ATmega8(L)
Table 52. Equations for Calculating Baud Rate Register Setting
Equation for Calculating
Baud Rate(1)
Equation for Calculating
UBRR Value
Operating Mode
Asynchronous Normal mode
(U2X = 0)
f
f
OSC
OSC
BAUD = --------------------------------------
16(UBRR + 1)
UBRR = ----------------------- – 1
16BAUD
Asynchronous Double Speed
Mode (U2X = 1)
f
f
OSC
OSC
BAUD = -----------------------------------
8(UBRR + 1)
UBRR = -------------------- – 1
8BAUD
Synchronous Master Mode
f
f
OSC
OSC
BAUD = -----------------------------------
2(UBRR + 1)
UBRR = -------------------- – 1
2BAUD
Note:
BAUD Baud rate (in bits per second, bps)
fOSC System Oscillator clock frequency
1. The baud rate is defined to be the transfer rate in bit per second (bps).
UBRR Contents of the UBRRH and UBRRL Registers, (0 - 4095)
Some examples of UBRR values for some system clock frequencies are found in Table
60 (see page 156).
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 descrip-
tion in this section refers to Figure 62 for details.
External clock input from the XCK pin is sampled by a synchronization register to mini-
mize 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 max-
imum external XCK clock frequency is limited by the following equation:
f
OSC
f
<
-----------
XCK
4
Note that fosc depends on the stability of the system clock source. It is therefore recom-
mended to add some margin to avoid possible loss of data due to frequency variations.
133
2486M–AVR–12/03
ATMEL [ ATMEL ]
