ATMEGA8L ATMEL [ATMEL Corporation], ATMEGA8L Datasheet - Page 104

• Manufacturer Part Number: ATMEGA8L
• Description: 8-bit AVR with 8K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Counter Unit
104
ATmega8(L)
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
Signal description (internal signals):
Depending on the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clk
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
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 108.
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.
count
direction
clear
clk
TOP
BOTTOM
T2
DATA BUS
TCNTn
Increment or decrement TCNT2 by 1.
Selects between increment and decrement.
Clear TCNT2 (set all bits to zero).
Timer/Counter clock.
Signalizes that TCNT2 has reached maximum value.
Signalizes that TCNT2 has reached minimum value (zero).
direction
count
clear
BOTTOM
Control Logic
T2
). clk
TOP
T2
T2
TOVn
(Int. Req.)
is present or not. A CPU write overrides (has
clk
can be generated from an external or internal
Tn
Prescaler
Oscillator
T/C
2486M–AVR–12/03
clk
I/O
TOSC2
TOSC1