AVR Freaks

However I need to generate pulses

What about PWM mode?  Anyhow...

 

You probably just need a little logic blip; width doesn't matter much  (but you want when the blip occurs controlled accurately).

 

You could simply run the pin toggling at 2x the desired pulse freq & then get 50% duty cycle "pulses" (a square wave).

 

If you want more of a "blip", set comxxx bits to TOGGLE the pin on a match--this will give "exact" timing.  Also have the ocr match interrupt turned on...it will also fire.  Maybe you can somehow use the force output compare bit to generate a toggle (back to low)

– FOC0A: Force Output Compare A  

A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR0A as TOP.

==============

Or even better(maybe)...set up toggle mode & the also the compare IRQ....in the IRQ set the irq for a small amount...you quickly get a toggle (downward)...then set a large number for a longer delay (upward).

In other words set a long delay for the next leading edge, then a short delay for the trailing edge.  like  97, 3, 97, 3, 97, 3....

Rather than using toggle mode, you might track things &  each IRQ alternate between the set  & Clear OC0B on Compare Match.

With toggling, if you lose software sync, your wave will be upside down (giving very wide pulses instead of very narrow).

 

 

 

 

He's simply saying:

if (PORTB & (1<< PB1)) usart0_putc('+');else usart0_putc('-');

should be:

if (PINB & (1<< PB1)) usart0_putc('+');else usart0_putc('-');

You read PINx not PORTx to detect the external state on pins that the DDR register has set as inputs.

Look at it this way.  When a pin is configured for output, its PORTxn bit sets the level to which that pin is driven by it's output buffer.  An analogy might be the accelerator pedal in a car.  Reading the state of the accelerator pedal (i.e. foot on, or foot off) tells you nothing about the speed of the vehicle.  If you want to know the speed, examine the speedometer.  The fact that most of the time with your foot off the accelerator the speed of the vehicle will be slow, and most of the time with your foot on the accelerator the speed of the vehicle will be fast, is not particularly useful.  If you doubt it, try telling the police officer 'but my foot wasn't even on the accelerator' next time you get pulled over for speeding ;-)

 

Your comments:

// this works on a regular pin
if (PORTB & (1<< PB0)) usart0_putc('/');else usart0_putc('*');
// this is required on a pin connected to the timer.  Strange but true.
if (PINB & (1<< PB1)) usart0_putc('+');else usart0_putc('-');

... especially the 'strange but true', suggest that you did not understand the difference, and that you are stabbing in the dark in your effort to understand.  This is not a disparagement.  It is meant to improve your understanding, and that of any others who come along in the future and read this thread.

Presumably including #21 and #22 ?? 

 

The fact is that the moderation we employ here is fairly lax (a Good Thing(tm) IMHO!), a bit of wander in thread topic doesn't really do anyone any harm (until it strays to religion or politics at which point moderation will happen!)

 

Moderator