Software Start One-Shot Mode

In One-Shot mode the timer resets and the ON bit is cleared when the timer value matches the PRx period value. The ON bit must be set by software to start another timer cycle. Setting MODE<4:0> = 01000 selects One-Shot mode which is illustrated in Figure 1. In the example, ON is controlled by BSF and BCF instructions. In the first case, a BSF instruction sets ON and the counter runs to completion and clears ON. In the second case, a BSF instruction starts the cycle, BCF/BSF instructions turn the counter off and on during the cycle, and then it runs to completion.

When One-Shot mode is used in conjunction with the CCP PWM operation the PWM pulse drive starts concurrent with setting the ON bit. Clearing the ON bit while the PWM drive is active will extend the PWM drive. The PWM drive will terminate when the timer value matches the CCPRx pulse width value. The PWM drive will remain off until software sets the ON bit to start another cycle. If software clears the ON bit after the CCPRx match but before the PRx match then the PWM drive will be extended by the length of time the ON bit remains cleared. Another timing cycle can only be initiated by setting the ON bit after it has been cleared by a PRx period count match.

Figure 1. Software Start One-shot Mode Timing Diagram (MODE = 01000)
  1. 1.BSF and BCF represent Bit-Set File and Bit-Clear File instructions executed by the CPU to set or clear the ON bit of TxCON. CPU execution is asynchronous to the timer clock input.