Design Ideas: April 27, 1995

Using the circuit in Fig 1, a 68HC11 µP's stop instruction can put the µP's external RC-oscillator clock, as well as the µP itself, into a low-power mode. On receiving an interrupt, the µP will exit the stop condition and enable the RC clock. The RC clock, being a low-Q circuit, will start up immediately. Crystal oscillators, on the other hand, can waste precious milliseconds coming up to speed and stabilizing.

The external RC oscillator uses simple Schmitt-trigger CMOS inverters. The clock frequency is

where V_LT and V_UT are the Schmitt triggers' lower and upper input-voltage switching thresholds. These voltages depend on the circuit's power-supply voltage.

The µP's XTAL pin gates the RC oscillator. The µP's stop instruction sets the XTAL pin to a logic high, disabling the RC oscillator. Remember to set the µP's Option Register bit #4=0 to disable the µP's crystal-oscillator-startup delay. (DI #1695)