ATMEGA645A-AU Atmel, ATMEGA645A-AU Datasheet - Page 185
![IC MCU AVR 64K FLASH 64TQFP](/photos/12/13/121332/313-64-tqfp_sml.jpg)
ATMEGA645A-AU
Manufacturer Part Number
ATMEGA645A-AU
Description
IC MCU AVR 64K FLASH 64TQFP
Manufacturer
Atmel
Series
AVR® ATmegar
Specifications of ATMEGA645A-AU
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
Processor Series
ATmega
Core
AVR
Data Bus Width
8 bit
Data Ram Size
4 KB
Interface Type
SPI, USART
Maximum Clock Frequency
16 MHz
Number Of Programmable I/os
54
Number Of Timers
3
Operating Supply Voltage
3.3 V
Maximum Operating Temperature
+ 85 C
Mounting Style
SMD/SMT
Minimum Operating Temperature
- 40 C
Operating Temperature Range
- 40 C to + 85 C
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Company:
Part Number:
ATMEGA645A-AUR
Manufacturer:
HONEYWELL
Quantity:
101
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19.8
19.8.1
19.8.2
8285B–AVR–03/11
Asynchronous Data Reception
Asynchronous Clock Recovery
Asynchronous Data Recovery
ATmega165A/165PA/325A/325PA/3250A/3250PA/6
The USART includes a clock recovery and a data recovery unit for handling asynchronous data
reception. The clock recovery logic is used for synchronizing the internally generated baud rate
clock to the incoming asynchronous serial frames at the RxD pin. The data recovery logic sam-
ples and low pass filters each incoming bit, thereby improving the noise immunity of the
Receiver. The asynchronous reception operational range depends on the accuracy of the inter-
nal baud rate clock, the rate of the incoming frames, and the frame size in number of bits.
The clock recovery logic synchronizes internal clock to the incoming serial frames.
on page 185
rate is 16 times the baud rate for Normal mode, and eight times the baud rate for Double Speed
mode. The horizontal arrows illustrate the synchronization variation due to the sampling pro-
cess. Note the larger time variation when using the Double Speed mode (U2Xn = 1) of
operation. Samples denoted zero are samples done when the RxD line is idle (i.e., no communi-
cation activity).
Figure 19-5. Start Bit Sampling
When the clock recovery logic detects a high (idle) to low (start) transition on the RxD line, the
start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in
the figure. The clock recovery logic then uses samples 8, 9, and 10 for Normal mode, and sam-
ples 4, 5, and 6 for Double Speed mode (indicated with sample numbers inside boxes on the
figure), to decide if a valid start bit is received. If two or more of these three samples have logical
high levels (the majority wins), the start bit is rejected as a noise spike and the Receiver starts
looking for the next high to low-transition. If however, a valid start bit is detected, the clock recov-
ery logic is synchronized and the data recovery can begin. The synchronization process is
repeated for each start bit.
When the receiver clock is synchronized to the start bit, the data recovery can begin. The data
recovery unit uses a state machine that has 16 states for each bit in Normal mode and eight
states for each bit in Double Speed mode.
data bits and the parity bit. Each of the samples is given a number that is equal to the state of
the recovery unit.
(U2X = 0)
(U2X = 1)
Sample
Sample
RxD
illustrates the sampling process of the start bit of an incoming frame. The sample
0
0
IDLE
0
1
1
2
3
2
4
5
3
6
Figure 19-6 on page 186
7
4
8
START
9
5
10
11
6
12
13
7
14
shows the sampling of the
15
8
16
1
1
2
BIT 0
Figure 19-5
3
2
185
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