XR16C854 Datasheet, PDF(19/51 Page) Exar Corporation – QUAD UART WITH RX/TX FIFO COUNTERS,128-BYTE FIFO

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XR16C854 Datasheet, PDF (19/51 Pages) Exar Corporation – QUAD UART WITH RX/TX FIFO COUNTERS,128-BYTE FIFO

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XR16C854

Single baud rate generator is provided for the

transmitter and receiver, allowing independent TX/

RX channel control. The programmable Baud Rate

Generator is capable of accepting an input clock up

to 24 MHz, as required for supporting a 1.5Mbps

data rate. The 854 can be configured for internal or

external clock operation. For internal clock oscilla-

tor operation, an industry standard microprocessor

crystal (parallel resonant/ 22-33 pF load) is con-

nected externally between the XTAL1 and XTAL2

pins (see figure ). Alternatively, an external clock

can be connected to the XTAL1 pin to clock the

internal baud rate generator for standard or custom

rates. (see Baud Rate Generator Programming).

The generator divides the input 16X clock by any

divisor from 1 to 216 -1. The 854 divides the basic

crystal or external clock by 16. Further division of this

16X clock provides two table rates to support low and

high data rate applications using the same system

design. After a hardware reset and during initializa-

tion, the 854 sets the default baud rate table according

to the state of the CLKSEL. pin. A logic 1 on CLKSEL

will set the 1X clock default whereas, logic 0 will set

the 4X clock default table. Following the default clock

rate selection during initialization, the rate tables can

be changed by the internal register, MCR bit-7. Setting

MCR bit-7 to a logic 1 when CLKSEL is a logic 1

provides an additional divide by 4 whereas, setting

MCR bit-7 to a logic 0 only divides by 1. (See Table 5

and Figure 11). Customized Baud Rates can be

achieved by selecting the proper divisor values for the

MSB and LSB sections of baud rate generator.

Programming the Baud Rate Generator Registers

DLM (MSB) and DLL (LSB) provides a user capability

for selecting the desired final baud rate. The example

in Table 5 below, shows the two selectable baud rate

tables available when using a 7.3728 MHz crystal.

Table 5, BAUD RATE GENERATOR PROGRAMMING TABLE (7.3728 MHz CLOCK):

Output

Baud Rate

MCR

BIT-7=1

Output

Baud Rate

MCR

Bit-7=0

User

16 x Clock

Divisor

(Decimal)

User

16 x Clock

Divisor

(HEX)

DLM

Program

Value

(HEX)

200

2304

900

300

1200

384

180

600

2400

192

1200

4800

2400

9600

4800

19.2K

9600

38.4k

19.2k

76.8k

38.4k

153.6k

57.6k

230.4k

115.2k

460.8k

DLL

Program

Value

(HEX)

Rev. 1.00P

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