18.4 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.

Figure 18-2. Counter Unit Block Diagram
Note: The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and the “x” indicates Output Compare unit (A/B).
Table 18-2. Signal Description (Internal Signals)
Signal NameDescription
countIncrement or decrement TCNT0 by 1.
directionSelect between increment and decrement.
clearClear TCNT0 (set all bits to zero).
clkTnTimer/Counter clock, referred to as clkT0 in the following.
topSignalize that TCNT0 has reached maximum value.
bottomSignalize that TCNT0 has reached minimum value (zero).

Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clkT0). clkT0 can be generated from an external or internal clock source, selected by the Clock Select bits (CS0[2:0]). When no clock source is selected (CS0=0x0) the timer is stopped. However, the TCNT0 value can be accessed by the CPU, regardless of whether clkT0 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 WGM01 and WGM00 bits located in the Timer/Counter Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B (TCCR0B). There are close connections between how the counter behaves (counts) and how waveforms are generated on the Output Compare outputs OC0A and OC0B. For more details about advanced counting sequences and waveform generation, see Modes of Operation.

The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM0[2:0] bits. TOV0 can be used for generating a CPU interrupt.