Servo - January 2012

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by

Dennis Clark

It’s 2012, a new year, with more to learn and more experiments to try. What new wonders will be made available by what seems to be an ever-expanding industry of robotics components suppliers and creators? I’m sure that I don’t know, but I’m just as sure I’m going to try to find out! I hope to hear from those of you who try experimenting with these new wonders and come up with questions about new ways to apply them.

This month, I’m going to finish up my generic discussion of timers and what we can do with them. The topic will be the PIC microcontroller and its various ways to deal with timers. Each PIC family uses timers; as the PIC has matured, the timer subsystems have changed a little and become simpler and easier to use, in my opinion. Older PIC16C parts made you set a period register, then set your match number, THEN set your counter. Most folks found that very confusing. Newer 16F parts just have a timer pre-scaler setting and the timer counter register (two registers for the 16-bit timer/counters). I’m going to start with the venerable PIC16F series, move to the PIC18F series, and end up with the PIC24 series. You should see the similarities between the timer modules of these very different PIC series variants.

Before I start though, I’m going to stand on my soapbox for a minute. I still from folks wanting advice with programming

16F<something> parts as a robot controller. My advice to them is “You need to move on!” For all but the simplest robot projects, these parts just won’t provide enough “bang for the buck” for you! Even the PIC18F parts should be passed over for the newer PIC24 or even PIC32 devices. These new 16-bit parts are under $4 usually and the 32-bit ones are typically under $7. Microchip has free compilers and a new IDE that runs under Windows, Mac, and Linux operating systems. There isn’t any reason not to step up! ‘Nuff said.

I’m going to choose a C compiler for each of the PIC variants rather than Assembly. While Assembly is fun and all, it isn’t the easiest way for newcomers to the craft to use an embedded processor like the PIC. For the PIC16 series, I’ll use the CCS PCM compiler. It is a low cost compiler that will integrate with the MPLAB PIC IDE. For the PIC18F series, I’ll use the Microchip C18 compiler which can be used by the hobbyist or student for free. For the PIC24F series, I’ll use the C24 (also called C30) compiler which is also free for hobbyists and students.

Timers for the PIC16/Fxxxx

Figure 1. PIC16F TIMER1 layout.

PIC16F parts typically top out at 20 MHz. The clock to the timers is then Fosc/4, which means when using a 20

MHz crystal your input clock to the timer is 5 MHz.

Remember that when you are setting the prescaler to get the timing you want. Figure 1 shows the hardware layout for the TIMER1 module in a PIC16F630; a very useful 16F series PIC. (All datasheet graphics used by permission of Microchip, Inc.)

The easiest way to use a timer is to set an interrupt on the overflow of the timer’s counter register. The timer register will overflow when the register increments from its maximum value (0xFF for an eight-bit timer; 0xFFFF for a 16-bit; and so on) to zero, where it starts counting all over again. To set the interval of that interrupt, you follow this procedure: