What is the fast and elegant way to generate PWM without a Microcontroller to control a servo motor? With potentiometer or other ways to control the duty cycle with fix period.
- sorry about the mess, I want to control a hobby servo.
Electronic – the simplest way to generate PWM without a Microcontroller
pwm
Related Solutions
If you have a suitable voltage output that you can control you can hook that up to a 555 instead of using a potentiometer.
If you read the 555 data sheet, you can find out what is required on the modulation input.
The delay() function causes the PIC to loop and prevents any other activity other than interrupt triggered routines. In your example it will turn on RA0 for 10 ms, pause (preventing servicing any other output) and then turn off RA0 for 10 ms with the same inhibiting action. The pause() function should really be hidden from beginners as it encourages this kind of poor programming practice.
Instead you should run a base timer for your PWM outputs and do something like this pseudo code.
while(1) {
timer += 1; // increment the timer.
if (timer > tmax) {
timer = 0; // reset
}
PORTA.RA0 = (timer < t0ontime); // Output on until ontime exceeded.
PORTA.RA1 = (timer < t1ontime);
PORTA.RA2 = (timer < t2ontime);
PORTA.RA3 = (timer < t3ontime);
PORTA.RA4 = (timer < t4ontime);
PORTA.RA5 = (timer < t5ontime);
PORTA.RA6 = (timer < t6ontime);
PORTA.RA7 = (timer < t7ontime);
}
It's been many years since I've used the PIC compiler so this may not work as written, but you get the idea. You should make the timer increment on a fixed timebase rather than the way I've shown it so that it won't vary if you change the chip oscillator, etc.
One other improvement you could make on this code is to avoid starting all the pulses at the same time. This would smooth out the demand on the power supply and may help reduce noise a little.
Best Answer
I recommend the (GASP!) 555 Timer in "astable" mode. You'll find everything you need in the link, but I copied them here just for you!
Astable mode gives you a variable PWM frequency, and allows for an adjustable duty cycle as well (high-time and low-time equations in the link).
The circuit:
Note: I'd add an electrolytic cap across Vcc (positive lead) and GND (negative lead) to reduce the effect of dips in power supply voltage.
The PWM frequency:
Some defense for my answer compared to others in this post. Most other answers require an intermediate waveform to generate a variable PWM signal, such as the common triangle wave/comparator method. I don't see much point in constructing triangle wave generator (a significant circuit in-and-of itself) just as an intermediate step to solve your problem.
The 555 is a great analog chip and does just what you need. I wish people didn't hate on them as much.