AM8530 datasheet(100/194 Pages) AMD

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Support Circuitry Programming

AMD

5–6

5.3

BAUD RATE GENERATOR (BRG)

Each channel in the SCC contains a programmable BRG. Each generator consists of two

8-bit, time-constant registers forming a 16-bit time constant, a 16-bit down counter, and a

flip-flop on the output that makes the output a square wave. On start-up, the flip-flop on

the output is set High so that it starts in a known state, the value in the time-constant reg-

ister is again loaded into the counter, and the counter begins counting down.

Upon reaching a count of zero, the output of the BRG toggles and the time-constant value

held in WR12 and WR13 is reloaded into the down counter and the process of counting

down starts over. When the zero count is reached, the output of the BRG toggles, and for

the duration of the zero count, the Zero Count status signal goes active to the External/

Status interrupt section. Refer to Zero Count Section for details on the Zero Count Status

bit in RR0. While the BRG is disabled the state of the Zero Count status bit in RR0 will

always read ‘0’ providing the Zero Count IE bit in WR15 is reset. While the Zero Count IE

bit is set, the Zero Count status bit in RR0 will be set to ‘1’ for as long as the BRG counter

is at the count of zero. This status bit is forced active by a hardware reset.

No attempt is made to synchronize the loading of a new time constant with the clock used

to drive the BRG. When the time-constant is to be changed, the generator should be

stopped by resetting bit D0 of WR14. This ensures the loading of a correct time constant.

The time-constant for the BRG is programmed in WR12 and WR13, with the least signifi-

cant byte in WR12. The formulas relating the baud rate to the time-constant and vice

versa are shown in Table 5–1 with an example. In these formulas the BRG clock fre-

quency is in Hertz, the desired baud rate in bits/second and the time-constant is dimen-

sionless. The example in Table 5–2 assumes a 2.4576 MHz clock frequency and shows

the time-constant for a number of popular baud rates.

Table 5–1. Time Constant Formulas

Time Constant =

Clock Frequency

–2

• (Clock Mode) • (Baud Rate)

Baud Rate =

Clock Frequency

• (Clock Mode) • (Time Constant + 2)

Table 5–2. Baud Rate Example

Baud Rate

Divider

Decimal

Hex

38400

0000H

19200

0002H

9600

0006H

4800

000EH

2400

001EH

1200

003EH

600

126

007EH

300

254

00FEH

150

510

01FEH

For 2.4576 MHz, X16 Clock Mode

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