MX016 - Measuring Time with Interrupts - Matrix Multimedia Ltd

© Matrix Multimedia 2011 MX016 - Measuring Time with Interrupts
Measuring Time with Interrupts
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by Ben Rowland, May 2009
Abstract
One problem with using or designing embedded systems is that it is hard to tie in real
world values like time or an incoming frequency to the operation of the embedded
system, the answer is to use onboard timers present in the majority of microcontrollers.
This article will attempt to explain interrupts and how you can use them in your
own programs.
Requirements
Software:
• Any licence type of Flowcode v3 or v4 for any variant.
Hardware:
• No specific hardware required.
Introduction
One problem with using or designing embedded systems is that it is hard to tie in real world values
like time or an incoming frequency to the operation of the embedded system. What is required is a
regular tick that will not vary over time and is slow enough to be collected within the main running
program.
To generate this tick we will be using a timer interrupt. Most microcontroller devices have at least one
or more onboard timers. The main difference between timers is the size of the counting register. This
can either be 8-bit, 16-bit or 32-bit. The PICmicro devices normally have a total of three timers consisting
of two 8-bit timers named timer0 and timer2 as well as a 16-bit timer named timer1.
When a timer interrupt is enabled the timer starts counting via the internal clock and the timer registers
then start to increment. As the registers increment there will be a point where the maximum
count value is reached (255 for 8-bit, 65535 for 16-bit). When this occurs, the timer count registers
reset to zero and the interrupt routine is triggered.
Note that the timer registers are still counting while you are processing the interrupt routine so the
main aim is to process your interrupt and return to your main program well before another timer overflow
occurs. If the interrupt routine carries on past the next timer rollover then you could either miss a
tick or worse still you could become trapped inside the interrupt service routine and never actually
process any of your main background program.
The timer interrupt enable settings normally show a clock source assigned to the timer count registers.
Unless you are providing an external clock to drive the timers this setting should always be set
to “CLKO” or “Internal Clock Out” in English.
So assuming we have designed our program to have a nice fast interrupt macro we will get a steady
tick that can be used for a great range of applications. Over the page are listed a few I can think of
off the top of my head.

© Matrix Multimedia 2011 MX016 - Measuring Time with Interrupts
• Clocks / Stopwatches
• Frequency generators / counters / analysers
• Data loggers
• Alarm systems
• Feedback / Closed-loop systems
• Real time operating systems
• Etc
Example Programs
Two example programs have been provided as part of this article. These can be found in the accompanying
downloadThe first deals with counting seconds to drive a clock based system. The second
uses interrupts to measure the frequency of an incoming digital waveform.
Example Program 1. Building a flexible and reliable second counter.
Firstly I added an interrupt icon to my program and double clicked it to access the properties.
I assigned the interrupt source to the TMR0 Overflow and created a new macro for the Interrupt to
call.
I then clicked properties to access the properties of the timer.
Inside the properties I changed the clock source to the Internal Clock CLKO
I also changed the Prescaler rate to give me a tick frequency of 150Hz or 150 ticks per second. This
is based on my clock speed of 19.6608MHz so if you are using a different clock speed then try and
find a nice whole number frequency so you are not loosing accuracy as your program runs. Also remember
that nothing will work correctly unless the clock speed setting in Flowcode matches your
actual hardware clock speed.
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© Matrix Multimedia 2011 MX016 - Measuring Time with Interrupts
I have made a couple of comments in the example about how I improved the functionality so as to
never miss a second even if the main loop were to take more than a second to complete.
I also started to hint at how a minute variable could be worked in to create a full working clock. If you
should want to implement a fully working clock then example TUT_22 in the Flowcode V3/Examples
directory contains a fully working version.
Example Program 2. Working out delays or frequencies based on timer overflows.
In this example our main program is monitoring the status of pin RA0. Whenever pin RA0 goes low
and then high we increment our frequency count value by 1. The Pin we are monitoring can be edited
by changing the input icon inside the Read_Input Macro. At the same time this is happening we
have an interrupt running at 75Hz. We count the number of interrupts and when a count value of 75
is reached we know that a second has passed. Seeing as frequency is measured in cycles per second
it seems like a good option to use this as our refresh interval. Also it means no further calculations
are necessary to work out the actual pulse frequency.
When the interrupt count value has reached 1 second the update display flag is set. This causes the
input checking while loops to exit immediately to go and service the display.
(in = 1) && (update_display = 0)
The double and (&&) is used to tell the compiler that we only want to stay in the loop if in is equal to
one AND update_display is equal to 0. If you wanted to check for this OR that then you can use the
double OR symbol which looks like this ‘||’. So we don’t get a false reading of one when there is no
incoming frequency we take a copy of the frequency variable before the while loops are forcibly exited
and this is the value that is then passed onto the LCD.
Once the display has been updated the program then clears the count variables and also clears the
timer interrupt count register. This means we are all ready to begin recoding the frequency reliably
with an accurate starting position.
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© Matrix Multimedia 2011 MX016 - Measuring Time with Interrupts
Further reading
Below are some links to other resources and articles on related subjects, and technical documentation
relating to the hardware used for this project...
Flowcode: http://www.matrixmultimedia.com/flowcode.php
Learning Centre: http://www.matrixmultimedia.com/lc_index.php
User Forums: http://www.matrixmultimedia.com/mmforums
Product Support: http://www.matrixmultimedia.com/sup_menu.php
Copyright © Matrix Multimedia Limited 2011
Flowcode, E-blocks, ECIO, MIAC and Locktronics are trademarks of Matrix Multimedia Limited.
PIC and PICmicro are registered trademarks of Arizona Microchip Inc.
AVR, ATMega and ATTiny are registered trademarks of the ATMEL corporation.
ARM is a registered trademark of ARM Ltd.
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