Re: EEPROM checksum error
From: Charles Edmondson (edmondson_at_ieee.org)
Date: 03/09/05
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Date: Wed, 09 Mar 2005 08:33:04 -0800
Dummy wrote:
> Robert Monsen <rcsurname@comcast.net> wrote in message news:<W8WdnfdK4bC_bLDfRVn-1w@comcast.com>...
>
>>Charles Edmondson wrote:
>>
>>>Take a GOOD look at power up and power down sequences. A few years ago,
>>>a vendor of mine was having problems with a similiar situation, where an
>>>EEPROM kept getting programmed to random bits here and there. Seemed
>>>that on start up (this was on a parallel port) there were voltage
>>>glitches that JUST HAPPENED to mimic the programming sequence on the
>>>device, which was not supposed to be field programmable! Since this was
>>>a security dongle, and the bits were sometimes the security ID codes,
>>>this was considered a very bad thing!
>>>
>>
>>Were they actually able to observe this, or was it assumed? Dealing with
>>hardware/software interfaces, it is quite common for programmers to
>>blame software bugs on hardware 'glitches'. I've seen this again and
>>again. It is usually a bug that just seems to come and go, possibly due
>>to some unrelated change in the software that changes the timing or
>>place in memory where a random pointer is hitting. I have made a living
>>out of consulting on these kinds of issues.
>>
>>
>>>So, take a look at what occurs during start up and shut downs, and see
>>>if there are any glitches then that can cause you problems!
>>>
>>
>>Yet another goblin to beware of. Thanks.
>
>
> Introducing random noise at supply line won't be able to cause any
> glitches at EEPROM lines because the noise was riding on the supply.
> However, when glitches are introduced to main supply line by creating
> a temporary dip of voltage at certain period, glitches can passed
> through to EEPROM lines.
>
> Previously, we have been able to see the EEPROM checksum error by
> introducing noise to EEPROM directly, bypassing the regulator. So I
> reckon that if the glitches get through the regulator, most probably
> checksum error will occur. We are checking on that. We thought of
> noise, but missed out the glitches.
>
> If that's the root cause, any method to prevent glitches? I guess
> regulator is only able to filter the noise that rides on the Vcc. Any
> sudden dip in voltage is not recoverable.
For you, this might be a design issue. As power ramps up/ramps down,
different components react differently. Some have internal caps that
make them hold state a little longer than others, or are just more
sensitive to power supply levels. Think about the programming sequence.
What could provide it in your circuit. What could PREVENT it in your
circuit!
I have often found that start up conditions are not fully considered in
design. You just assume that the power comes up all at once, smoothly.
In reality, different voltage rails come up differently. Filter caps
take time to charge up to voltage. Good design takes that into account,
sometimes adding POR circuits to make sure that power is steady before
starting things up, and quick shut down sequences to turn everything off
before the power goes below limits. It's like preventing race
conditions and logic glitches. Sometimes, you just have to take a good
look at the failure modes...
-- Charlie -- Edmondson Engineering Unique Solutions to Unusual Problems
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