Table 18-1. Equations for Calculating Baud Rate Register Setting
Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRRn
Operating Mode
Value
Asynchronous Normal
mode (U2Xn = 0)
f
OSC
f
OSC
BAUD = -----------------------------------------
UBRRn = ----------------------- – 1
16BAUD
16(UBRRn + 1)
Asynchronous Double
Speed mode (U2Xn = 1)
f
OSC
f
OSC
BAUD = --------------------------------------
UBRRn = -------------------- – 1
8(UBRRn + 1)
8BAUD
Synchronous Master
mode
f
OSC
f
OSC
BAUD = --------------------------------------
UBRRn = -------------------- – 1
2(UBRRn + 1)
2BAUD
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)
fOSC
System Oscillator clock frequency
UBRRn
Contents of the UBRRHn and UBRRLn Registers, (0-4095)
Some examples of UBRRn values for some system clock frequencies are found in Table 18-9
(see page 195).
18.2.2
Double Speed Operation (U2Xn)
The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. 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.
18.2.3
External Clock
External clocking is used by the synchronous slave modes of operation. The description in this
section refers to Figure 18-2 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 intro-
duces a two CPU clock period delay and therefore the maximum 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 recommended to
add some margin to avoid possible loss of data due to frequency variations.
172
ATmega169P
8018A–AVR–03/06
ATMEL [ ATMEL ]
