C167CS datasheet(277/517 Pages) INFINEON

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C167CS

Derivatives

The Asynchronous/Synchronous Serial Interface

User’s Manual

11-11

V2.0, 2000-07

11.4

ASC0 Baud Rate Generation

The serial channel ASC0 has its own dedicated 13-bit baud rate generator with 13-bit

reload capability, allowing baud rate generation independent of the GPT timers.

The baud rate generator is clocked with the CPU clock divided by 2 (

fCPU/2). The timer

is counting downwards and can be started or stopped through the Baud Rate Generator

Run Bit S0R in register S0CON. Each underflow of the timer provides one clock pulse to

the serial channel. The timer is reloaded with the value stored in its 13-bit reload register

each time it underflows. The resulting clock is again divided according to the operating

mode and controlled by the Baudrate Selection Bit S0BRS. If S0BRS = ‘1’, the clock

signal is additionally divided to 2/3rd of its frequency (see formulas and table). So the

baud rate of ASC0 is determined by the CPU clock, the reload value, the value of S0BRS

and the operating mode (asynchronous or synchronous).

S0BG returns the content of the timer (bits 15 … 13 return zero), while writing to S0BG

always updates the reload register (bits 15 … 13 are insiginificant).

An auto-reload of the timer with the content of the reload register is performed each time

S0BG is written to. However, if S0R = ‘0’ at the time the write operation to S0BG is

performed, the timer will not be reloaded until the first instruction cycle after S0R = ‘1’.

Asynchronous Mode Baud Rates

For asynchronous operation, the baud rate generator provides a clock with 16 times the

rate of the established baud rate. Every received bit is sampled at the 7th, 8th and 9th

cycle of this clock. The baud rate for asynchronous operation of serial channel ASC0 and

the required reload value for a given baudrate can be determined by the following

formulas:

<S0BRL> represents the content of the reload register, taken as unsigned 13-bit integer,

<S0BRS> represents the value of bit S0BRS (i.e. ‘0’ or ‘1’), taken as integer.

The tables below list various commonly used baud rates together with the required

reload values and the deviation errors compared to the intended baudrate for a number

of CPU frequencies.

Note: The deviation errors given in the tables below are rounded. Using a baudrate

crystal (e.g. 18.432 MHz) will provide correct baudrates without deviation errors.

BAsync =

fCPU

× (2 + <S0BRS>) × (<S0BRL> + 1)

S0BRL = (

fCPU

× (2 + <S0BRS>) × B

Async

) – 1

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