On Mon, 1 Dec 1997 15:08:11 +0200 Gordon Zerf writes: >>PTM: Whay spend two xtals ?. Use 2^x Xtal and use counter timeout >>interrupt. > >I could use RC to drive the chip and a watch crystal to generate >accurate >time, but I don't know which "TWO" pins they are on about > The pins are RC0/T1OSO/T1CKI and RC1/T1OSI/CCP2. They are found only on 28 and 40-pin PICs. On a PIC16C63, they are pins 11 and 12 respectively. When the chip is set to do so, the pins form an oscillator which clocks TMR1 directly at up to 200 KHz. The crystal needs to be connected possibly with loading capacitors and a series resistor just as for the LP option on the main oscillator. For precise timekeeping making one of the capacitors adjustable (or providing a software adjustment) would be useful. The primary advantage of this feature is that the timer will count while the PIC is asleep, and it can generate a wake-up interrupt when it overflows (at least every 2 seconds with a 32.768 KHz crystal). Thus the PIC can spend most of its time shut down, except for the low-power, low-speed oscillator. For applications where the PIC will be doing absolutely nothing for hours at a time though, probably lower power consumption could be realized with a seperate RTC chip.