Power consumption for microSD cards ?

[Rodo]

Hi all,

I'm thinking of using a microSD card to store data in my next project at
work (Microchip PIC18 or PIC24). I was looking at the specs from a mfr (I
think it was Sandisk) for the regular SD card and it says that standby power
was about 250uA. This is good. But the read and write cycle take a max of
100mA. This is a killer. I'll be running off a 3.6 C cell with about 7Ah
storage. This sounds like a lot but the device has to work for several years
on that battery and the 100mA will just kill it. I realize that it won't
draw 100mA all the time but if I write every minute about 25 bytes to it...
well... it will kill my battery way sooner than sales/marketing like.

Does anyone know the real power consuption for writting and reading a
microSD card ?

Thanks

 

[[email protected]]

Hi all,

I'm thinking of using a microSD card to store data in my next project at
work (Microchip PIC18 or PIC24). I was looking at the specs from a mfr (I
think it was Sandisk) for the regular SD card and it says that standby power
was about 250uA. This is good. But the read and write cycle take a max of
100mA. This is a killer. I'll be running off a 3.6 C cell with about 7Ah
storage. This sounds like a lot but the device has to work for several years
on that battery and the 100mA will just kill it. I realize that it won't
draw 100mA all the time but if I write every minute about 25 bytes to it...
well... it will kill my battery way sooner than sales/marketing like.

Does anyone know the real power consuption for writting and reading a
microSD card ?

Thanks

 

[Bob]

Hi all,

I'm thinking of using a microSD card to store data in my next project at
work (Microchip PIC18 or PIC24). I was looking at the specs from a mfr (I
think it was Sandisk) for the regular SD card and it says that standby power
was about 250uA. This is good. But the read and write cycle take a max of
100mA. This is a killer. I'll be running off a 3.6 C cell with about 7Ah
storage. This sounds like a lot but the device has to work for several years
on that battery and the 100mA will just kill it. I realize that it won't
draw 100mA all the time but if I write every minute about 25 bytes to it...
well... it will kill my battery way sooner than sales/marketing like.

Does anyone know the real power consuption for writting and reading a
microSD card ?

Thanks

How long is a write cycle and how often do you need to repeat it?
Perhaps a large low leakage capacitor would supply enough current.

Guessing some numbers. Lets say 100mA for 0.01s with 0.2V voltage
drop being acceptable.

i=C(dv/dt)
works out to 50,000uF
An electrolytic that big can have 100uA leakage current, though
it sounds like your device will have power all the time which helps.

If the write cycle is less than 0.01s or you can get a way with
a bit more voltage drop you may be able to use a smaller cap.

I used 0.01s as that is standard for little EEPROM's.
Flash may well be much shorter.

If you need multiple writes wihtout time for the battery to charge
the cap you may need more.

I havn't looked up the leakage of current supercaps. Last
time I looked I was amazed to see they available up to thousands
of farads. I'm told the current kilofarad caps don't contain
horribly toxic stuff like the early ones.

Bob

 

[linnix]

Hi all,

I'm thinking of using a microSD card to store data in my next project at
work (Microchip PIC18 or PIC24). I was looking at the specs from a mfr (I
think it was Sandisk) for the regular SD card and it says that standby power
was about 250uA. This is good. But the read and write cycle take a max of
100mA. This is a killer. I'll be running off a 3.6 C cell with about 7Ah
storage. This sounds like a lot but the device has to work for several years
on that battery and the 100mA will just kill it. I realize that it won't
draw 100mA all the time but if I write every minute about 25 bytes to it...
well... it will kill my battery way sooner than sales/marketing like.

Does anyone know the real power consuption for writting and reading a
microSD card ?

SD card has a controller, probably more powerful than a PIC.
So, you have to pay for it in power.
If you don't mind a few more pins (8 to 10),
you can interface to NAND chip directly.
They are 10 to 20 mA on average.

 

[Donald]

Hi all,

I'm thinking of using a microSD card to store data in my next project at
work (Microchip PIC18 or PIC24). I was looking at the specs from a mfr (I
think it was Sandisk) for the regular SD card and it says that standby power
was about 250uA. This is good. But the read and write cycle take a max of
100mA. This is a killer. I'll be running off a 3.6 C cell with about 7Ah
storage. This sounds like a lot but the device has to work for several years
on that battery and the 100mA will just kill it. I realize that it won't
draw 100mA all the time but if I write every minute about 25 bytes to it...
well... it will kill my battery way sooner than sales/marketing like.

Does anyone know the real power consuption for writting and reading a
microSD card ?

Thanks

There are a bunch of question I have for you.

But where to start.

OK

Lets make some assumptions, for info you did not give.

