> 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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