external interrupts with RTOS

LiamLiam · Joined: 21 Sep 2008 Posts: 7

Hi All

I am a little confused about the use of external interrupts while using a RTOS.

In my application I'd like to count an incoming pulse, but at the same time drive an LCD in response to a user input via buttons (and a couple of other minor things) - it seems like I finally have to use a RTOS. After a little reading, these are my thoughts:

1/ I would have to have the input state checked (polled) as a very quick to execute function. This would be set as a task with a rate of execution that would satisfy the maximum frequency of the pulsed input.

2/ If I were to just have this pulsed input on a pin that has an external interrupt, I could not actually use the interrupt because of the RTOS? I would normally use this approach if I was not going to use a RTOS.

3/ My understanding is that there is no other hardware on any of the PIC's that I can offload the pulse counting to - the input capture features are to count the pulse widths etc. Am I correct?

Any comments would be greatly appreciated

Many thanks

LiamLiam · Joined: 21 Sep 2008 Posts: 7

Hi again

sorry if I was a little vague with the question, does anybody have suggestions or even just comments on this application?

Many thanks

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

First, I think hardly anyone on this forum uses the CCS RTOS.
That's because there are very few questions and/or answers on it.
Also, you only get the RTOS if you buy PCW, PCWH, etc.
You don't get the RTOS if you buy the command line compilers and
integrate them with MPLAB. So a lot of us don't have it.

If you have a problem with counting pulses, then we need to ask:

1. What device is providing the pulses ?

2. What's your purpose in counting these pulses ? Why are you doing it ?
What information do the pulses contain ? Are they encoded ?
ASCII ? Manchester ? Etc.

3. What is the frequency and duty cycle (or high and low times)
of the pulses ? What are the voltage levels ? Is this a rectangular
waveform, or is it sinusoidal ? Are the pulses spaced at specific
intervals ? What is the length of the interval ?

LiamLiam · Joined: 21 Sep 2008 Posts: 7

Thanks for the reply PCM programmer.

1/ The incoming pulse is simply a square wave from a flow sensor - from 0V-5V. I then put this through a potential divider to take it to 2.5V for the PIC. It has a duty cycle of 50%.

2/ The frequency of the pulses is directly proportional to the flow rate.

3/ The maximum frequency of this is 100Hz, with levels as descibed above. The minimum will be approximate 0.01Hz

All I need to do though is increment a 32 bit unsigned integer each time a rising edge appears. As I previously mentioned I would normally do this with a external interrupt however I am unsure if this is possible while using a RTOS.

I noticed that there was little discussion of CCS' RTOS on this forum too... do you have any personal experiences with any? any suggestions on a suitable RTOS for use with CCS C?

Many thanks

EDIT: FYI I am using the PIC24F256GB108

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

LiamLiam · Joined: 21 Sep 2008 Posts: 7

This counter will never be reset - so it will be greater than 65K. I will periodically back this value up to EEPROM.

The PIC / the hardware / levels and interfacing is all sorted, this isn't part of the question. I can easily implement what I want, although without a RTOS, simply with an external interrupt set to trigger from low to high.

My question is how best to integrate this functionality with a RTOS, I was hoping somebody with some RTOS experience could shed some light on how best to achieve this.

thanks for your replies.