ATmega8A Atmel Corporation, ATmega8A Datasheet - Page 140

• Manufacturer Part Number: ATmega8A
• Manufacturer: Atmel Corporation

Specifications of ATmega8A

• Flash (kbytes): 8 Kbytes
• Pin Count: 32
• Max. Operating Frequency: 16 MHz
• Cpu: 8-bit AVR
• # Of Touch Channels: 12
• Hardware Qtouch Acquisition: No
• Max I/o Pins: 23
• Ext Interrupts: 2
• Usb Speed: No
• Usb Interface: No
• Spi: 1
• Twi (i2c): 1
• Uart: 1
• Graphic Lcd: No
• Video Decoder: No
• Camera Interface: No
• Adc Channels: 8
• Adc Resolution (bits): 10
• Adc Speed (ksps): 15
• Analog Comparators: 1
• Resistive Touch Screen: No
• Temp. Sensor: No
• Crypto Engine: No
• Sram (kbytes): 1
• Eeprom (bytes): 512
• Self Program Memory: YES
• Dram Memory: No
• Nand Interface: No
• Picopower: No
• Temp. Range (deg C): -40 to 85
• I/o Supply Class: 2.7 to 5.5
• Operating Voltage (vcc): 2.7 to 5.5
• Fpu: No
• Mpu / Mmu: no / no
• Timers: 3
• Pwm Channels: 3
• 32khz Rtc: Yes
• Calibrated Rc Oscillator: Yes

19.3.2
19.3.3
19.3.4
8159D–AVR–02/11
Double Speed Operation (U2X)
External Clock
Synchronous Clock Operation
Table 19-1.
Note:
Some examples of UBRR values for some system clock frequencies are found in
(see
The transfer rate can be doubled by setting the U2X bit in UCSRA. Setting this bit only has effect
for the asynchronous operation. Set this bit to zero when using synchronous 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 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 (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
opposite XCK clock edge of the edge the data output (TxD) is changed.
Operating Mode
Asynchronous Normal mode (U2X
= 0)
Asynchronous Double Speed
Mode (U2X = 1)
Synchronous Master Mode
page
BAUD
f
UBRR
OSC
1. The baud rate is defined to be the transfer rate in bit per second (bps).
osc
161).
depends on the stability of the system clock source. It is therefore recommended to
Equations for Calculating Baud Rate Register Setting
Figure 19-2
Baud rate (in bits per second, bps)
System Oscillator clock frequency
Contents of the UBRRH and UBRRL Registers, (0 - 4095)
for details.
BAUD
BAUD
BAUD
Equation for Calculating
f
XCK
Baud Rate
=
=
=
-------------------------------------- -
16 UBRR
<
---------------------------------- -
8 UBRR
---------------------------------- -
2 UBRR
(
(
f
---------- -
(
OSC
4
f
f
f
OSC
OSC
OSC
(1)
+
+
+
1
1
1
)
)
)
Equation for Calculating
UBRR
UBRR
UBRR
UBRR Value
ATmega8A
=
=
=
----------------------- - 1
16BAUD
------------------- - 1
8BAUD
------------------- - 1
2BAUD
f
f
f
OSC
OSC
OSC
Table 19-9
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–
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