ATMEGA323-8AC Atmel, ATMEGA323-8AC Datasheet - Page 88
![IC AVR MCU 32K 8MHZ COM 44TQFP](/photos/6/70/67042/313-44-tqfp_sml.jpg)
ATMEGA323-8AC
Manufacturer Part Number
ATMEGA323-8AC
Description
IC AVR MCU 32K 8MHZ COM 44TQFP
Manufacturer
Atmel
Series
AVR® ATmegar
Datasheet
1.ATMEGA323L-4PC.pdf
(247 pages)
Specifications of ATMEGA323-8AC
Core Processor
AVR
Core Size
8-Bit
Speed
8MHz
Connectivity
I²C, SPI, UART/USART
Peripherals
Brown-out Detect/Reset, POR, PWM, WDT
Number Of I /o
32
Program Memory Size
32KB (16K x 16)
Program Memory Type
FLASH
Eeprom Size
1K x 8
Ram Size
2K x 8
Voltage - Supply (vcc/vdd)
4.5 V ~ 5.5 V
Data Converters
A/D 8x10b
Oscillator Type
Internal
Operating Temperature
0°C ~ 70°C
Package / Case
44-TQFP, 44-VQFP
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Other names
ATMEGA3238AC
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Asynchronous Data
Reception
Asynchronous Clock
Recovery
Asynchronous Data Recovery
88
ATmega323(L)
The USART includes a clock recovery and a data recovery unit for handling asynchro-
nous 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 samples and low pass filters each incoming bit, thereby improv-
ing the noise immunity of the Receiver. The asynchronous reception operational range
depends on the accuracy of the internal 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. Fig-
ure 49 illustrates the sampling process of the start bit of an incoming frame. The sample
rate is 16 times the baud rate for normal mode, and 8 times the baud rate for Double
Speed mode. The horizontal arrows illustrate the synchronization variation due to the
sampling process. Note the larger time variation when using the double speed mode
(U2X = 1) of operation. Samples denoted zero are samples done when the RxD line is
idle (i.e. no communication activity).
Figure 49. 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-sam-
ple as shown in the figure. The clock recovery logic then uses samples 8, 9 and 10 for
Normal mode, and samples 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-transi-
tion. If however, a valid start bit is detected, the clock recovery 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 8 states for each bit in Double Speed mode. Figure 50 shows the sampling of
the data bits and the parity bit. Each of the samples is given a number that is equal to
the state of the recovery unit.
Figure 50. Sampling of Data and Parity Bit
(U2X = 0)
(U2X = 1)
(U2X = 0)
(U2X = 1)
Sample
Sample
Sample
Sample
RxD
RxD
0
0
IDLE
0
1
1
1
1
2
2
3
2
3
2
4
4
5
3
5
3
6
6
7
4
7
4
8
8
START
BIT n
9
5
9
5
10
10
11
11
6
6
12
12
13
13
7
7
14
14
15
15
8
8
16
16
1457G–AVR–09/03
1
1
1
1
2
BIT 0
3
2
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