AVR-Chat

Several of the data sheets have comments about changing
port direction. If you don't do it "right", you can get an
unwanted single-instruction transient on the port pin.

At the moment, I don't recall the details but it might be
worth checking one of the newer data sheets (such as
Mega48).

Jim


On Tue, 23 Jan 2007 15:15:26 -0600
 David Kelly <dkelly@hiwaay.net> wrote:
> On Tue, Jan 23, 2007 at 09:02:43PM -0000, billsputters
> wrote:
> > What is the considered wisdom regarding changing a port
> from output to
> > input? I am trying to interface to a LCD display and
> attempting to
> > read the busy flag which is available on the data lines
> of the device. 
> > I've looked around and some code just changes the DDR
> as required and 
> > some set the outputs of the port to all 0's before this
> so as not to 
> > get a flase reading. Is this a belts and braces
> approach or a 
> > requirement. BTW, I've tried both versions of changing
> the port over 
> > and the LCD does not function how I want, but it's a
> case of 
> > eliminating errors. Thanks in advance.
> 
> Just clobber the DDR bits to toggle between input and
> output. I get CPUs
> confused sometimes so check the documentation for
> "internal pull ups" on
> the ports. No need to change the output value to
> something else unless
> the output value in input mode enables pullups that you
> need. You
> shouldn't need internal pullups, "But what do I know?
> (about your
> hardware)".
> 
> And that is likely why you are having problems: when you
> change the
> output values you tell the LCD to do something unexpected
> and its off
> doing that rather than the next thing you tell it to do.
> Make sure your
> R/-W, E, and -CS lines are stable and reasonable for
> changing data line
> values when you change the DDR.
> 
> -- 
> David Kelly N4HHE, dkelly@HiWAAY.net
>
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