Contents

Introduction

A common task for a robot is measuring distance as it travels.  For a wheeled robot, this is often accomplished by using encoders that produce a pulse when the wheels move a small distance.  This example shows a TEA program that may be used to keep track of two digital encoders. 

A typical encoding scheme for a small robot is an optoelectronic switch that stares at a pattern of black and white sectors on an encoder wheel.  As the wheel turns, the switch turns on and off.  A microcontroller can count these pulses and determine the distance traveled. 

Source Code

In the example program below, each transition on an encoder input (low-to-high or high-to-low) increments a counter.  The encoder inputs are digital IO pins 0 and 1.  When a transition occurs on pin 0, counter 0 is incremented.  When a transition occurs on pin 1, counter 1 is incremented.  Counters 0 and 1 are two-byte reflex counters.  A host computer may use the cmdCTR_SET command to read the current count or set the current count.  A concurrent TEA process running within the same module may access the counters via their TEA I/O ports. 

The rate at which this program can process encoder transitions depends on the processor load.  When the processor is busy, it can not count as many pulses.  The program was coded by hand using virtual machine (VM) opcodes to keep the code as efficient as possible. 

In the worst case, the program executes 24 TEA opcodes per iteration.  The typical execution rate for VM opcodes is approximately 9000 instructions per second.  At this rate, the program can process a maximum of about 375 digital transitions per second for each encoder.  If other TEA processes are running, the number of digital transitions that this program can process will be divided by the number of active processes. 

This program may be used instead of reflexes to monitor incoming pulses.  Firmware versions prior to Build 9 may not handle digital transition reflexes properly. 

Revision History:

• 2003-08-08: Example Created.