Servo - July 2011

CPLDs — Part 5

A CPLD Project complex programmable logic devices

by David A. Ward

For the final article in this series, I decided to incorporate a CPLD to control a mobile robotics platform or tank-bot (see Photo 1). Normally, a microcontroller is used to control the tank-bot, but I thought that it might be fun to see if a CPLD could be used to accomplish some of the same tasks that the microcontroller does. The tank-bot chassis consists of a Tamiya track kit and double gear box mounted on three pieces of aluminum. See Table 1 for the details and sources for some of the parts used to build the tank-bot chassis.

APCB (printed circuit board) is located on the bottom front of the tank-bot that holds an infrared LED in the center and two infrared phototransistors with one on each side (see Photo 2). The LED and phototransistors are used as sensors so the tank-bot can follow a black line of electrical tape laid out on a light colored table top. All of the circuitry involved is shown in the schematic diagram in Figure 1. The two phototransistors produce more of an analog output, so Schmitt trigger inverters were placed between their outputs and the CPLD. A L293 quad half H-bridge IC is used to handle the 6 VDC used to energize the two drive motors.

To start a truth table for this project, let’s first look at how the infrared phototransistors will behave. When the LED’s light reflects off of a light colored object — such as a light colored table top — back to the two phototransistors, they will be in an ON state. When these transistors are ON, they will take their collectors to ground and the CPLD pins high since their signals are being inverted through the Schmitt inverters. This would be the case if the tank-bot were placed on a table top without any black line under the