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From: Silvar Beitel on 24 Mar 2010 14:50
Custom datalogger circuit using SD card, running on switched
automotive power.

It's cheerfully writing data to the card and the power quits (user
turns key off, accidently pulls off power connector, engine stalls,
car crashes, whatever - this could even be the preferred method for
stopping it if it's robust). Up to three blocks worth of cached data
need to be saved in the card (data block, FAT block, block w/
directory entry) before the system can die cleanly.

Easy solution if there's a connection to unswitched (battery) power
too, but that makes using the device as a consumer plug-in impossible.

So, you need some short-term energy storage. Choices, as I see it:

1) Batteries. You'd need some circuitry to keep them from discharging
after power-down. Expensive unless heavy (some cellphone-like thing
vs AA's, for instance), requires user replacement occasionally.

2) Super-capacitor. Enough energy storage to do the job for a
reasonable volume and cost, but poor ESR - may not be good enough to
hold the supply voltage up above minimum required by SD card.

3) Standard electrolytics. Good ESR, but physically big ones required
to supply enough power over required shutdown (SD card block write x
3) time.

Looking for guidance from anyone who's familiar with flash cards and
powering (and unpowering!) devices that use them in automotive
applications.

--
Silvar Beitel
From: D Yuniskis on 24 Mar 2010 15:16
Hi Silvar,

Silvar Beitel wrote:
> Custom datalogger circuit using SD card, running on switched
> automotive power.
>
> It's cheerfully writing data to the card and the power quits (user
> turns key off, accidently pulls off power connector, engine stalls,
> car crashes, whatever - this could even be the preferred method for
> stopping it if it's robust). Up to three blocks worth of cached data
> need to be saved in the card (data block, FAT block, block w/
> directory entry) before the system can die cleanly.

Presumably, that is because you "just" want the user to
(later) plug the SD card into a "PC" and read an intact
filesystem? (i.e., you aren't planning on having any
"special" application that can read the card *specifically*).
Otherwise, you could just update the data in an "expected"
manner and have that special application find it and
respond accordingly (e.g., even "fixing up" the SD card
to make everything look like a typical filesystem). Cuts
this down to a single block update (?)

> Easy solution if there's a connection to unswitched (battery) power
> too, but that makes using the device as a consumer plug-in impossible.
>
> So, you need some short-term energy storage. Choices, as I see it:
>
> 1) Batteries. You'd need some circuitry to keep them from discharging
> after power-down. Expensive unless heavy (some cellphone-like thing
> vs AA's, for instance), requires user replacement occasionally.
>
> 2) Super-capacitor. Enough energy storage to do the job for a
> reasonable volume and cost, but poor ESR - may not be good enough to
> hold the supply voltage up above minimum required by SD card.
>
> 3) Standard electrolytics. Good ESR, but physically big ones required
> to supply enough power over required shutdown (SD card block write x
> 3) time.

All of the above will *eventually* screw the user (e.g., what if the
flash write fails and has to be retried? What if the user has
forgotten to replace the battery?) When that happens, your device
"looks bad". OTOH:

4) You tell user he has to *do* something before removing the device.
E.g., just like you have to tell your "PC" that you want to unmount the
volume. (i.e., what happens if he removes the SD card from your device
while the device is operating??)

In that case, if the device misbehaves, it is because of an
action on the user's part -- something *he* can control.

> Looking for guidance from anyone who's familiar with flash cards and
> powering (and unpowering!) devices that use them in automotive
> applications.
From: Vladimir Vassilevsky on 24 Mar 2010 15:17


Silvar Beitel wrote:
> Custom datalogger circuit using SD card, running on switched
> automotive power.
>
> It's cheerfully writing data to the card and the power quits (user
> turns key off, accidently pulls off power connector, engine stalls,
> car crashes, whatever - this could even be the preferred method for
> stopping it if it's robust). Up to three blocks worth of cached data
> need to be saved in the card (data block, FAT block, block w/
> directory entry) before the system can die cleanly.
>
> Easy solution if there's a connection to unswitched (battery) power
> too, but that makes using the device as a consumer plug-in impossible.
>
> So, you need some short-term energy storage. Choices, as I see it:
>
> 1) Batteries.
> 2) Super-capacitor.
> 3) Standard electrolytics.

After the power fault is detected, you will need to stay up for at least
100ms (likely 500ms or so); that's substantial amount of energy.

4) Journaling fail-safe filesystem and/or RAID-like arrangement.

5) Keep the most critical data in autostore nvRAM or FeRAM.

6) Run checkdisk at system startup.

7) All of the above.

> Looking for guidance from anyone who's familiar with flash cards and
> powering (and unpowering!) devices that use them in automotive
> applications.

