8-bit AVR Microcontrollers

Normal Mode

The simplest mode of operation is the Normal mode (TCCRnA.WGMn[3:0]=0x0). In this mode, the counting direction is always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its maximum 16-bit value (MAX=0xFFFF) and then restarts from BOTTOM=0x0000. In normal operation, the Timer/Counter Overflow Flag (TIFRn.TOV) will be set in the same timer clock cycle as the TCNTn becomes zero. In this case, the TOV flag in behaves like a 17th bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt that automatically clears the TOV flag, the timer resolution can be increased by software. There are no special cases to consider in the Normal mode, a new counter value can be written any time.

The input capture unit is easy to use in Normal mode. However, observe that the maximum interval between the external events must not exceed the resolution of the counter. If the interval between events are too long, the timer overflow interrupt or the prescaler must be used to extend the resolution for the capture unit.

The output compare units can be used to generate interrupts at some given time. Using the output compare to generate waveforms in Normal mode is not recommended since this will occupy too much of the CPU time.