microcontroller ROM copy trouble ?

One possibility is that the speed grade of the EPROM is slower than
that of the original ROM chip. There should be a couple of digits
after a dash on each chip, like -12 or -70. What are they?

ROM
M27C256B)

than
digits

Thanks for reply Rich,

I have a variety of ROMs i have tested

i have a set of uv eraseable and OTP
both of the 10 12 15  variety  (ie. 100/120/150 ns)

the original equipment is from 1987-89 ...so i do not think it is
terribly fast,
the clock is 12 Mhz. I could not locate an exact datasheet for
the (NEC D23256AC  016  8437k9) but the one 23256ac i did find
was rated at 150 ns
i tried to match or better that number.

faster than spec chip is OK i presume ?

the 8031  processor has a   Time of Address Valid to Instruction
Read Valid of 300 ns (max value)

do any of these numbers sound close (bad)  ?

****And more info****
 ... There is no funny business with 8031 processor because i was
able to sub in a **brand new/ not used ** Atmel 8031 processor in
place of the original 8031 and worked great ???? with the
original ROM if that means anything

Thanks again for your help i am really stuck here and wondering
if there is some security measure installed on the original ROM
but that just seems unlikely ???? unless that funny stuff with
the Address line has something to do with it ???

but my reconing on that is.... if i have the same raw image on
the two ROM chips then the processor sould see no difference in
the data it reads from either chip using the same interface to
those two chips ? yes /no /maybe ?

Thanks again for help Rich,
robb

My guess is that here is a subtle pin difference between the eproms... speed
is definitely NOT an issue.

Active HIGH versus Active LOW on CS and OE pins? Maybe the EPROM programmer
is able to compensate for that and "do the right thing", but your target
circuit can't? Been through that recently with some EPROM/ROMs and it is
a pain.

Get data sheets for both chips and look closely.

It *looks* like the memory chips are faster than the processor, so
that may not be the issue. The original was probably 160 ns.

The note at the back of the TMM23256P data sheet (I'm guessing that
is the one that you also found) does reference a required initialization
sequence. If you can, scope out which of the two sequences are used
and perhaps determine whether that sequence is disturbing the alternate
chip in some way.

find
so
that
initialization
used
alternate

Thanks Rich,
I scoped the working chip in operation and i saw wave forms i was
not expecting ?  the (pin 1) was particulalry different
/peculiar.

i will ooku pthe sequence next and see if i can decipher what i
see.
thanks again ,
robb

robb wrote:



What is pin 1 of the ROM connected to ? It may simply be floating.

Try tying it low.

Graham

find
Instruction
so
that
initialization
used
alternate

uhmm....
any ideas on how to scope an initializing sequence ?
it's to fast or my adjusting or triggering is a problem ???

robb

[condense]


Thinking there might be some MCU 8031 funny business/security i
swapped a ** brand new / not used**  (Atmel 80C31x2-UM) in place
of original 8031 and works like a charm...

    ........with the **Original  ROM ONLY ***


could really use some ideas , i am stumped,
robb

John Fields wrote:



You didn't read what he wrote did you ?

He has an *external* ROM.

Graham

On Fri, 28 Dec 2007 04:14:21 +0000, Eeyore

if memory serves me right the 27... is a progamable chip and you
apply a programming voltage to a certain pin. To read that chip
you must change "that" pin to hi or low.
I got on datasheets here.

Been there (or somewhere similar).  Is Vpp connected? (to Vcc?).
In the case I saw EPROM's programmed & verified (in programmer),
but the output disappeared almost instantly in the real circuit
where Vpp wasn't connected (to Vcc).  (2716's).

ROM
M27C256B)

Vcc?).

programmer),


well oddly on this board the (pin 1) is hardwired to (pin 27)

for the  original chip that is ( NC to Addr 14)  on the **new**
27c256 that would be (Vpp to Addr 14)

so i **tried** insert new 27C256 into socket with (pin 1) not
making contact(bend pin and slide to outside of socket) then
attach micro-jumper hooks from (pin 1) to Vcc

and this did not have any effect ??


so now i am wondeing if i am even reading the original Chip
correctly it is ***NOT*** directly supported /mentioned in my ROM
reader but it has similar equivalents by my judgement  but the
data does not look like it is thr 8051 processor instructions as
i would expect.

thanks for help ,
robb

robb wrote:



You could try running the ROM contents through a disassembler. That should
tell you if it's junk or not.

Graham

[snip...snip...]


*Reading* is (nearly) universal for memory chips with a standard
pinout. *Programming* is often vendor-specific and not well
documented in end-user data sheets.

Have you tried running the object code through a disassembler?
I've used D52, which is available at http://www.8052.com.

"Nearly" tell that to Apple and Sinclair.

Apple II Europlus O/S ROMS wouldn't read on my EPROM programmer without
some fiddling about to come up with CS (Chip select) HIGH and VPP/OE LOW.
(Normal would be BOTH low)

Sinclair QL OS ROMS are odder still: OE required to be HIGH, CS is active
LOW on one chip, active HIGH on another. The CS lines are tied together
in the computer, so it makes sense. However, for an EPROM programmer,
it's backwards.



Also run it through an ASCII dump, look for any copyright strings that
make sense.

That doesn't sound odd as pin 1 would be A15 on the next
size up ROM.



OK, so that's not the problem.



Have you looked at a hex dump of the ROM?    Perhaps
it didn't really enabel the ROM when reading it and you
"read" all FF or something like it from the original ROM and
it programmed your new EPROM with that.

Mask ROMs can have additional chip selects (if pins are
available) and these can be (mask programmed) to select
on high or low.   There may or may not be an OE (possibly
just the chip select(s)).

It's a 28 pin chip and has to have gnd, Vcc, 8 data,
15 address so that leaves 3 pins.  The eprom
has CE, OE, and Vpp; the ROM might have
3 more chip selects or some number of chip selects
plus OE?

It sounds solvable...