ATmega16 datasheet(147/357 Pages) ATMEL

147 / 357 page

147

2466S–AVR–05/09

ATmega16(L)

Note:

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

BAUD Baud rate (in bits per second, bps)

OSC

System Oscillator clock frequency

UBRR Contents of the UBRRH and UBRRL Registers, (0 - 4095)

Some examples of UBRR values for some system clock frequencies are found in Table 68 (see

page 168).

Double Speed

Operation (U2X)

The transfer rate can be doubled by setting the U2X bit in UCSRA. 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.

External Clock

External clocking is used by the synchronous Slave modes of operation. The description in this

section refers to Figure 70 for details.

External clock input from the XCK pin is sampled by a synchronization register to minimize the

chance of meta-stability. The output from the synchronization register must then pass through

an edge detector before it can be used by the Transmitter and receiver. This process introduces

a two CPU clock period delay and therefore the maximum external XCK clock frequency is lim-

ited by the following equation:

Note that f

osc depends on the stability of the system clock source. It is therefore recommended to

add some margin to avoid possible loss of data due to frequency variations.

Table 60. Equations for Calculating Baud Rate Register Setting

Operating Mode

Equation for Calculating

Baud Rate(1)

Equation for

Calculating UBRR

Value

Asynchronous Normal Mode

(U2X = 0)

Asynchronous Double Speed Mode

(U2X = 1)

Synchronous Master Mode

BAUD

fOSC

UBRR 1

()

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

UBRR

fOSC

BAUD

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

–

BAUD

fOSC

UBRR 1

()

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

UBRR

fOSC

BAUD

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

–

BAUD

fOSC

UBRR 1

()

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

UBRR

fOSC

BAUD

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

–

fXCK

fOSC

-----------

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