real time clock

ljbeng · Joined: 10 Feb 2004 Posts: 205

I want to use a 16F628 to be a real time clock on 2AA batteries. I am connecting a 32.768KHz crystal to the Timer1 oscillator inputs. I want to use the internal RC oscillator for the main program and sleep a lot (the chip, not me). Will my clock be accurate in this configuration? Has anyone done this? Thanks.

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

It will be as accurate as your 32.768 kHz crystal. I think 20ppm is typical for these crystals. Crystal accuracy is made up from two parts: absolute accuracy and drift because of the environment (temperature).
You can include a trimmer cap to fine tune the frequency, though you cannot avoid drift caused by temperature change. That is why many manufacturers produce temperature compensated RTC oscillators. You can search the internet for actual accuracies of these solutions.