ATmega8 Atmel Corporation, ATmega8 Datasheet - Page 70

• Manufacturer Part Number: ATmega8
• Manufacturer: Atmel Corporation

Specifications of ATmega8

• 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
• Output Compare Channels: 3
• Input Capture Channels: 1
• Pwm Channels: 3
• 32khz Rtc: Yes
• Calibrated Rc Oscillator: Yes

Timer/Counter
Clock Sources
Counter Unit
Operation
Timer/Counter
Timing Diagrams
70
ATmega8(L)
The Timer/Counter can be clocked by an internal or an external clock source. The clock source
is selected by the clock select logic which is controlled by the clock select (CS02:0) bits located
in the Timer/Counter Control Register (TCCR0). For details on clock sources and prescaler, see
“Timer/Counter0 and Timer/Counter1 Prescalers” on page
The main part of the 8-bit Timer/Counter is the programmable counter unit.
block diagram of the counter and its surroundings.
Figure 27. Counter Unit Block Diagram
Signal description (internal signals):
The counter is incremented at each timer clock (clk
or internal clock source, selected by the clock select bits (CS02:0). When no clock source is
selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be accessed by the
CPU, regardless of whether clk
counter clear or count operations.
The counting direction is always up (incrementing), and no counter clear is performed. The
counter simply overruns when it passes its maximum 8-bit value (MAX = 0xFF) and then restarts
from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set
in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves
like a ninth bit, except that it is only set, not cleared. However, combined with the timer overflow
interrupt that automatically clears the TOV0 Flag, the timer resolution can be increased by soft-
ware. A new counter value can be written anytime.
The Timer/Counter is a synchronous design and the timer clock (clk
clock enable signal in the following figures. The figures include information on when Interrupt
Flags are set.
figure shows the count sequence close to the MAX value.
count
clk
max
Tn
DATA BUS
TCNTn
Increment TCNT0 by 1
Timer/Counter clock, referred to as clk
Signalize that TCNT0 has reached maximum value
Figure 28 on page 71
count
T0
is present or not. A CPU write overrides (has priority over) all
contains timing data for basic Timer/Counter operation. The
Control Logic
max
TOVn
(Int. Req.)
clk
T0
Tn
T0
). clk
in the following
T0
73.
can be generated from an external
Clock Select
( From Prescaler )
Detector
Edge
T0
) is therefore shown as a
Figure 27
2486Z–AVR–02/11
Tn
shows a