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Accurately measuring duty cycle

Table of Contents

  1. Specification
  2. Implementation

Specification

  • To measure the average duty cycle in a PWM/pulse train
  • To accept a minimum high of 3uS
  • To measure to the highest accuracy easily attainable with the mbed
  • To measure signals with frequency 1Hz - 100kHz

Implementation

The LPC1768 (and indeed many others) include Counter/Timer peripheral blocks which make this task rather easy. They have the facility to capture the timer value on certain pin toggle events (the CAPn.0/1 inputs). The timer can be configured to count up by ones every clock; record when the signal goes low and record when it goes high again, and have interrupts on either of these events.

Unfortunately the two capture functions (cap 1 and 0) can only work on separate pins (with a register for storing the value for each) so to measure both high and low going times one is required to short two pins together. In this example we are using Timer 2 and therefore pins 30 and 29. To test pin 23 was used for generating a PWM signal of various frequencies.

A class was written to do the heavy lifting: PWMAverage

Import library
PWMAverage - PWMAverage Class Reference

Public Member Functions

PWMAverage (PinName cap0, PinName cap1)
Create a PWMAverage object.
void reset ()
Reset the counters and values.
void start ()
Start the timer.
void stop ()
Stop the timer.
float read ()
Read the duty cycle measured.
double avg_up ()
Read the average length of time the signal was high per cycle in seconds.
float avg_UP ()
Read the average length of time the signal was high per cycle in seconds.
double avg_down ()
Read the average length of time the signal was low per cycle in seconds.
double period ()
Read the average period in seconds.
int count ()
Read the number of cycles counted over.

Here is a test program:

Import programPWMAverage_test

A test program for the PWMAverage library. This program will print the average duty cycle of a signal (1Hz-100kHz) after a button is pressed for a few seconds.

Last commit 29 Aug 2012 by Giles Barton-Owen

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