Re: [avr-chat] Getting started

[Rick Altherr]

On Jan 4, 2008, at 3:50 PM, J D Freeman wrote:

> On Fri, Jan 04, 2008 at 03:09:07PM -0800, Rick Altherr wrote:
> > If your switch is betwen PD0 and +5V, why do you need the pullups  
> > enabled?
> > That would make it always be +5V since when the switch is off, it  
> > would be
> > pulled up by the pull-ups.
>
> Reading the datasheet, setting PORTD to zero should disable the  
> pullups.
> To be sure, I tried adding a line to force it:
>
> MCUCR = _BV(PUD);
>
> This makes no difference.
>
> So, either I am not setting the pin's up properly. OR, I am not  
> reading
> from them correctly. Or both.

I was basing my comment on the code comments.  They said the pullups  
were being enabled.  They should not be.  In fact, you should have a  
pull-down externally for robustness otherwise the inputs will float  
and could cause spurious triggering.  You could also use the internal  
pullups and connect the switch between PD0 and GND which would invert  
the logic, but would avoid an extra external component.

> > So, every iteration through the loop, PD0 is checked to see if it is
> > 1.
> > Because of the above, it always is.  Then, it toggles PC4.  Depending
> > on
> > how fast the clock is going, this could easily be toggling at a rate
> > that
> > the naked eye cannot see.  For a 1MHz part, it could take 4 cycles to
> > run
> > an iteration of the loop, so the LED would be toggling at 250KHz, or
> > stated
> > another way, would have a 50% duty cycle at 125KHz.
>
> That makes sense. Sticking a delay in the loop just makes it sit there
> and flash.
>
> J
>
>
>
> _______________________________________________
> AVR-chat mailing list
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/avr-chat

--
Rick Altherr
address@hidden

"He said he hadn't had a byte in three days. I had a short, so I split  
it with him."
 -- Slashdot signature

smime.p7s
Description: S/MIME cryptographic signature