.0625 (1/
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
Bit-2
, 16C550
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
Bit-1
WITH
WITH
Table 12
8XMODE [7:0]
THRRHR1
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
Bit-0
8XMODE [7:0]
COMPATIBLE
Table 12
. At 8X sampling
XR17V258
IS
shows the
IS
1
0