Nuts and Volts - January 2014

Since the clock is so important, PIC microcontrollers offer a broad range of options to choose from for your own applications. You can have clocks that run rapidly when a lot has to happen in a short span of time, or ones that consume negligible power for battery-operated rigs. Maybe synchronizing the PIC to a stable real time clock is important to you. In fact, there are eight or more clock options for the typical PIC.

However, if you grab the datasheet and try to sort out all the possibilities, you'll quickly come to rue that old phrase, "Can't see the forest for the trees." Datasheets for PICs run in the hundreds of pages and aren't necessarily organized for best learning. And, of course, the occasional error works its way in, making it even harder for the newcomer to get up to speed.

As someone who has spent his entire adult life in teaching, I've always intuitively valued the importance of a well organized presentation. You begin with the

"forest" and only approach the "trees" afterwards. That's what we'll do here — getting the big picture in mind first and then tackling the details when they're finally needed.

So, if you've been dismayed in the past at how complicated the clock options for PICs seem to be, tag along now and see how the right approach can make all the difference. To turn this into an active learning experience, we’ll conclude with some actual experiments you can conduct on the breadboard. By the end of our session together, you should be all set to start using PIC clocks with confidence.

Primary Clocks

To keep things concrete, I'll focus on the PIC16F88 which is one of the most popular of all microcontrollers among DIYers. However, other PICs will sport many of the same options — even the smaller eight-pin chips.

All microcontrollers require a clock or oscillator to guide a program through its paces. It's the duty of such a module to indicate when an instruction should be fetched from program memory, decoded, and acted upon. Actually, even the simplest instruction is made up of a number of operations that must be sequenced in just the right order and at just the right moment. The clock, then, is like the conductor of an orchestra, coordinating all of the parts that make up the whole.