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Philips SRAM, not

Started by Tom Walsh June 26, 2006
Has anyone else seen this? I'm pretty much convinced that the reference
to 'SRAM' in the philips LPC2000 family is not 'Static-RAM', but
pseudo-static RAM. I'd tried to keep values in RAM between resets and
the LPC2136 won't keep all of them.

The RAM appears to be pattern sensitive as to what it is able to keep:

fill 0x40000000-0x400001000 with 0xffffffff, 0x00000000, or 0x55aa55aa
and it will keep that value thru reset. But put a pattern such as
0x55aa55aa, 0xaa55aa55, 0x55aa55aa, 0xaa55aa55 into RAM and it won't
keep that pattern intact.

The RESET chip I'm using has approx 1second pulse. I've seen dynamic,
self-refresh RAM behave as this SRAM of philips does: maintain some
values in columns, but other columns won't keep data intact.

I'm thinking that if you need to keep values in RAM across RESET, that
you have to use a RESET pulse close to the minimum width?

Anyone care to comment and share your experience?

Regards,

TomW

--
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
----------------

An Engineer's Guide to the LPC2100 Series

--- In l..., Tom Walsh wrote:
>
> Has anyone else seen this? I'm pretty much convinced that the
reference
> to 'SRAM' in the philips LPC2000 family is not 'Static-RAM', but
> pseudo-static RAM. I'd tried to keep values in RAM between resets
and
> the LPC2136 won't keep all of them.
>
> The RAM appears to be pattern sensitive as to what it is able to
keep:
>
> fill 0x40000000-0x400001000 with 0xffffffff, 0x00000000, or
0x55aa55aa
> and it will keep that value thru reset. But put a pattern such as
> 0x55aa55aa, 0xaa55aa55, 0x55aa55aa, 0xaa55aa55 into RAM and it
won't
> keep that pattern intact.
>
> The RESET chip I'm using has approx 1second pulse. I've seen
dynamic,
> self-refresh RAM behave as this SRAM of philips does: maintain some
> values in columns, but other columns won't keep data intact.
>
> I'm thinking that if you need to keep values in RAM across RESET,
that
> you have to use a RESET pulse close to the minimum width?
>
> Anyone care to comment and share your experience?
>
> Regards,
>
> TomW
>
> --

I think you may be catching the paranoia some people have regarding
Philips. I doubt that when they say the chips have "on-chip static
RAM" they are trying to mislead anyone: I'd assume they do have.

There's a note in the LPC213x User Manual regarding the contents of
the SRAM through resets: maybe this is what you're seeing? If not,
I'd be inclined to put together a simple program that exhibits the
unexpected behaviour and pass it on to Philips for comment.

Brendan.

brendanmurphy37 wrote:

> dynamic,
> > self-refresh RAM behave as this SRAM of philips does: maintain some
> > values in columns, but other columns won't keep data intact.
> >
> > I'm thinking that if you need to keep values in RAM across RESET,
> that
> > you have to use a RESET pulse close to the minimum width?
> >
> > Anyone care to comment and share your experience?
> >
> > Regards,
> >
> > TomW
> >
> > --
>
> I think you may be catching the paranoia some people have regarding
> Philips. I doubt that when they say the chips have "on-chip static
> RAM" they are trying to mislead anyone: I'd assume they do have.
>
> There's a note in the LPC213x User Manual regarding the contents of
> the SRAM through resets: maybe this is what you're seeing? If not,
> I'd be inclined to put together a simple program that exhibits the
> unexpected behaviour and pass it on to Philips for comment.
>
Hi Brendan,

I was contacted by Philips, they assured me that the SRAM is truely
"static RAM".

I'm not sure what it is that happened, ghosts? It appeared, happened
somewhat consistently for an interval of a few minutes, then was gone.
Then a few hours later it briefly appeared again. Checked all my solder
joints, reflowed each pin, looked at the clock levels, looked for supply
noise, inventoried all the pins on the board.

The only thing that I can think of is that one of the power pins is
"trapped" and has several inches on the power plane before it
interconnects with two others of the MCU. But, you know, when you start
looking for power loops with an processor that runs at 20..50ma, that is
reaching for an answer. :-/

Regards,

TomW

--
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
----------------