Datasheet

211
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
Clock Generation The Clock Generation logic generates the base clock for the Transmitter and Receiver.
The USARTn supports four modes of clock operation: Normal asynchronous, Double
Speed asynchronous, Master synchronous and Slave synchronous mode. The UMSELn
bit in USART Control and Status Register C (UCSRnC) selects between asynchronous
and synchronous operation. Double Speed (asynchronous mode only) is controlled by
the U2Xn found in the UCSRnA Register. When using synchronous mode (UMSELn =
1), the Data Direction Register for the XCKn pin (DDR_XCKn) controls whether the
clock source is internal (Master mode) or external (Slave mode). The XCKn pin is only
active when using synchronous mode.
Figure 83 shows a block diagram of the clock generation logic.
Figure 83. Clock Generation Logic, Block Diagram
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.
f
OSC
XTAL pin frequency (System Clock).
Prescaling
Down-Counter
/2
UBRR
/4 /2
fosc
UBRR+1
Sync
Register
OSC
XCK
Pin
txclk
U2X
UMSEL
DDR_XCK
0
1
0
1
xcki
xcko
DDR_XCK
rxclk
0
1
1
0
Edge
Detector
UCPOL