Glad I asked, cause I forgot that the primary oscillator is disabled =20 during sleep. The 16F883 does not have an IDLE mode, but it seems the =20 18F (which I'll be moving to eventually) does, and that when I'll need =20 this clock feature. Those external RTC's are generally expensive, though now I see some =20 (even from our own Microchip) that are a bit of a buck each. Prob =20 here is that these use I2C, and I'll either spend a lot of development =20 time to bit-bang I2C, or I'm faced with the same problem of needing a =20 couple Port-C pins to use the I2C peripheral. Perhaps I can find a =20 small, low-cost RTC that just gives me an interrupt every second. This is a very low-volume project (couple dozen units), so I'm =20 cringing at any significant development. It's a spin-off of something =20 else I've done so most of the code is already developed, and I'd =20 prefer not to invest too much in coding changes for now. Adding a =20 secondary oscillator with watch crystal would be simple. Bit-banging =20 will take up much time debugging etc (past experience). Cheers, -Neil. Quoting peter green : > PICdude wrote: >> What I'm not clear on, is that if I run the PIC from an external >> oscillator (say 12Mhz, 20ppm crystal) instead, can I use that >> instruction clock to wake the PIC periodically? > You can but you will have to use "idle" mode rather than "sleep" mode. > Sleep mode disables the main clock while idle mode doesn't. I dunno > if the power consumption in idle mode will be acceptable to you. > >> Or is there some simpler method to do this? >> > Have you considered simply using a seperate RTC chip to track the time? > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .