[Diy_efi] Re: Reversing EPROM decryption board

[Don Paauw]

You will need to play with the NVRAM's /CS.  If there is a separate /RD line,
tie that high and try tying /CS low (and pull it high before shutting down to
prevent spuriuous writes).  If that doesn't work, you can try tying /CS and /W
together but it would be best to poke around looking for another signal that
would correspond to the EPROM /CS, maybe one of the PEEL inputs.
It's usually better to have /WR framed by all the other signals.
If the NVRAM has one RD/WR line rather than separate /RD /WR lines,
tie RD/WR low and tie /CS to /OE.
If the NVRAM has any /OE input, tie that high to prevent bus contentions.
These are general rules-of-thumb.  Let me know your NVRAM part number and
an URL for the data sheet, if you have that.

-- Don 



At 12:17 PM 6/14/06 +0100, you wrote:
>Don,
>   
>  just checked: /CS is kept low all the time, /OE is the only one
moving... Whenever the EPROM area is accessed, the /OE goes low... Does it
change something? I mean, as long as NVRAM's /W is connected to the /OE, it
should be fine.. or?
>   
>  Unfortunatelly, I am not able to control the address lines, I was
controlling them only by software...
>   
>  The rest is clear... I will make the board over weekend and let you know
how it went. Hopefully I won't fry the ECU :)).
>   
>  Regards,
>NG
>
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>  Conceptually this is correct but check all of the timing specs. For NVRAM,
>make sure it is fast enough and if there is only one bank of EPROMs, make
>sure the /CS is actually moving. You will be loading the address & data
>busses with more capacitance but only one or two more chips should be fine.
>You'll probably want to gate the RAM /CS for a clean shutdown.
>It sounds like you are already driving the address lines, so if you can
>control
>the control signals as well, then there should be no speed or timing
problems.
>
>Address sequencing may be checked by rules like: address are sequential
unless
>a branch or data access is seen, ISR vectors are constant, etc.
>
>-- Don
>
>
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