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.

Signal description (internal signals):

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

The counter is incremented at each timer clock (clk_T0). clk_T0 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 clk_T0 is present or not. A CPU write overrides (has priority over) all counter clear or count operations.