131
2486AA–AVR–02/2013
ATmega8(L)
Figure 62. 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
fosc
XTAL pin frequency (System Clock)
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 62.The USART Baud Rate Register (UBRR) and the down-counter connected to it function as a
programmable prescaler or baud rate generator. The down-counter, running at system clock
(fosc), is loaded with the UBRR value each time the counter has counted down to zero or when
the UBRRL Register is written. A clock is generated each time the counter reaches zero. This
clock is the baud rate generator clock output (= fosc/(UBRR+1)). The Transmitter divides the
baud rate generator clock output by 2, 8, or 16 depending on mode. The baud rate generator
output 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
UMSEL, U2X and DDR_XCK bits.
for calculating the UBRR value for each mode of operation using an internally generated clock
source.
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
0
Edge
Detector
UCPOL