Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. The following figure shows a block diagram of the counter and its surrounding environment.

Figure 1. Counter Unit Block Diagram

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).
clkT2
Timer/Counter clock.
TOP
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 (clkT2). clkT2 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 clkT2 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, refer to Modes of Operation .

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.