Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
Page 341
341
8266A-MCU Wireless-12/09
ATmega128RFA1
page 331 shows a block diagram of the clock generation logic.
Figure 23-2. Clock Generation Logic, Block Diagram
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
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
System clock frequency.
23.3.1 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
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 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
UMSELn, U2Xn and DDR_XCKn bits.
contains equations for calculating the baud rate (in bits per second)
and for calculating the UBRRn value for each mode of operation using an internally
generated clock source.
Table 23-1. 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)
)
1
(
16
+
=
UBRRn
f
BAUD
OSC
1
16
−
=
BAUD
f
UBRRn
OSC
Asynchronous Double Speed
Mode (U2Xn = 1)
)
1
(
8
+
=
UBRRn
f
BAUD
OSC
1
8
−
=
BAUD
f
UBRRn
OSC