New Tutorial : PWM on ATmega168

[airman00]

New Tutorial - http://www.societyofrobots.com/member_tutorials/node/226

Can be used to give PWM capabilities to the 50 dollar robot board or any ATmega168.

feedback appreciated.

[mbateman]

Good info, but you should make it clear in the tutorial that this is useful for PWM that does not care too much about the base freqency and can use the whole range of 0-100% duty cycles. It is not suitable for servo control since it is not the correct base freqency and with the 8 bit timers, does not have sufficient granularity for the 5-10% range that servos require. For things like your LED dimmer and direct H-bridge motor control, it is very useful, though.

[airman00]

Good info, but you should make it clear in the tutorial that this is useful for PWM that does not care too much about the base freqency and can use the whole range of 0-100% duty cycles. It is not suitable for servo control since it is not the correct base freqency and with the 8 bit timers, does not have sufficient granularity for the 5-10% range that servos require. For things like your LED dimmer and direct H-bridge motor control, it is very useful, though.

I'll copy and paste that into the tutorial right now

[Ro-Bot-X]

Too much cut and paste

For both Timer0 and Timer 2:

// 00   A off, toggle on compare
should be:
// 00  A Off, pin works as general I/O
same for B...
// 11   Low part of WGM: PWM, phase correct
should be:
// 11   Low part of WGM: PWM, Fast PWM

Then, when you set the pin On, you comment:
// 10   A on, toggle on compare
should be:
// 10 - A On, set at Bottom, clear at compare match

// 5   B on, toggle on compare
here instead of pin number 5, there should be the way you are setting the 2 bits for the channel B:
// 10 - B On, set at Bottom, clear at Compare match

There is no "Toggle on Compare", they should all be "Set at Bottom, Clear at Compare match" for all timers in the setup you are using.

Other than that, the functionality is good, just the comments are wrong. We don't want people to get confused, right?

[Webbot]

@airman00

Hey, I've started doing a tutorial on PWM as well. Didn't mean to step on your toes but my idea was to cover the subject generically - not just Mega168, Roboduino etc. Just that there seem to be loads of questions/misunderstandings about it.

Its up there but unfinished and is still work in progress.

Will eventually also cover doing PWM via software rather than hardware

[airman00]

yea you caught me RobotX , I'll edit the comments sometime today .

@Webbot
OK cool Looking forward to it

[Ro-Bot-X]

Can you also add a formula to convert the 0-255 value to 0-100 percentage? That way it doesnt matter if the timer is 8 or 16 bit...

So instead of writing:
 pwmSet5(255); //pin6   which is 8 bit PWM
to write:
pwmSet5(100); //pin6   which is 8 bit PWM

I know this reduces the granularity, but sometimes is more usefull. Maybe a pwmPercentSet5(value) command?

[Webbot]

I recommend a general linear interpolation routine such as:

Code:

/*
* Interpolate between two numbers
* value - the current value to be used
*   minVal - the minimum that 'value' can be
*   maxVal - the maximum that 'value' can be
*   minRtn - the return value if 'value = minVal'
*   maxRtn - the return value if 'value = maxVal'
*   return a value in the range minRtn to maxRtn
*/
int interpolate(int value, int minVal, int maxVal, int minRtn, int maxRtn){
long int lRtnRange = maxRtn - minRtn;
long int lValRange = maxVal - minVal;
long int lRelVal = value - minVal;
lRtnRange =  minRtn + ( lRtnRange * lRelVal / lValRange );
return (int)lRtnRange;
}

[Admin]

I just added it to the $50 Robot tutorial.