1.8v level convert

[HaQue]

Hi, I want to play around with reading some eMMC chips. First I'm going to try 1-bit mode due to the simplicity. I should be able to connect CLK,CMD, DAT0 & GND to a compatible SD Card Reader, and power the chip with 1.8v.

Which brings me to my question. As I don't have a 1.8v source, would it be fine to voltage divide a 3.3v or 5v ? can anyone see any issues with doing so using one of these circuits? And if they are ok, would one or the other be preferable.. or both work equally ok?

1.8v.jpg (9.92 KiB) Viewed 11604 times

[fzabkar]

Those circuits are simple potential dividers. They work fine as long as you don't use them as power supplies.

Seriously though, when you draw current from the 1.8V terminals, the output voltage drops.

[MarkGreenwood (KE)]

sounds like a job for the good old LDO.. I use these for many of my projects

including reading emmc

http://cpc.farnell.com/burr-brown/reg11 ... dp/SC08278

[HaQue]

Thanks Franc, I might do some tests and see how it goes then at least I can blog it and other poor homebrewers might be able to have some fun.

Thanks Mark, I am going to pickup a few components today, including something like those. I originally wanted a circuit I could do straight away with parts I had at home, rather than wait until I could get something posted, but the I had to forget it and come to Melbourne.. so I am here waiting till things open and will try and pickup something like a variable lab power supply.. Adelaide is useless for buying anything electronic.

[fzabkar]

The best way to see that testing your potential divider circuit would be pointless would be to reduce it to its Thevenin equivalent.

http://en.wikipedia.org/wiki/Thévenin's_theorem

For example, your first circuit is equivalent to ...

Code:

           2.076K
            ___
    +------|___|----o
    |
  + |
   --- 1.854V
    -
    |
    +---------------o

Following up on Mark's idea, I would suggest that the adjustable version of the same LDO may be a better option. I say this because there may be times when you need to vary the voltage slightly above or below 1.8V in order to deal with bit errors. The voltage could then simply be selected via a multi-position make-before-break rotary switch. The total cost, including a jiffy box, shouldn't exceed $10. I could help you with that.

If you prefer to rummage through your parts bin or examine some junk electronics, then you may find a suitable chip. Otherwise, if you have an expendable car charger for an old mobile phone, I may be able to help you adapt it with only two resistors.

[fzabkar]

Brian, if you can tolerate bad regulation, and assuming that your eMMC doesn't draw more than 10mA from the 1.8V supply, and if you are desperate to give it a go, then the following circuit may get you out of trouble.

Code:

            0.5W
           27 ohms
            ___
      +----|___|---+-------o   1.786V
      |            |
     +|           .-.
  5V ---          | | 15 ohms
      -           | |  0.5W
120mA |           '-'
      |            |
      +------------+-------o   0V

Here is its Thevenin equivalent:

Code:

           9.64 ohms
            ___
    +------|___|----o 1.8V @ no load
    |                 1.7V @ 10mA
  + |
   --- 1.786V
    -
    |
    +---------------o

[jeremyb]

Why are you doing it this way? Much better ways, seems like your asking for trouble dividing the power to signals via resistors. Idk though, I never did it that way.

[jeremyb]

This is what I do..
Get a 1.8v LDO and a MAX3013 put them together and be amazed by that sexy beast.. Oh yeah..

1.8v is handled by the LDO and the signals are automatically converted to 1.8 using the LDO as the reference voltage..

The MAX3013 is for 3.6v and under, they have slower models for 5v.

Cool solution aye?

If you don't want to use a LDO use a variable powersuppy the MAX3013 will convert signals down to 1.2v

BTW: In my tests with NAND going below 1.8v made results worse than reading at 3.3v.. :-\

Enjoy

[HaQue]

Thanks Jeremy, yes it wasn't my preferred way.. I have a tablet to recover, and of course doing it around midnight, AND wanting a soulution straight away.. IE not waiting for things to be shipped. The timeframe has blown out anyway and I just ended up ordering a variable 0-30vdc lab desktop power supply.

BTW this is only for eMMC and is 1.8v supply anyway.

Just looked at the MAX3013 datasheet.. yes that's a nice chip, definitely need a few of those in kit, thanks

I probably could have powered the chip with the actual tablet.. as it powers on, but is bricked. Still not even sure reading thru eMMC is going to work, I have seen people in forums say that the eMMC chip is the cause of the brick.. but don't know how they verified it. Seems plausible though.

[jeremyb]

I'm thinking the signals are also 1.8v and will need to be converted to, not sure what the i/o voltage on your reader is

[HaQue]

Good point, I will check that. Now the MAX is looking like a better choice than the lab power supply!