Timer 0 and Timer 1 may also be operated as counters. When functioning as a counter, a counter/timer register is incremented on each high-to-low transition at the selected input pin (T0 or T1). Events with a frequencyof up to one-fourth the system clock's frequency can be counted. The input signal need not be periodic,but it should be held at a given level for at least two full system clock cycles to ensure the level is properly sampled.