ATMEGA645A-MUR Atmel, ATMEGA645A-MUR Datasheet - Page 173

• Manufacturer Part Number: ATMEGA645A-MUR
• Description: IC MCU AVR 64K FLASH 64QFN
• Manufacturer: Atmel
• Series: AVR® ATmegar

Specifications of ATMEGA645A-MUR

• Core Processor: AVR
• Core Size: 8-Bit
• Speed: 16MHz
• Connectivity: SPI, UART/USART, USI
• Peripherals: Brown-out Detect/Reset, POR, PWM, WDT
• Number Of I /o: 54
• Program Memory Size: 64KB (32K x 16)
• Program Memory Type: FLASH
• Eeprom Size: 2K x 8
• Ram Size: 4K x 8
• Voltage - Supply (vcc/vdd): 1.8 V ~ 5.5 V
• Data Converters: A/D 8x10b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 64-TQFP Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

19.3.3
19.3.4
8285B–AVR–03/11
External Clock
Synchronous Clock Operation
ATmega165A/165PA/325A/325PA/3250A/3250PA/6
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling
the transfer rate for asynchronous communication. Note however that the Receiver will in this
case only use half the number of samples (reduced from 16 to 8) for data sampling and clock
recovery, and therefore a more accurate baud rate setting and system clock are required when
this mode is used. For the Transmitter, there are no downsides.
External clocking is used by the synchronous slave modes of operation. The description in this
section refers to
External clock input from the XCK pin is sampled by a synchronization register to minimize the
chance of meta-stability. The output from the synchronization register must then pass through
an edge detector before it can be used by the Transmitter and Receiver. This process intro-
duces a two CPU clock period delay and therefore the maximum external XCK clock frequency
is limited by the following equation:
Note that f
add some margin to avoid possible loss of data due to frequency variations.
When synchronous mode is used (UMSELn = 1), the XCK pin will be used as either clock input
(Slave) or clock output (Master). The dependency between the clock edges and data sampling
or data change is the same. The basic principle is that data input (on RxD) is sampled at the
opposite XCK clock edge of the edge the data output (TxD) is changed.
Figure 19-3. Synchronous Mode XCK Timing.
The UCPOLn bit UCRSC selects which XCK clock edge is used for data sampling and which is
used for data change. As
rising XCK edge and sampled at falling XCK edge. If UCPOLn is set, the data will be changed at
falling XCK edge and sampled at rising XCK edge.
UCPOL = 1
UCPOL = 0
osc
depends on the stability of the system clock source. It is therefore recommended to
Figure 19-2
RxD / TxD
RxD / TxD
XCK
XCK
Figure 19-3
for details.
shows, when UCPOLn is zero the data will be changed at
f
XCK
<
f
---------- -
OSC
4
Sample
Sample
173