An SD card has 512 byte sectors. So writing 25 bytes at a time would be
writing the same sector 20 times in 20 minutes.

Most larger micros, let alone systems, have at least 512 bytes of
battery backed RAM.

So writting a 512 byte sector every 20 minutes would extend your battery
by 20 times, by your calculations.

You would write 3 times an hour, instead of 60 time an hour.

I hope this help you with your battery analysis.

donald

PS: All this goes out the window if you want FAT16/FAT32 compatability !
But using a battery backed RAM buffer and writing to the SD when
enough data have been collected will extended your battery life by
years.

PPS: Good Luck

 

[Rene Tschaggelar]

Hi all,

I'm thinking of using a microSD card to store data in my next project at
work (Microchip PIC18 or PIC24). I was looking at the specs from a mfr (I
think it was Sandisk) for the regular SD card and it says that standby power
was about 250uA. This is good. But the read and write cycle take a max of
100mA. This is a killer. I'll be running off a 3.6 C cell with about 7Ah
storage. This sounds like a lot but the device has to work for several years
on that battery and the 100mA will just kill it. I realize that it won't
draw 100mA all the time but if I write every minute about 25 bytes to it...
well... it will kill my battery way sooner than sales/marketing like.

Does anyone know the real power consuption for writting and reading a
microSD card ?

A write cycle is a few 10us to a few ms for a sector.
This current is easily supplied by a ceramic 10uF
capacitor close to the pins. You should do some
measurements on how the current averages under the
assumed temporal writing density

Rene

 

[Rodo]

Could you tell me why "All this goes out the window if you want FAT16/FAT32
compatability !
" ? I do need the data to be readable by inserting the card into a pc.

Thanks and thanks to everyone for the replies.

 

[Donald]

Could you tell me why "All this goes out the window if you want FAT16/FAT32
compatability !
" ? I do need the data to be readable by inserting the card into a pc.

Thanks and thanks to everyone for the replies.

Oh boy, now I've done it.

Well, this is something you are going to have to learn on your own.

There are lots of discussions on the web about FAT16/32 directories,
tables, Boot Sectors, and other things related to "DOS" formats.

Google will find tons out there, so spend the weekend studing and reading.

By Monday you will either understand it or you won't.


Hmmm,

In a nut shell, for every 512 byte data block you write , you will also
have to write at least 2 FAT tables, read blocks of the directory
structure from the begining to find an empty location. Read blocks from
the FAT table from the begining till you find an empty location. Worst
case is reading all the directdory/FAT table blocks ( as few as 128
blocks to 1024 blocks),( reading a block off an SD card is almost as
much current as writing a block )

So, please do yourself a favor and truly understand what you are
getting this project into. And you won't get this done in a months time,
without really understanding all the little details of FAT file systems.

Good luck, its not magic, but it is work.

donald

 

[jasen]

Could you tell me why "All this goes out the window if you want FAT16/FAT32
compatability !

because then everytime you extend the file you should update the
directory entry and possibly the FAT too.

" ? I do need the data to be readable by inserting the card into a pc.

It's OK to write the FAT and directory only immediately before the
card is removed, you'll have to provide a way to detect that.

Bye.
Jasen

 

[jasen]

In a nut shell, for every 512 byte data block you write ,

no, for every cluster, which can be upto 128 blocks, it's determined when
the media is formatted.

you will also have to write at least 2 FAT tables,

you can cache these writes and only write each FAT sector when it's full,
also, there's no requirement to have two FATs,

read blocks of the directory structure from the begining to find an empty
location. Read blocks from the FAT table from the begining till you find
an empty location. Worst case is reading all the directdory/FAT table
blocks ( as few as 128 blocks to 1024 blocks),

easiest way is to cheat and reformat each new card so that only a subset of
FAT-FS needs to be implemented.

Bye.
Jasen

 

[Donald]

no, for every cluster, which can be upto 128 blocks, it's determined when
the media is formatted.




you can cache these writes and only write each FAT sector when it's full,
also, there's no requirement to have two FATs,




easiest way is to cheat and reformat each new card so that only a subset of
FAT-FS needs to be implemented.

Bye.
Jasen

Thank you Jason for extending my off-the-cuff description.

There are all kind of tricks to cheat FAT.

My point and yours is, understand the details of FAT before starting.

FAT is not as simple as so would think.

donald

PS: or google for FFS.

 

[Michael A. Terrell]

Google will find tons out there, so spend the weekend studing

Don't you need a license and a health certificate for that job?


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Donald]

Don't you need a license and a health certificate for that job?

Depends what you do on your weekends. ;-)

donald

 

[Michael A. Terrell]

Depends what you do on your weekends. ;-)

donald

"The Old Engineer's Stud Farm", where we work day and night to
produce out next generation of Engineers! ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida