Re: More CPLD explorations

Date: Tue, 25 Feb 2014 20:28:51 +0100
Message-ID: <etPan.530ceef3.79e2a9e3.40ee@szaman.lan>
On 2014-02-25 at 18:51:52, ( wrote:

> > Then the interesting part is how to switch between them without  
> breaking
> > compatibility to existing Software in plain C64 mode. The dilemma  
> the
> > C128 designers already faced and _almost_ solved.
> […]
> However, I would propose using the CHAREN/LORAM/HIRAM inputs  
> to the PLA, but
> using them like an SPI mechanism with a "lock code" that must be  
> sent first. […] The key is to create a lock code that is  
> long enough that it cannot be accidentally triggered.

Exactly - I called it “magic sequence” but it’s the same idea. And if done well, it should be “robust enough” to prevent accidental triggering. “Cannot" is rather incorrect but the chances are negligible.


> * Provide a switch on the unit (or a jumper) to absolutely override 
> any enhanced
> behavior. That way, if someone does manage to find a program that  
> triggers the
> new features accidentally, there is a quick fix to continue operation.

I very much like this approach. The only drawback is that the switching code has to be relatively lengthy and placed properly before running it.


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Received on 2014-02-25 20:01:17

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