[volt-nuts] Prema 6048 Cal switch confusion

Randy Evans randyevans2688 at gmail.com
Fri Nov 3 13:16:44 EDT 2017


Dave,

Note the following from Maxim AN 202:

I Replaced My Standard SRAM with an NV SRAM and Now My
System Doesn't Work at All. What Caused This?

In general, this is caused by one of two things:
First, the designer may not have considered the recovery time, or tREC, of
the particular NV SRAM
selected. On power-up, an internal power monitor disables the NV SRAM until
a power-good situation
and then holds it disabled for an additional 2ms (max) or 125ms (max),
depending on the NV SRAM
Page 5 of 9

after power-good. If the microcontroller attempts to access the memory
before tREC times out, it will not
be able to access the device's memory to read or write, so the system
fails. Either a software loop on
power-up to extend the access time past tREC, or moving the NV SRAM access
somewhere later in the
power-on initialization sequence in the microcontroller's firmware will
resolve the problem. This problem
often can be corrected by selecting a CPU supervisor that has a reset time
longer than the recovery time
of the NV SRAM.

Second, selecting the voltage levels at which the NV SRAM and the
microcontroller become active is
critical. If the microcontroller becomes active below 4.5V, and the NV SRAM
becomes active above
4.75V, the same problem of the microcontroller trying to access a disabled
NV SRAM occurs. The
power-good threshold for the two devices should force the system to enable
the NV SRAM first and then
the processor. This involves selecting the NV SRAM with the appropriate
power-good level and pairing
that with a CPU supervisor that enables the processor at a higher voltage.

Some NV SRAMs have an active-low RESET output that is synchronous with its
own internal reset. If
this is used to reset the microcontroller, the possibility of trying to
access a disabled NV SRAM is
removed.

and:

Are Any NV SRAMs Not Recommended for Future Designs?

Yes. The DS1220Y and DS1225Y are not recommended for new designs. These
older devices used a
battery reference to determine the power-valid trip-point during power
cycles. Newer designs use a band
gap reference. The battery-referenced devices had a trip point that
decreased during the life of the
device. Devices using the band gap have a trip point that is stable for the
life of the product.

The DS1220AD and DS1225AD are recommended for new designs needing the
functionality of a 16kb
or 64kb NV SRAM. For existing designs, the DS1220AD or DS1225AD may be
considered as
replacements; however, the "Y" parts had a reset timeout on the order of
milliseconds while the "AD"
parts have a timeout of 125ms. When replacing the Y part with the AD part,
it must be determined that
the controlling processor does not become active during a power-up cycle
for at least 125ms to ensure
that the NV SRAM is available before the processor attempts a memory access.

Randy Evans

On Fri, Nov 3, 2017 at 2:54 AM, David C. Partridge <
david.partridge at perdrix.co.uk> wrote:

> This gets odder and odder.   I put the new DS1220AD into the burner and
> read it.   Pretty random rubbish.
>
> So I cleared it to all FF and inserted into the meter set to Cal mode and
> powered on - it apparently copied the default Cal constants to the NVRAM.
>
> However when I took it out again and looked at the content it didn't look
> anything like what I'd read from the original NVRAM, and was mostly still
> x'FF' but with blocks of ten bytes of zeros at x'100' boundaries, followed
> by zeroes starting at x'6C7' to x'7ef' and what looks like a checksum at
> x'7f0'.   I can't believe that these are cal constants specific to this
> meter!  Though they might be a set of default constants for any 6048!
>
> Filling it with all 00 resulted in an Error 8 (Checksum failure).
>
> I then programmed it with the data I had read from the DS1220Y (and had
> verified).   That also gave an Error 8.
>
> The data I read is attached in Intel Hex format.
>
> I'm wonder whether the meter is failing to read the original NVRAM in a
> way that somehow passes checksumming????
>
> Dave
> -----Original Message-----
> From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Tom
> Miller
> Sent: 02 November 2017 19:05
> To: Discussion of precise voltage measurement
> Subject: Re: [volt-nuts] Prema 6048 Cal switch confusion
>
> How about copying the contents of the old nvram to the new one?
>
>
> ----- Original Message -----
> From: "David C. Partridge" <david.partridge at perdrix.co.uk>
> To: "'Discussion of precise voltage measurement'" <volt-nuts at febo.com>
> Sent: Thursday, November 02, 2017 2:58 PM
> Subject: Re: [volt-nuts] Prema 6048 Cal switch confusion
>
>
> > Update - when I tried this I was using a brand new DS1220AD-100IND
> (blank)
> > to replace the DS1220Y.
> >
> > Reinstalling the DS1220Y and it worked as expected!  Processor must
> detect
> > cal mode by writing to NVRAM - if succeeds, then in Cal mode??? Maybe
> >
> > Is there some obscure incompatability between the DS1220AD and the
> > DS1220Y???
> >
> > Dave
> >
> > -----Original Message-----
> > From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of David
> C.
> > Partridge
> > Sent: 02 November 2017 18:23
> > To: 'Discussion of precise voltage measurement'
> > Subject: [volt-nuts] Prema 6048 Cal switch confusion
> >
> > I've got a Perma 6048 that I've trying to put into Cal mode.  It just
> > won't!
> >
> > The switch on the rear panel works OK, AFAICT when in the Cal position it
> > connects ~WE on the DS1220 (pin 21) and ~W on the $864 SRAM (pin 27) to a
> > 4K7 pullup to +5V and to the output of an LS00 which should be strobing
> > the
> > Write lines low as needed.   I can't see any way in which the processor
> > can
> > be informed of the switch setting!  In measure mode it just connects ~WE
> > on the DS1220 to a 10k pullup to +5V.
> >
> > The schematic is on page 120 (Figure 11.4) of the user manual and also
> > page
> > 9-7 of the service manual.
> >
> > Clearly I'm missing something - as a friend reports that his does go into
> > cal mode.
> >
> > Confused ...
> > Thanks
> > Dave
> >
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