Re: [volt-nuts] Fall of SRAM voltage in a 3457A without external power

[Andrea Baldoni]

On Mon, Jan 25, 2016 at 11:46:19AM +, Dr. David Kirkby (Kirkby Microwave 
Ltd) wrote:

> V). and well above the 2.0 V needed to hold the SRAM contents. Assuming the
> SRAM takes a constant current one would expect the voltage to fall linearly
> with time. If so, it would take 46 minutes to fall to 2.0 V even without
> battery power.

Hello David.
I think you can probably assume the SRAM idle current be proportional
to e(k*v) (an experimental k for some microcontrollers with SRAM I used
was 0.44), so it should decay even more slowly.

Best regards,
 Andrea Baldoni
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[volt-nuts] Fall of SRAM voltage in a 3457A without external power

[Dr. David Kirkby (Kirkby Microwave Ltd)]

I decided to try a little experiment on my 3457A, When on mains the SRAM
gets 4.8 ~ 5 V. This does not seem to be well regulated, so I assume
depends on mains voltage. Once power is removed, the voltage on the SRAM
stays well above the battery voltage for some considerable time, which I
assume is due to a decoupling capacitor. My 10 M Ohm input Z multimeter is
loading the circuit too much to continuously monitor the voltage, but a few
checks indicated the voltage across the SRAM is falling quite slowly.
Starting at 4.8 V from mains power, after 23 minutes of no mains power, the
voltage on the SRAM was at 3.4V, which is above the battery voltage (3.03
V). and well above the 2.0 V needed to hold the SRAM contents. Assuming the
SRAM takes a constant current one would expect the voltage to fall linearly
with time. If so, it would take 46 minutes to fall to 2.0 V even without
battery power.

ESD and leakage of the human body would probably make screw this up, so I'm
not suggesting replacing the battery that way if you want to preserve the
contents of the SRAM, but there's a fairly good chance the contents would
remain in RAM if one was reasonably quick, especially if you topped the
voltage up from the mains just before removing it from the chassis.

Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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Re: [volt-nuts] Fall of SRAM voltage in a 3457A without external power

[Poul-Henning Kamp]

In message 
, "Dr. David Kirkby (Kirkby Microwave Ltd)" writes:

>ESD and leakage of the human body would probably make screw this up, so I'm
>not suggesting replacing the battery that way if you want to preserve the
>contents of the SRAM, but there's a fairly good chance the contents would
>remain in RAM if one was reasonably quick, especially if you topped the
>voltage up from the mains just before removing it from the chassis.

I usually hook up a separate battery while doing such surgery.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Re: [volt-nuts] Fall of SRAM voltage in a 3457A without external power

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 25 January 2016 at 11:50, Poul-Henning Kamp  wrote:

> 
> In message <
> canx10hcamahhhxqiao9bdzu03c2bbt787mr10rmnywk3oa8...@mail.gmail.com>
> , "Dr. David Kirkby (Kirkby Microwave Ltd)" writes:
>
> >ESD and leakage of the human body would probably make screw this up, so
> I'm
> >not suggesting replacing the battery that way if you want to preserve the
> >contents of the SRAM, but there's a fairly good chance the contents would
> >remain in RAM if one was reasonably quick, especially if you topped the
> >voltage up from the mains just before removing it from the chassis.
>
> I usually hook up a separate battery while doing such surgery.
>


Yes, it is the logical thing to do, but there's a reasonable chance one
could get away with it, but obviously if its important to keep the data,
one would use an external supply. I'm tempted to purposely let the data
become corrupted for the reasons I gave in another thread.

Dave
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