ATMEGA8_08 Datasheet, PDF (106/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
46 shows a block diagram of the counter and its surrounding environment.
Figure 46. Counter Unit Block Diagram
DATA BUS
TOVn
(Int. Req.)
TCNTn
count
clear
Control Logic
direction
clk _Tn
Prescaler
T/C
Oscillator
TOSC1
TOSC2
BOTTOM
TOP
clk
I/O
Signal description (internal signals):
count
Increment or decrement TCNT2 by 1.
direction Selects between increment and decrement.
clear
Clear TCNT2 (set all bits to zero).
clk_T2
TOP
Timer/Counter clock.
Signalizes that TCNT2 has reached maximum value.
BOTTOM Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clk_T2). clk_T2 can be generated from an external or internal clock source,
selected by the clock select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the
timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of
whether clk_T2 is present or not. A CPU write overrides (has priority over) all counter clear or
count operations.
The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in
the Timer/Counter Control Register (TCCR2). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the Output Compare Output
OC2. For more details about advanced counting sequences and waveform generation, see
“Modes of Operation” on page 110.
The Timer/Counter Overflow (TOV2) Flag is set according to the mode of operation selected by
the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.
106 ATmega8(L)
2486T–AVR–05/08