Re: BASIC and System ROM replacements

From: Jim Brain <>
Date: Tue, 04 Jun 2013 16:01:53 -0500
Message-ID: <>
On 6/4/2013 12:57 PM, Gerrit Heitsch wrote:
> 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.
I'm struggling with that.  ahct138 is 12nS worst case, and hct688 is 
34nS worst case (@25C) That's over 46nS of delay, not including the 
ahct14 delay of 8ns for ahct14.  54ns is quite a bit.  Doesn't the VIC-I 
use the other half of the cycle?

>>> 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)
39sf010 datasheet specified !OE must be Vih (High) during write cycle.


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Received on 2013-06-04 22:00:03

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