Datasheet
ATmega164A/PA/324A/PA/644A/PA/1284/P
2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 178
Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps)
BAUDBaud rate (in bits per second, bps).
f
OSC
System Oscillator clock frequency.
UBRRnContents of the UBRRnH and UBRRnL Registers, (0-4095).
Some examples of UBRRn values for some system clock frequencies are found in Table 19-9 on page 198.
19.4.2 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.
19.4.3 External Clock
External clocking is used by the synchronous slave modes of operation. The description in this section refers to
Figure 19-2 on page 177 for details.
External clock input from the XCKn 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
Table 19-1. Equations for calculating baud rate register setting
Operating mode Equation for calculating baud rate
(1)
Equation for calculating UBRR value
Asynchronous Normal
mode (U2Xn = 0)
Asynchronous Double
Speed mode (U2Xn = 1)
Synchronous Master mode
BAUD
f
OSC
16 UBRRn 1+
------------------------------------------=
UBRRn
f
OSC
16BAUD
------------------------ 1–=
BAUD
f
OSC
8 UBRRn 1+
---------------------------------------=
UBRRn
f
OSC
8BAUD
-------------------- 1–=
BAUD
f
OSC
2 UBRRn 1+
---------------------------------------=
UBRRn
f
OSC
2BAUD
-------------------- 1–=