Re: [stella] FE, 3F, Bankswitching, and Data lines

Subject: Re: [stella] FE, 3F, Bankswitching, and Data lines
From: mickey_m@xxxxxxxxx
Date: Thu, 19 Dec 2002 17:38:09 +0000
Most bankswitching relies on A12 (cart select) so I have a small feedback 
circuit that uses A12 and a delayed A12.  The delay is through a counter and 
20MHz clock setup outside the CPLD.  What I am trying to do, is setup a gate 
chain delay in the CPLD.  But, I couldnt figure out how to get it to work, 
more on that below.  Once the circuit goes high then the address is valid and 
I check to see if its what I want.  Data works the same.  The only reason I go 
outside the chip is because I need the room on chip, and it will be easier to 
switch to a chain delay once I figure it out.  The only reason I want a delay 
chain is because the original carts did it without clock counting, I should 

For 3F and FE I go off an address line I know is going to be high or low when 
the right address come in.  Like 3F, looks to A6 being off, because I guess 
technically 3F's hotspot is 00 through 3F. 

I am new to CPLDs and dont really know the ins and outs, so for two days I 
tried to setup a chain delay and failed.  I cant get any delay what so ever 
with a chain.  It took me a while to realize that the software was optimizing 
my circuit and removing the chain gates because they have no effect on the 
logic.  So I used the NOREDUCE attribute on the gates and I still get the same 
thing.  Thinking that the chip is a magical no gate switch time chip, I looked 
at the datasheet.  As far as I can tell, gates should have a 3ns switch time.  
But all my post fit simulations show no delay.  I figured that since the 
simulator has picosecond resolution that it would show something, nope.  I 
even setup some circuits that were a cascade of gates from multiple inputs, 
and there was no way to optimize the gates out, but still the same, magically 
all gates switch the same time.  Unfortunately all I have to go on is the 
simulator.  I dont have access to specialty tools to do circuit debugging to 
see if there is any real delay.  I only have a Shack multimeter.  I have asked 
Santa for many years for an osciliscope and a logic analyzer and others, but 
he never brings them (I must be bad), maybe this year.

If anyone can help with this chain thing, please do.  I know this is possible, 
it has to be.  I am to the point of thinking that if I run my car over it a 
few times it just might start working the way I want it to.

And anyone know were I can get the real 2600 6502 datasheet.  The wait times I 
got through trial and error because all the datasheets I found through google 
have different values for 1MHz.  It wasnt real hard though, they seemed to be 
all around the same area.

> Vern:
> What are you using to generate the timing for when the address lines and 
> data lines are stable?
> Chad
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