Nuts & Volts - June 2007

going to the same pins. The only problem is that the direction of the LEDs are reversed, but that’s just a code change for the PIC. We just reverse the level on the outputs on the same pins. If you want a segment to light, you still just apply a positive to the anode (which is on the same pin as a cathode for the common anode design) and a negative to the cathode (which is on the same pin as an anode on the common anode design). I know it sounds a bit strange, but look at Figure 2 and reverse the direction of the diodes. Then pick a segment and a digit and apply the logic levels. I’ve allowed for either in the code (more on that later).

I have a collection of 16F648A PIC microcontrollers. I bought these at a quantity 25 price break and they are my little “go to” microcontrollers. The design could also use the 16F627 and 16F628

parts. The -A models just have twice as much program Flash memory and they only cost a couple of pennies more than the lower capacity parts, so I typically use these. But if you have some of the other parts, by all means, change the configuration word and the processor type in the source code and use those.

For little designs like this, I often do the printed circuit board design first and then do the schematic. It’s a little backwards, but allows for some much simpler layouts. I first lock out the special pins of the PIC (like MCLR, the RS-232 pins, RB4, power, and ground). Then with the pins left over, I connect to the display. To keep the small form factor for the board, I mounted the display on the solder side of the board and the rest of the components on the other side. This means that I could put the PIC inside

the footprint of the display. With a little bit of thought, I got a single-sided board without a jumper.

After doing the layout, I generated the schematic (see Schematic 1). There are four current-limiting resistors for each of the anodes, the microcontroller, and a simple inverter made from a 2N2222 or other general-purpose NPN transistor. This is required for the PIC’s serial input. The signal must be inverted (have a low true signal for the start bit). Note that this inverter circuit is not within RS-232 specs, however I have used this circuit with many different serial interfaces without a problem. The power supply is just a full bridge linear regulator design, and without it, there is only the PIC, one transistor, the display, and seven resistors. That’s a pretty low parts count!