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Bolier control application. PWM output with a PIC16F876 to control valve.

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Mitchy190

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Hello everyone, bellow shows a question with regards to micro-controllers which I am currently interested in, but stuck on, and need some (a lot) of help!

A boiler has an analogue inlet and outlet temperature sensor and the output controls
a valve in the gas supply to determine the heat input. This valve must be activated
within 1ms from a change in demand. Pulse width modulation with a fundamental
period of 10ms is used to perform the activation and the temperature difference
between the two sensors is used to represent the on period of the valve.

I need to use the PIC16F8761 data sheet to study the subsystems that will be required and
develop a procedure to determine if the time response would be adequate for the application.

data sheet found at "http://ww1.microchip.com/downloads/en/DeviceDoc/30292c.pdf"

It can be assumed that:
i) 8-bit resolution is adequate for the analogue inputs
ii) A 20MHz crystal is used for the PIC

Basically I want to use the assembler language to create the appropriate code needed to perform the control of the valve within the time specifications shown above, and what I understand from the above so far is that:

duty cycle = outlet sensor - inlet sensor

and that the PIC I am using has a PWM module in which I have no clue on how to set it up. I would much appropriate some help in how to go about this question, and where to start first!

Many thanks Mitch! :)
 

Greetings,

If your pic has a pwm module then this is fairly easy to set up. You will have to configure the timer, then set up the PWM module, and then maybe write a function or two to output specific duty cycles.

I have included the link to the user guide for the capture/compare/pwm module. This is a fairly useful guide that will show you the steps that need to be taken in order to create a PWM pulse...
https://ww1.microchip.com/downloads/en/DeviceDoc/31014a.pdf
The instructions are in assembly, but it should still give you a pretty general idea of what needs to happen.

Regards,
Willis
 

using Microchp's PICDEM mechatronics kit
**broken link removed**

the following code is used to control motor speed using PWM
Code: 
// initialise PWM using Capture Compare PWM Module 2
void PWMinitialise(void)
{
        // assume Fosc uses oscillator 8MHz - call systemInitialise() first
        TRISD2=0;                // set RD2/CCP2 is output
        TRISD7=0;                // set RD7 is output
        PR2=0x3f;                // set PR2 to 31.2 kHz PWM (PWM value will be 8 bits)
        // Configure Capture Compare PWM Module 2
        CCPR2L=0;                // set PWM duty cycle to 0
        CCP2CON=0xC;             // set PWM mode
        TMR2ON=1;                // Turn on Timer 2 PWM
}

// set PWM duty cycle - value between 0 and 100
void PWMcontrol(int dutyCycle)
{
        // get 8 bit value for PWM control at 31.2 kHz PWM
        dutyCycle = (int) (dutyCycle * 255L / 100L);             // get 8 bit value
        RD7=1;                                                   // switch on motor
        // load most significant 6 bits in CCPR2l and least significant two bits in CCP2CON
        CCPR2L=dutyCycle >> 2;                          
        CCP2CON = (CCP2CON & 0xF) + ((dutyCycle << 4) & 0x30);   // set bits 4 and 5
}
 
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