Counter Unit

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

Figure 1. Counter Unit Block Diagram

Signal description (internal signals):

Increment TCNT0 by 1.
Timer/Counter clock, referred to as clkT0 in the following.
Signalize that TCNT0 has reached maximum value.

The counter is incremented at each timer clock (clkT0). clkT0 can be generated from an external or internal clock source, selected by the clock select bits (CS02:0). When no clock source is selected (CS02:0 = 0) 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.