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