That's no simple problem. I have experience with flash cards and
automotive controllers; If you are interested in my services, the
contact is at the web site.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: Silvar Beitel on 24 Mar 2010 15:46
On Mar 24, 2:16 pm, D Yuniskis <not.going.to...(a)seen.com> wrote:
> Hi Silvar,
>
> Silvar Beitel wrote:
> > Custom datalogger circuit using SD card, running on switched
> > automotive power.
>
> > It's cheerfully writing data to the card and the power quits (user
> > turns key off, accidently pulls off power connector, engine stalls,
> > car crashes, whatever - this could even be the preferred method for
> > stopping it if it's robust). Up to three blocks worth of cached data
> > need to be saved in the card (data block, FAT block, block w/
> > directory entry) before the system can die cleanly.
>
> Presumably, that is because you "just" want the user to
> (later) plug the SD card into a "PC" and read an intact
> filesystem? (i.e., you aren't planning on having any
> "special" application that can read the card *specifically*).
> Otherwise, you could just update the data in an "expected"
> manner and have that special application find it and
> respond accordingly (e.g., even "fixing up" the SD card
> to make everything look like a typical filesystem). Cuts
> this down to a single block update (?)

Excellent observation. "Special" application to read the card would
be OK, narrowing down the write to a single block (that is in fact
what is happening in the prototype - real file system support within
the unit is "on the table".) But even that one-block write consumes
some significant juice.

> > Easy solution if there's a connection to unswitched (battery) power
> > too, but that makes using the device as a consumer plug-in impossible.
>
> > So, you need some short-term energy storage. Choices, as I see it:
>
> > 1) Batteries. You'd need some circuitry to keep them from discharging
> > after power-down. Expensive unless heavy (some cellphone-like thing
> > vs AA's, for instance), requires user replacement occasionally.
>
> > 2) Super-capacitor. Enough energy storage to do the job for a
> > reasonable volume and cost, but poor ESR - may not be good enough to
> > hold the supply voltage up above minimum required by SD card.
>
> > 3) Standard electrolytics. Good ESR, but physically big ones required
> > to supply enough power over required shutdown (SD card block write x
> > 3) time.
>
> All of the above will *eventually* screw the user (e.g., what if the
> flash write fails and has to be retried? What if the user has
> forgotten to replace the battery?) When that happens, your device
> "looks bad".

I only see 1) as having an issue, if 2) or 3) would be designed robust
enough to ensure that the card ends up being consistent upon shutdown.

> OTOH:
>
> 4) You tell user he has to *do* something before removing the device.
> E.g., just like you have to tell your "PC" that you want to unmount the
> volume. (i.e., what happens if he removes the SD card from your device
> while the device is operating??)
>
> In that case, if the device misbehaves, it is because of an
> action on the user's part -- something *he* can control.

Good. If it were that kind of device and there was some form of
communication to the user. In this case, it's a (cheap, invisible)
datalogger, with little (OK, no) user interface. Or at least that's
the way I/they want it to be.

--
Silvar Beitel
From: Silvar Beitel on 24 Mar 2010 15:57
On Mar 24, 2:17 pm, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
> Silvar Beitel wrote:
> > Custom datalogger circuit using SD card, running on switched
> > automotive power.
>
> > It's cheerfully writing data to the card and the power quits (user
> > turns key off, accidently pulls off power connector, engine stalls,
> > car crashes, whatever - this could even be the preferred method for
> > stopping it if it's robust). Up to three blocks worth of cached data
> > need to be saved in the card (data block, FAT block, block w/
> > directory entry) before the system can die cleanly.
>
> > Easy solution if there's a connection to unswitched (battery) power
> > too, but that makes using the device as a consumer plug-in impossible.
>
> > So, you need some short-term energy storage. Choices, as I see it:
>
> > 1) Batteries.
> > 2) Super-capacitor.
> > 3) Standard electrolytics.
>
> After the power fault is detected, you will need to stay up for at least
> 100ms (likely 500ms or so); that's substantial amount of energy.

I've managed to figure that out myself, thus the question about *how*
to do it :-) I do appreciate your response, Vladimir.

> 4) Journaling fail-safe filesystem and/or RAID-like arrangement.
>
> 5) Keep the most critical data in autostore nvRAM or FeRAM.
>
> 6) Run checkdisk at system startup.
>
> 7) All of the above.

I'm hoping that these things are unnecessary in a typical micro, flash-
storage, simple I/O type of datalogging gadget/accessory, but I will
consider including them as needed - and as costs allow.

> That's no simple problem. I have experience with flash cards and
> automotive controllers; If you are interested in my services, the
> contact is at the web site.

Thanks. How does a $50K retainer up front sound to you?

> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
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