Timing + PWM on same MCU ?

educasoft · Joined: 02 Feb 2009 Posts: 82

Hi,

For a laser engraver machine I am writing a little bit of code to handle the laser power (PWM) and the length of 1 single laser shot (aprox 3ms)

The project will have 1 input line called SHOOT and 1 full port(8bits) will be used to atach dipswitch for pwm value select.
The project also will have 2 output pins being 1 SHOOT pin and 1 pwn signal

If the controler detects an active 1 on the input shoot pin, then it will set its output shoot pin to 1, but after 3ms will automatically reset it back to 0. This will repeat as soon as the controller receives anther 1 on the input shoot line

I was thinking of using timer1 for this, because its the highest resolution timer.

But then I also need in my main program loop (since the SHOOT will be in interrupt routine) some code to read the 8 bits of data from the 8 input pins (this will be done continuously, but speed isn't crucial as this value will not change a lot and only when it changes the pwm value will change)

I was thinking of using TIMER0 for this, but in the documentation I cannot find info about pwm on timer0.

Does this mean I cannot do pwm on timer0 ?

Or am I thinking totally wrong and should I run the PWM on my timer1 and then time that 3ms delay on timer0 ? Even at 8bits and using a bigger prescaler I guess aprox 3 ms would be easy to achieve ?

Fact is both pwm and 3ms delay would benefit if they are fairly accurate. That doesn't mean it should be exactly 3ms, but is can als be 3.0012154 ms , as long as it stays stable and doesn't change to much. The difference of 2.9ms , 3ms or 3.1ms of laser shooting is qute significant when

It would not be a problem if PWM only has 256 possible values, since I want to control laser power in percentages and that only needs 100 different steps, so even at 8bits I am 2.5 times more accurate then this.

I hope somebody can tell me if my thoughts are right ?

I also think an attiny2313 would be enough for this, since it won't need a lot of instructions.

albertsm · Joined: 09 Apr 2004 Posts: 5916 Location: Holland

timer0 has pwm output. (pb2). have a look in the pin out viewer in bascom.
so yes a tiny2313 can do the job nicely. leave it to someone else to give a better detailed answer Very Happy

i.dobson · Posted: Tue Feb 14, 2012 2:34 pm Post subject:

Hi,

Have a look at the CPU you want to use, if the cpu supports pwm then you'll have a pin called something like OC0A where 0 is the timer number used to generate the PWM signal and A is the output (Some chips support several PWM outputs from one TIMER so OC0A,OC0B etc).

Then have a look at what other timers are available.

Unless your looking at a huge production (Several K chips/boards) use a larger chip than you think you need.

I usually start with a mega8, then mega162, then mega32. The price difference between a tiny chip and a mega chip is so small that it doesn't make sense to try and save afew dollars. Also when you start a project do you know exactly what the final result (In terms of code/functionallity) should be? I'm currently working on a project for my company, the first version of the software was about 1K lines and 10Kb in size. After the salesman saw what was possible they came up with new ideas/improvements which after afew "discussions" I've implemented, and now the code is 1.5K lines and 14Kb in size.

Regards
Ian Dobson

educasoft · Joined: 02 Feb 2009 Posts: 82

Hi again,

I am a little further now in my code and until now I got an external intrerrupt working on INT0 + Timer1 also helps me to generate a very precisely timed ON time for an output pin.
Using serial communication I can set this timer1 value, so that all works.

The only thing which seems not to work yet is the fact that I want to use timer0 (or timer2 if timer0 can't do it) to generate a pwm output signal on a pin of the mcu.

I do understand that since I allready use my timer1 for something else, timer0 and timer 2 are only 8 bits , so only 256 possible pwm values, but I have no problem at all with that since it will be more then enough. The purpose of this pwm signal is to input the signal into the analog input of a laser tube power supply to determine the power with which the laser will shoot.

So I need to make a 0->5V signal for that ant thats why I need PWM

Here is my code until now

educasoft · Joined: 02 Feb 2009 Posts: 82

Is there anybody who could help me a little bit with this one.

Today I struggled another 5+ hours trying to solve this but somehow I must be missing something crucial ?

The chip I want to let this work on is a ATMEGA328-PU

Timer1 and INT0 in my code all work flawlessly, but I cannot seem to get any pwm response at all with this code on an atmega328-PU PDIP chip

If it isn't possible with timer0 then timer2 is also not used at the moment so can also be used. This is the only function left which should get into my code so it doesn't matter if the pwm output is done with timer0 or timer2 as long as I have 1 working 8bit settable pwm output on the chip.

Kind regards,

Bart

educasoft · Joined: 02 Feb 2009 Posts: 82

Yes!!!!!!!! I found a solution to my problem

By taking the source code above and removing all code related to timer0, I then added only these 3 lines of code

Tccr0a = &HA1
Tccr0b = &H02
Ocr0a = &H7F

And suddenly PWM output became available on the atmega328. I don't know why it didn't work with CONFIG TIMER0 ?

So it does work now, but if somebody knows why my other code didn't work then this could be nice to know for other people searching this forum with similar problems.

Kind regards,

Bart

hgrueneis · Joined: 04 Apr 2009 Posts: 902 Location: A-4786 Brunnenthal

Your timer0 config:
Config Timer0 = Pwm , Prescale = 1 , Compare A Pwm = Clear Up , Compare B Pwm = Clear Down
Pwm0a = 128
Maybe it is because there is no port config for "B-output" and no compare value set for B but declared in the timer config.
(did not test it)
Hubert