Servo - September 2009

In the Nuts & Volts PICAXE Primer column, we just completed a three-installment series that focused on methods of interfacing various inexpensive character-based LCDs with PICAXE microcontrollers. Since LCDs have been on my mind lately, they are the first thing I thought of when I sat down to plan this month’s TankBot article. In fact, when I started testing the EZ1 ultrasonic range finder that we worked with last time, it was hard to resist the temptation to stop in the middle of things to add an LCD to the TankBot so that I could display real-time, distance-to-obstacle data right on the TankBot itself. Now that the EZ1 article is finished, that’s exactly what we’re going to do. This month, we’ll focus on the hardware details of the project and, next time, we’ll explore two real-time software applications: distance-to-obstacle measurement and battery voltage monitoring.

Because the basic details of LCD interfacing are already available in the PICAXE Primer series in Nuts & Volts (April, June, and August, 2009), I won’t repeat them here. However, a brief overview will still be helpful. Our discussion will be limited to LCDs based on the Hitachi HD44780 controller chip because they are readily available and relatively inexpensive. These displays all use a parallel data transfer scheme that requires a minimum of six data lines. It’s certainly possible to directly interface an HD44780-based LCD with the TankBot’s PICAXE-14M processor. However, doing so would tie up six I/O lines and the necessary software would require almost all of the 14M’s program memory. We solved this problem in the Primer articles by dedicating a separate 14M as a peripheral processor for the LCD. The software we developed simply accepts standard serial data from a “master processor” and converts it to the parallel format required by an HD44780 LCD.

Constructing a Serial LCD Board

If you enjoy the challenge and satisfaction of developing your own hardware projects, you may want to construct one of the stripboard circuits we developed in the Primer series. If not, you can also use the

printed circuit board (PCB) version of the project that’s available on my site www.jr hackett.net/LCD08.shtml). Either way, we are going to mount a completed eight-character by two-line serial LCD to our TankBot and interface it with the BrainAlpha PCB.

Mounting the LCD to the TankBot is a simple procedure. (Figure 1 shows the mounting arrangement viewed from the back of the TankBot.) To mount the LCD board on the TankBot, you will need two #4 or #6 one inch bolts, six nuts, and two lock-washers. If you use #4 hardware, you’ll also need a 1/8th inch drill bit; for #6 hardware, a 9/64th inch drill bit is necessary. Here’s the way I did it:

Before assembling the

LCD board, place the bare board on the small horizontal cross-piece of the TankBot’s chassis (above the servo motors; see Figure 1) so that its bottom edge is about 1/16th inch forward of the back edge of the cross-piece.