Re: LCD controlling with comparators

From: Danny T (danny_at_nospam.oops)
Date: 01/16/05 

Date: Sun, 16 Jan 2005 13:28:09 +0000

Anthony Fremont wrote:

> I'm bad about not finishing my projects all the way. I breadboard them
> up, tinker with them till they work and then get bored and move on to
> something else. I'm going to solder up my clock listening project since
> I actually have some use for that. ;-)

What's that do?

I'm the same with software... I've written about 30,000 "personal"
websites, and none of them ever make it to the web, because by the time
I've finished, I've thought of a better way to do it... It's good for
learning, but gets me a reputation!! I've decided if I can do this clock
properly (replace these 4 pics with cheap shift registers or something)
with alarm, I might start using it - just because I can! :)

>>For parallel data, I needed 7 outputs (7 segments) and 8 inputs (7
>>segments plus clock/latch (what's the right word?))
>
> On your slave PICs you might consider it to be a "chip enable" pin, or
> even a "read".

I meant in a more general sense. From looking at shift registers, they
seemed to have clock and latch - clock seemed to be to set data (for
serial use), and latch was to move the collected input to the output
pins... I guess mine's a combi! ;)

> The reason I ask is that the RA4 pin is an open collector when used as
> output. This means it needs a pullup resistor when used as an output
> pin. Therefore, you'll see allot of it being used as an input pin
> whenever possible. It's also useful for talking to Dallas 1-wire
> devices. ;-)

Hmmm, I wasn't aware of this - and everything seems to work without a
resistor. What are the implications - what difference would I see in
high/low with and without an external resistor?

> You have to give time for the slave PIC to "see" the data. If your
> slave PIC has a worst case timing of say 10 cycles to recognize its
> "enable" pin and retreive the data, then the master PIC has to allow at
> least that much time every time it presents a digit. Then you wont have
> random failures.
>
> I'm not sure what you mean by "new" copy of the data.

As well as holding the "enable" pin for 10 cycles, to be safe (if it
where 10), I'd also have to hold the "enable" pin low while maintaining
the same data to be "safe". Imagine if I hold the enable pin high for
100 cycles, we know my chip has read the data successfully, so I then
drop *all* pins down to 0. Since my "shift register" is running very
quickly, all it takes is for this moment (when all pins go low) to be
between the "is enable pin high" and "move input to output" and 0h would
be copied to my output!

Does that make sense? 

-- 
Danny

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