ATA6602-PLQW ATMEL [ATMEL Corporation], ATA6602-PLQW Datasheet - Page 192

• Manufacturer Part Number: ATA6602-PLQW
• Description: Microcontroller with LIN Transceiver, 5V Regulator and Watchdog
• Manufacturer: ATMEL [ATMEL Corporation]

4.17.2
4.17.2.1
192
ATA6602/ATA6603
Clock Generation
Internal Clock Generation – The Baud Rate Generator
The Clock Generation logic generates the base clock for the Transmitter and Receiver. The
USART supports four modes of clock operation: Normal asynchronous, Double Speed asyn-
chronous, 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 4-70
Figure 4-70. Clock Generation Logic, Block Diagram
Signal description:
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
the UBRRnL Register is written. A clock is generated each time the counter reaches zero. This
clock is the baud rate generator clock output (= f
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_XCKn bits.
osc
), is loaded with the UBRRn value each time the counter has counted down to zero or when
txclk
rxclk
xcki
xcko
fosc
DDR_XCKn
XCKn
Pin
shows a block diagram of the clock generation logic.
xcko
xcki
OSC
Down-Counter
Transmitter clock (Internal Signal).
Receiver base clock (Internal Signal).
Input from XCK pin (internal Signal). Used for synchronous slave
operation.
Clock output to XCK pin (Internal Signal). Used for synchronous master
operation.
XTAL pin frequency (System Clock).
Prescaling
Register
UBRRn
Sync
UBRRn+1
foscn
Detector
UCPOLn
Edge
/2
Figure
osc
/(UBRRn+1)). The Transmitter divides the
/4
4-70.
/2
DDR_XCKn
U2Xn
0
1
0
1
0
1
1
0
4921C–AUTO–01/07
UMSELn
txclk
rxclk