Re: [stella] 7800 programs -- potential problems?

Subject: Re: [stella] 7800 programs -- potential problems?
From: Kevin Horton <khorton@xxxxxxxxxx>
Date: Wed, 19 Jan 2000 18:29:39 -0500 (EST)
At 10:52 1/19/00 -0500, you wrote:
>
>> Kevin Horton wrote:
>> > I call it the "ROMblaster" because it does just that- blasts the ROM in the
>> > 7800 and temporarally over-powers it enough to get its reset vectors in
>> > and totally bypass the 7800's ROM by overpowering it.
>> 
>> I inquired about doing something similar in 12/98 and the
>> feedback was "don't do it".  "Two devices outputting data
>> on the bus at the same time would cause damage no matter
>> how short of duration."
>> 
>> So how does Kevin's proposal differ?  :-)
>
>Well, he's KEVIN...  :)
>
>I was probably one of the people who said "don't do it".  I stand by that.
>Even for you, Kevin.  :P

I don't say "don't do it". :-) It's perfectly safe since the time duration is
extremely short... the thermal time constant of the chip is extremely high
compared to the duration the output drivers are being overridden.  All memory
chips I know of (RAMs, EPROMs, ROMs) can withstand direct output buffer shorts
according to their datasheets for several seconds or more.  The main problem
you run into is thermal rise of the part due to the current drawn.  And since
the time the ROM is being overridden is so short (on the order of maybe a 
couple hundred uS) thermal rise will be neglegible.

---

Yes, the "ROMblaster" would be placed inside the cart and is designed to 
"unlock" a normal US 7800.  

---

As for replacing the ROM in the 7800, sure it can be done.  The problem is
on some 7800's it is soldered in and hence would be tough to remove for most
people without desoldering equipment.

---

Wow, I didn't know there would be so much intrest in it.  I guess I could
draw up some plans and junk about it over the next couple days.  In fact
I could do some work on it tonight.  Just gotta bust out my 7800 docs again.
(been about a year since I looked into this last)






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