The Timer1 16-bit incrementing counter is accessed through the TMR1H:TMR1L register pair. To preload the timer, these registers can be updated directly with writes to the TMR1H or TMR1L registers. Timer1 is incremented by either the system clock, instruction clock, or an external clock signal. When used with a system or instruction clock source, the module is a timer and increments on every instruction cycle. When used with an external clock source, the module can be used as either a timer or counter and increments on every selected edge of the external source.

Timer1 must be enabled to operate. This is done by setting the Timer1 On (TMR1ON) bit in the Timer1 Control (T1CON) register. This starts incrementing the counter on each pulse from the selected clock source.