21.4 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Below is the block diagram of the counter and its surroundings.
| Signal name | Description |
|---|---|
| count | Increment or decrement TCNT2 by 1. |
| direction | Selects between increment and decrement. |
| clear | Clear TCNT2 (set all bits to zero). |
| clkTn | Timer/Counter clock, referred to as clkT2 in the following. |
| 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 (CS2[2:0]). When no clock source is selected (CS2[2:0]=0x0) 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 (TCCR2A) and the WGM22 bit located in the Timer/Counter Control Register B (TCCR2B). There are close connections between how the counter behaves (counts) and how waveforms are generated on the Output Compare outputs OC2A and OC2B. For more details about advanced counting sequences and waveform generation, see "Modes of Operation".
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by the TCC2B.WGM2[2:0] bits. TOV2 can be used for generating a CPU interrupt.