Re: BASIC and System ROM replacements

From: Jim Brain <>
Date: Mon, 03 Jun 2013 21:14:28 -0500
Message-ID: <>
On 6/3/2013 8:32 PM, Jim Brain wrote:
> On 5/30/2013 1:55 AM, Gerrit Heitsch wrote:
>> On 05/30/2013 08:22 AM, Jim Brain wrote:
>>> Speaking (writing) of bank registers, anyone know of a TTL option that
>>> has a falling edge trigger?  Yes, I can do an inverter, but would 
>>> rather
>>> not.  Use case is VIC-20, which won't work on rising edge of Phi,
>>> because not all of the data is valid on rising edge.
>> The 74LS373 or 74LS573 does that. But it's a latch, not a register. 
>> They use it in the C64 to de-multiplex the VIC address so it can 
>> access the char ROM. Trigger is the falling edge of _RAS.
>> Now you have to check if a latch works in your case.
> A latch (in general) would work in this case, as the outputs don't 
> need to be valid until PHI2 goes high again, which would be 500nS later.
> Thus, I appreciate the suggestion.  But, as I look at replacing, I 
> realize I forgot the latch needs an enable line as well.  Neither of 
> those offer one.
> I'm trying to create a "register" at Address 38920 (on the VIC) to 
> hold the FLASH ROM bank.  I have 38920 decoded as active low, and then 
> an inverted PHI into the CLK line of a '173 is latching the bank.  
> But, I'd rather use a single IC solution.

My apologies for following up to my own post, but I thought a schematic 
might help.

As I look at it, I can;t help but wonder if some juggling of the address 
lines and such would simplify things and remove the requirement for an 
inverter altogether, but I am not seeing an easy way to do it.

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Received on 2013-06-04 03:01:38

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