Programming voltage is 12.5V
If you have an old EPROM will be probably un-erasable. Use a powerfull UV lamp
(at least 100W) and prepare for a long erase time (5-10 cycles of 2
minutes at 2cm distance from the lamp)
Programming first with FF is a good choice. This will fresh the cell.
Or better forget it. UV Eprom days has gone.

Vasile

On 7/3/07, James Holland <j_holland@btopenworld.com> wrote:
> > Subject: Re: [EE] Vpp on 27C256
> > To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> > Message-ID: <328792.69830.qm@web83709.mail.sp1.yahoo.com>
> > Content-Type: text/plain; charset=iso-8859-1
> >
> >
> > --- James Holland <j_holland@btopenworld.com> wrote:
> >
> > > I'm reverse engineering a DSM ECU. The programme
> > > memory is a TMS27C256. I
> > > have looked at the datasheet and it specifies that
> > > 13V (ish ) is required
> > > for programming. On the board that I have Vpp is
> > > tied to the 5V (Vcc)
> > > supply. Now I would have expected it to be tied to
> > > GND as there is no
> > > circuitry to switch it higher for ICP. Is there
> > > something clever that I'm
> > > missing? I'm sure that a few of you must have
> > > experience with this IC.
> >
> >
> > Hi James,
> >
> > I'm sure you'll get a few replies on this, but I'll
> > cover a few of the specifics:
> >
> > 1. 27C256 is EPROM, it requires UV to erase. So why
> > would it need to program the chip if it can't erase
> > it?
>
> When I first saw that it was not connected to 0V I assumed that the
> alternative was to a switchable 12V source that would allow ICP. That would
> have allowed the hardware to be programmed before delivery with the latest
> firmware. I rather hoped that it would be like this so I could modify it for
> ICP with a 28HC256. I'm rewiring to use the EEPROM device anyway to make it
> easier to experiment with the code.
>
> >
> > 2. +5 is the standard connection for the Vpp pin
> > during non-programming operation.
> >
>
> You are absolutely right. I have re-read the datasheet, it does state under
> operating modes, Read - Vpp=Vcc. I shouldn't work on stuff late at night, I
> guess that I just assumed that if programming was 13V, not programming would
> be 0V! I won't worry about that then ;-)
>
> > 3. User program or data is stored elsewhere, perhaps
> > in battery-backed static RAM.
> >
> This is a 68HC11 type processor running in expanded mode. The EPROM contains
> the programme code. RAM is internal.
>
>
> > Cheerful regards,
> >
> > Bob
>
> Thanks for your reply.
> Cheers
> James
>
>
>
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