Re: BASIC and System ROM replacements

From: Gerrit Heitsch <>
Date: Tue, 04 Jun 2013 19:57:56 +0200
Message-ID: <>
On 06/04/2013 07:23 PM, Jim Brain wrote:
> On 6/4/2013 11:59 AM, Gerrit Heitsch wrote:
>> On 06/04/2013 04:14 AM, Jim Brain wrote:
>>> 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.
>> You have a few unused inputs on the LS688. Can't you use them to
>> include PHI2 into the _SEL signal already? That would mean you could
>> free up some inputs on the other logic simplify the circuit there.
> My concern there is that it delays PHI2 all over, and it's typically the
> last signal to switch.  I felt putting it through logic would create
> timing hazards.

That shouldn't be much of an issue with the current logic chips and 
become less so if you can reduce the logic further.

>> Hm, if you're willing to use _IO2 completly, you could get rid of the
>> LS688 and use the LS138 to decode the I/O-Space for the UART and
>> Banking register. It has a high active input, meaning you could add
>> PHI2 here as well.
> Well, I rant on cartridge designers who under-decode the IO address
> space, and thus it would be inconsistent for me to create a design that
> breaks my own passionate philosophy.

Commodore did it too... And then think how many people will want more 
than one cartridge in use at the same time. Since you will only use 2 
outputs of the LS138, there's still leftover space for other things.

>> Then you should read the datasheet for the Flash since for most RAMs,
>> _WE overrides _OE, meaning you can tie _CE and _OE together. If that's
>> the same for the flash, you can free up an inverter for use elsewhere.
> That. I did not know.  I will check that tonight.

Checked the datasheet for a µPD43256 from NEC, as soon as _WE goes low, 
_OE becomes 'x' (don't care)


       Message was sent through the cbm-hackers mailing list
Received on 2013-06-04 18:03:39

Archive generated by hypermail 2.2.0.