pwm

[Einly]

Dear all,

I would like to use pwm to control the speed of my dc motor, so that I can start the motor immediately/ faster when I switch it on. However, when I control the speed by sending pulses, eg:

do
{
portb=0xC0; //high
delay_us(40);
portb=0;
delay_us(40);
}while( );

I couldn't do anything else. I would like to continuously sense the movement of an optical mouse using pic 16f876, and move the motor whenever movement occurs. How can I do these 2 actions (sensing and control the speed of the motor by sending pulses) in the same time?

Is it any other better ways to do a fast switching for the motor?

Thanks for your help.
___________________________
This message was ported from CCS's old forum
Original Post ID: 10637

[R.J.Hamlett]

:=Dear all,
:=
:=I would like to use pwm to control the speed of my dc motor, so that I can start the motor immediately/ faster when I switch it on. However, when I control the speed by sending pulses, eg:
:=
:=do
:={
:=portb=0xC0; //high
:=delay_us(40);
:=portb=0;
:=delay_us(40);
:=}while( );
:=
:=I couldn't do anything else. I would like to continuously sense the movement of an optical mouse using pic 16f876, and move the motor whenever movement occurs. How can I do these 2 actions (sensing and control the speed of the motor by sending pulses) in the same time?
:=
:=Is it any other better ways to do a fast switching for the motor?
:=
:=Thanks for your help.
Yes.
Use the hardware PWM.
The chip contains two hardware PWM modules, designed to perform exactly this sort of task. Set up something like this (I am only posting the 'PWM' guts, so you will have to add your own code 'round' this, to give the required effect):

int power;

#int_TIMER2
TIMER2_isr() {
//Update the motor pulse width
set_pwm1_duty(power);
}

//Then in main
//The timing values here will need to be set
//according to the clock you are using, and the
//PWM rate you require. These values give a PWM
//pulse that is at about 4KHz with a 4MHz clock,
//((clock/16)/64)=3906Hz, and an 'update' interval
//at 244Hz.
setup_timer_2(T2_DIV_BY_4,63,16);
setup_ccp1(CCP_PWM);
set_pwm1_duty(0); //disable PWM to start.
power=0;
enable_interrupts(INT_TIMER2);
enable_interrupts(global);


Now loop monitoring your inputs, and when you want to change the power, simply set 'power', equal to the value you require - 0=power off, 256=max. In the example given, the PWM 'resolution', is 8 bits. Look at Microchip AN539, for a good description of how this all works.
When you change 'power', the next time the TIMER2 interrupt occurs, this value, will be applied as the value to use. This happens every 16 pulses of the output, so there is a maximum of just over 4mSec delay (again at 4MHz), before the power changes. The PWM output is on pin C2.
The actual frequencies available, resolution, and update intervals you can use, depend on the clock rate you are running your chip. For instance if you change the setup line to:

setup_timer_2(T2_DIV_BY_4,15,16);

On the same 4MHz clock, the PWM rate, now rises to 15625Hz, and the 'update' interval becomes just under 1KHz. However the available resolution falls to only 6bits (0-64). Hence the choice of values, depends on the frequency you need to make the drive smooth for your motor, and the number of resolution steps required.
You can update the PWM frequency without using the interrupt, by just using 'set_pwm1_duty'. The use of the interrupt driven version, is especially useful, when you are performing a lot of arithmetic to set the speed required, and want to keep the update 'rate' constant, especially since you can generate a 'tick' counter in the insterrupt code, to keep the rest of the code 'in sync'. Whether this is the better solution for you will depend on the details of your application.

Best Wishes
___________________________
This message was ported from CCS's old forum
Original Post ID: 10641

[sliders_alpha]

i've tried your program but it's not working.

[Gerhard]

Firstly just try a small program that just switches on your output as a pwm to the motor and check that this is working.

#define P1C PIN_B1

[sliders_alpha]

B1 and B2 not C2 ans C3? well, i've got nothing on them with that

[Gerhard]

Check the data sheet.

[sliders_alpha]

[Gerhard]

You need to read up about this functions. Look at the header file and data sheet p61-63 and read the ccs manual