AVR-Chat

Hi Martin.

I played with PWM on the Mega 8 for hours on the simulator trying to find 
what I was doing wrong.  I finally found somewhere that the PWM does not 
work for the Mega 8 (or even the 168) with the simulator(studio 4).  I 
finally did what I thought must be right and stuck it in the chip and it 
worked correctly.

Good luck

Loren





Dave Hylands <dhylands@gmail.com> 
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Nov 09, 2005 03:35 PM
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Re: [AVR-Chat] Phase and Frequency Correct PWM on a Mega 8






Hi Martin,

> I'm having what I can only classify as annoying.  I implemented a simple 
servo controller with a tiny 15 ( two position ) and it worked perfectly. 
I've also done some simple PWM stuff with Timer 2 on the Mega 8.  Now, 
however I need to use the 16 bit timer to generate PWM.  I need a bare 
minimum of 12 bits precision but more would be better ( 256 servo 
positions, 60 Hz update rate ).  To that end I've tried to get 16 bit 
phase and frequency correct PWM working and I've had no luck.  In the 
simulator the timer counts up to TOP (in my case 0xFFFE ) and promptly 
overflows to zero.  Now it's my understanding that it is supposed to count 
up then down ( certainly that's how it works in 8 bit mode ).  It wouldn't 
bug me so much, it overflowing, if I was able to get something useful out 
of it.  However because it is overflowing the output pin is only getting 
set or cleared ( depending on the setting of the COM1nX pins ).  Following 
is the code I use to initialize it.

I wrote some code for the ATMega128 which uses Timer3, but it should
work on Timer1 with a minor tweak (changing the SERVO_1 and SERVO_2
definitions to be 1A and 1B rather than 3A and 3B).. It uses the Fast
PWM mode runs the timer at 2MHz, which means it will overflow every
32.7 msec (or 30.5 times per second).

The sample code can be found here:
<http://websvn.gumstix.com/listing.php?repname=Buildroot&path=%2Fbranches%2Fprojects%2Frobostix%2F&rev=0&sc=0>

You'll be prompted for a username and password, use the word gumstix for 
both.

To get the complete example to build, you'll need the Common
directory, the Simple-Servo directory, and the Rules.mk file.

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/



 
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