Specifications

Serial Communications (V40)

Baud Rate

The SCU baud rate is determined by the output of counter/timer 1.

Counter/timer 1 must be initialized for a specific mode of operation

and programmed with a count that defines the required baud rate. The

discussion below explains initialization and how to calculate the

count.

To use counter/timer 1 as a baud rate generator, the TCKS register ust

specify that counter/timer 1 has an internal clock with a divisor

determined by the formula shown below. The TCU Counter/Timer

Mode register must also be programmed to configure counter/timer 1

for binary operation as a square wave generator (Mode 3). Different

baud rates are obtained by programming counter/timer 1 with

different counts. The relationship between the baud rate and the count

is given below:

Count = V40 Clock / (Prescale x Baud Rate x Baud Factor)

This formula includes all the factors that determine the baud rate as a

function of the V40 clock. The best way to understand the formula is

to trace the clock signal from the V40 clock to the SCU. The V40

clock starts with a value of 8.0 x 10

Hz. This frequency is Prescaled

as defined by the PS bits in the TCKS V40 configuration register. At

this point the clock is input to the counter/timers. Counter/timer 1

further divides the signal by the programmed count before it is input

to the SCU. The last division is done internal to the SCU by the baud

factor programmed in the BF bits of the Serial Mode register.

As an example, assume the following parameters are specified:

V40 Clock = 8.0 x 10

Prescale = divide by two (TCKS PS bits = 00)

Baud Rate = 9600

Baud Factor = divide by 16 (SMD BF bits = 10)

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