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ST16C1550

2.97V TO 5.5V UART WITH 16-BYTE FIFO

Exar Corporation 

áç

REV. 4.2.0

ST16C1550/51

2.97V TO 5.5V UART WITH 16-BYTE FIFO

FIGURE 4. TYPICAL OSCILLATOR CONNECTIONS

XTAL1

C1

22-47pF

XTAL2

R2

500K - 1M

R1

0-120

(Optional)

1.8432 MHz

Y1

to

24 MHz

C2

22-47pF

2.4 Programmable Baud Rate Generator

The UART has its own Baud Rate Generator (BRG) with a prescaler. 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 clock output of the prescaler goes to the BRG. The BRG further divides this clock by a

programmable divisor between 1 and (216 -1) to obtain a 16X 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 and DLM registers) defaults to a random value upon power up or a reset. 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 of selecting the operating data rate. Table 2 shows the standard data

rates available with a 14.7456 MHz crystal or external clock at 16X clock rate. When using a non-standard data

rate crystal or external clock, the divisor value can be calculated for DLL/DLM with the following equation.

divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 16)

9

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