AT90SCR100 datasheet(234/433 Pages) ATMEL

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234

TPR0327AY–SMS–30Jan09

AT90SCR100

Table 18-1 contains equations for calculating the baud rate (in bits per second) and for calculat-

ing the UBRR0 value for each mode of operation using an internally generated clock source.

Some examples of UBRR0 values for some system clock frequencies are found in Table 18-9 on

page 254.

18.4.2

Double Speed Operation (U2X0)

The transfer rate can be doubled by setting the U2X0 bit in UCSR0A. Setting this bit only has

effect for the asynchronous operation. Set this bit to zero when using synchronous operation.

Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling

the transfer rate for asynchronous communication. Note however that the Receiver will in this

case only use half the number of samples (reduced from 16 to 8) for data sampling and clock

recovery, and therefore a more accurate baud rate setting and system clock are required when

this mode is used. For the Transmitter, there are no downsides.

Table 18-1.

Equations for Calculating Baud Rate Register Setting

Operating Mode

Equation for Calculating Baud

Rate

(1)

The baud rate is defined to be the transfer rate in bit per second (bps)

BAUD

Baud rate (in bits per second, bps)

OSCSystem Oscillator clock frequency injected in UART block: clk

UBRRn

Contents of the UBRRH0 and UBRRL0 Registers, (0-4095)

Equation for Calculating UBRR

Value

Asynchronous Normal

mode (U2X0 = 0)

Asynchronous Double

Speed mode (U2X0 = 1)

Synchronous Master

mode

BAUD

OSC

16 UBRR01

()

------------------------------------------

UBRR0

OSC

16BAUD

------------------------

–

BAUD

OSC

8 UBRR01

()

---------------------------------------

UBRR0

OSC

8BAUD

--------------------

–

BAUD

OSC

2 UBRR01

()

---------------------------------------

UBRR0

OSC

2BAUD

--------------------

–

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