STV3550 datasheet(122/144 Pages) STMICROELECTRONICS

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STV3550

7.8.5.1 Baud Rates

The baud rate generator provides an internal oversampling clock at 16 times the external baud rate.

This clock only ticks if the Run bit of the ASC_n_Control register is set to 1. Setting this bit to 0 will

immediately freeze the state of the ASCs transmitter and receiver.

Mode 0

When the BaudMode bit in the ASC_n_Control register is set to 0, the baud rate and the required

reload value for a given baud rate can be determined by the following formula:

where: ASCBaudRate represents the content of the ASC_n_BaudRate reload value register, taken

as an unsigned 16-bit integer and fCPU is the frequency of the CPU.

The baud rate counter is clocked by the subsystem clock. It counts downwards and can be started

or stopped by the Run bit in the ASC_n_Control register. Each underflow of the timer provides one

oversampling baud rate clock pulse. The counter is reloaded with the value stored in its 16-bit

reload register each time it underflows.

Writes to the ASC_n_BaudRate register update the reload register value. Reads from the

ASC_n_BaudRate register return the current value of the counter.

Mode 1

When the BaudMode bit in the ASC_n_Control register is set to 1, the baud rate is controlled by

the following circuit:

Writes to ASC_n_BaudRate go to the reload register. Reads from ASC_n_BaudRate return the

value in the accumulator register. Both registers are 16 bits wide and are clocked by the subsystem

clock.

If the system clock frequency is fCPU, writing a value of ASCBaudRate to the ASC_n_BaudRate

so the baud rate is given by:

BaudRate =

16 x ASCBaudRate

ASCBaudRate =

16 x BaudRate

fCPU

ASCBaudRate

Carry-out

Oversampling Clock

(Reload)

ASCBaudRate

(Accumulator)

fCPU

216

ASCBaudRate x fCPU

BaudRate =

16 x 216

ASCBaudRate x fCPU

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