[Coco] Simple Coco bus buffer for HW folksO

[lciotti1 at gmail.com]

OK so when setting up the fgpa, for pins have to be tri-state... Got it...  
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-----Original Message-----
From: Matthew D Stock <stock at csgeeks.org>
Sender: coco-bounces at maltedmedia.com
Date: Mon, 20 May 2013 19:02:58 
To: <coco at maltedmedia.com>
Reply-To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Subject: Re: [Coco] Simple Coco bus buffer for HW folks

On 5/20/2013 10:22 AM, Louis Ciotti wrote:
> Being green on FPGA stuff please explain?  I am sure it is just the term
> drive them high that has me confused, which happens all to easy when I look
> at FPGAs right now.
>
Louis,

This isn't an FPGA-specific description.  The idea is that since there 
is already a pullup resistor taking that line to a 5v "high" value, that 
means that there is no need, and in fact it can damage the FPGA chip if 
it also attempts to drive the line to a 3.3v "high."  At this point in 
my experimentation, I have left these lines in a high impedance state, 
meaning that they behave as if the lines are disconnected (I'm 
simplifying a bit).  If I want to use the FPGA to drive these lines, 
I'll set the output to 0.  There will never be a need to set them to 1 - 
that's the default.  You can get away with this because of the pullup 
resistor - when the line is pulled low/0, that resistor ensures that you 
aren't causing a high current short, etc.

I'm not an expert, so if anyone has additional pedantic goodness to add, 
I'm happy to learn something new.  :-)
   -Matt


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