Re: BASIC and System ROM replacements

From: Jim Brain <brain_at_jbrain.com>
Date: Tue, 04 Jun 2013 12:23:40 -0500
Message-ID: <51AE229C.9020802@jbrain.com>
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.
>>
>> http://jbrain.com/vicug/gallery/vicmidi/VIC_MIDI_Schematic
>
> 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.
>
> Like this: Hook up PHI2 to Q6 and tie P6 high. Then the _SEL signal 
> will already include PHI2 and you no longer need it at the UART.
>
> Also why do you hook up CR/_W to the 74LS138? You could use that on G2 
> of the 74LS173.
That is an idea.  I will look into that.
>
> 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.
>
>
> 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.
>
> Maybe some of this is of use to you...
No, all great ideas.  I appreciate them.

Jim
>
>  Gerrit
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-- 
Jim Brain
brain@jbrain.com
www.jbrain.com


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Received on 2013-06-04 18:02:21

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