
172
8285B–AVR–03/11
ATmega165A/165PA/325A/325PA/3250A/3250PA/645A/645P/645
Signal description:
txclk
Transmitter clock (Internal Signal).
rxclk
Receiver base clock (Internal Signal).
xcki
Input from XCK pin (internal Signal). Used for synchronous slave
operation.
xcko
Clock output to XCK pin (Internal Signal). Used for synchronous master
operation.
fosc
XTAL pin frequency (System Clock).
19.3.1
Internal Clock Generation – The Baud Rate Generator
Internal clock generation is used for the asynchronous and the synchronous master modes of
operation. The description in this section refers to
Figure 19-2.The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a
programmable prescaler or baud rate generator. The down-counter, running at system clock
(f
osc), is loaded with the UBRRn value each time the counter has counted down to zero or when
the UBRRLn Register is written. A clock is generated each time the counter reaches zero. This
clock is the baud rate generator clock output (= f
osc/(UBRRn+1)). The Transmitter divides the
baud rate generator clock output by 2, 8 or 16 depending on mode. The baud rate generator out-
put is used directly by the Receiver’s clock and data recovery units. However, the recovery units
use a state machine that uses 2, 8 or 16 states depending on mode set by the state of the
UMSELn, U2Xn and DDR_XCK bits.
Table 19-1 contains equations for calculating the baud rate (in bits per second) and for calculat-
ing the UBRRn value for each mode of operation using an internally generated clock source.
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)
f
OSC
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 19-419.3.2
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.
Table 19-1.
Equations for Calculating Baud Rate Register Setting
Operating Mode
Equation for Calculating Baud
Equation for Calculating UBRRn
Value
Asynchronous Normal
mode (U2Xn = 0)
Asynchronous Double
Speed mode (U2Xn = 1)
Synchronous Master
mode
BAUD
fOSC
16
UBRRn 1
+
()
------------------------------------------
=
UBRRn
fOSC
16
BAUD
------------------------
1
–
=
BAUD
fOSC
8
UBRRn 1
+
()
---------------------------------------
=
UBRRn
fOSC
8
BAUD
--------------------
1
–
=
BAUD
fOSC
2
UBRRn 1
+
()
---------------------------------------
=
UBRRn
fOSC
2
BAUD
--------------------
1
–
=