ATMEGA128A-ANR Atmel, ATMEGA128A-ANR Datasheet - Page 178
ATMEGA128A-ANR
Manufacturer Part Number
ATMEGA128A-ANR
Description
IC MCU AVR 128K FLASH 64TQFP
Manufacturer
Atmel
Series
AVR® ATmegar
Specifications of ATMEGA128A-ANR
Core Processor
AVR
Core Size
8-Bit
Speed
16MHz
Connectivity
EBI/EMI, I²C, SPI, UART/USART
Peripherals
Brown-out Detect/Reset, POR, PWM, WDT
Number Of I /o
53
Program Memory Size
128KB (64K x 16)
Program Memory Type
FLASH
Eeprom Size
4K x 8
Ram Size
4K x 8
Voltage - Supply (vcc/vdd)
2.7 V ~ 5.5 V
Data Converters
A/D 8x10b
Oscillator Type
Internal
Operating Temperature
-40°C ~ 85°C
Package / Case
64-TQFP
Core
AVR8
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
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Manufacturer
Quantity
Price
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20.3.3
20.3.4
8151H–AVR–02/11
External Clock
Synchronous Clock Operation
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 introduces
a two CPU clock period delay and therefore the maximum external XCK clock frequency is lim-
ited 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 (UMSEL = 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 oppo-
site XCK clock edge of the edge the data output (TxD) is changed.
Figure 20-3. Synchronous Mode XCK Timing.
The UCPOL 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 UCPOL 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 20-2
RxD / TxD
RxD / TxD
XCK
XCK
Figure 20-3
for details.
shows, when UCPOL is zero the data will be changed at
f
XCK
<
f
---------- -
OSC
4
Sample
Sample
ATmega128A
178
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