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RG1 1.8v regulator

Former Member

Ok, so in a different thread I threatened to remove RG1 and do some current measurements on it's output after seeing those thermal images that show it's not generating any heat...

 

Well, I did it tonight. Some photos here: https://picasaweb.google.com/selsinork/RPi18v

 

The jumper pins in the output let me either just put a jumper on and verify the Pi boots ok, or wire a multimeter in series to get some current readings.

 

The results were interesting to say the least. I had to go back and check I was reading the multimeter correctly, that it wasn't broken etc.

 

On initial power up I see a negative current for a second or so which then reverses to about 0.5mA (yes half a milliamp, that's not a typo) for a few seconds while we get the first sd-card accesses. Once we're booted and sitting at the login prompt the current reading fluctuates from around 0.001mA to maybe 0.04mA. 

 

I'm using the 40mA range on a decent Fluke multimeter, so I've no reason to doubt the results. There's obviously going to be some inaccuracy down at that level due to length of meter leads etc, but the result is fairly clear.  You'll understand why I was checking the meter was working and I was reading it correctly though image

 

 

So from there onto the next test, lets try completely disconnecting RG1 and see if the Pi boots while using the LAN9512 1.8v 'output'.  Yes it does! 

 

I think that's reasonably good indication that jamodio got it spot on, the lan9512 shouldn't be connected to the 1.8v plane and it's heat problems are going to be largely due to supplying current on it's 1.8v filter pin that it was never designed to do.

 

So anyone willing to pull RG1 off a Pi and verify my results ?

Parents

  • As shown in Troy Mackay's post on Jul 28 it seems to me that we could mod our PI's to work more as the chips were designed.

     

    I think that this could make the PI more stable, from the looks of it.

     

    I am not a designer Just an old tech, but I think we need to find a way to FIX what we now know is an error in the board.

     

    So could someone do some practical testing to see if there is something that can be done to easly fix the current board, rather than wait for RPI foundation to fix it by waiting for a board redesign?

Reply

  • As shown in Troy Mackay's post on Jul 28 it seems to me that we could mod our PI's to work more as the chips were designed.

     

    I think that this could make the PI more stable, from the looks of it.

     

    I am not a designer Just an old tech, but I think we need to find a way to FIX what we now know is an error in the board.

     

    So could someone do some practical testing to see if there is something that can be done to easly fix the current board, rather than wait for RPI foundation to fix it by waiting for a board redesign?

Children
  • in reply to electron2

    i will love to try out the hack Troy mackay has done also and then test again with that fix on the board, but i have looked into this and i most say it is very well done by Troy as i think it is to small for me todo and i done have an microscope as need for this.

     

    so yes if some one can findout where to make an cut to split the LAN9512 1v8 from the lod 1v8 then i will try this also.

     

     

    Tooms

  • in reply to electron2

    So could someone do some practical testing to see if there is something that can be done to easly fix the current board, rather than wait for RPI foundation to fix it by waiting for a board redesign?

    I think we'd all like an easy fix, but with the information we have available today what Troy did is likely the best we can do. Cutting three tracks and some fiddly at best soldering on of some wire links isn't for everyone though.

     

    Beyond that we need to wait to see what Pete comes up with.  If the full gerbers were available someone would have a look and see if there was an easier way to seperate the lan9512's 1.8v from the rest of the board, but even then from what the schematics suggest there's probably a 1.8v plane on layer 5 which could make it essentially impossible...  If we're very lucky there might be a single via in a favourable place that could be carefully drilled out, but that's just about as bad as Troys wire links.

     

    Tooms idea of a marginally higher 1.8v feed from an external switching regulator may be the simplest after-market mod to accomplish even if it's not ideal. There seems to be a fair bit of interest all round in replacing the linear regulators, several designs have surfaced already.

  • in reply to Former Member

    It would help to have a nice set of Gerber files with the current pcb layout to take a look a the traces and vias to chose what is the best course of action

    before cutting stuff.

     

    One option could be to just lift the two VDD18CORE pins, put togeher a small board that can be placed on top where the logo is with the 3.3 and 1.8

    swtiched regulators, there are some very efficient and small ones like the LMZ10501 that can handle up to 1A and includes the FET and inductor

    on the chip, add the decoupling caps for VDD18CORE on this board and run a wire to those pins. With some work you could manage to have

    through hole pads to match the pads for the existing regulators.

     

    I think there are two or three tiny ceramic caps that will get hidden under the add-on board. It may work ...

     

    My .02

    -J

  • in reply to Former Member

    I'm certainly no engineer, so take what I have to say next with a grain of salt. It might be that you could get away with just cutting the 3 traces. The VDD18CORE pins of IC3 are linked internally and I have gotten away with running regulators without the decoupling/filter capacitors in the past. This would certainly be running out of spec, whether or not it is worse than forcing in current from an external higher potential source I don't know. I'll do some more testing when my IR Thermometer arrives.

  • in reply to Former Member

    Even if you are able to place the decoupling caps not close as recommended to the pins, something is better than nothing.

     

    The internal regulator has no internal capacitance at all, running LDOs without or with the wrong caps may produce oscillation and not a reliable

    power source for the rest of the device.

     

    Here is an article that talks a little bit about the subject.

     

    PS. While datasheets are not always 100% accurate, complete or comprehensive, if a part says it needs a capacitor you should put the capacitor.

     

    -J

  • in reply to jamodio

    maybe you could replace RG1 with a red LED.    A 1.4v

    forward voltage drop, leaving 1.9v, might be perfect to

    overpower the LAN chip, but still within 10% tolerance

    of 1.8v.

     

    Or maybe element14 will accept a RMA for a hot LAN chip

    and replace it with a board that runs cooler.

     

    p.s.

      I see this issue has made it to the other forum.

     

    http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=12387&start=23

     

    edited to add:

     

    jamodio, I see your 25 July 2012 post in the other forum in this thread:

    http://www.raspberrypi.org/phpBB3/viewtopic.php?t=6050&p=144439

  • in reply to Former Member

    I reported the problem while ago on the "other forum" but since I'm a concern troll probably didn't count ....

     

    -J

  • in reply to Former Member

    Tooms wrote:

     

    i will love to try out the hack Troy mackay has done also and then test again with that fix on the board, but i have looked into this and i most say it is very well done by Troy as i think it is to small for me todo and i done have an microscope as need for this.

     

    so yes if some one can findout where to make an cut to split the LAN9512 1v8 from the lod 1v8 then i will try this also.

     

     

    Tooms

    The microsope is really handy (and didn't cost me a cent), you'd be surprised what you can do when you can see what it is you are doing. I haven't even done a lot of SMD work. I'd love one of those IR cameras though... They'd be a bit harder to find in someones junk bin.

  • in reply to Former Member

    I really don't know how much effort to put on modifying the existing boards, what should happen is the RPF people acknowledge the problem, come up with a workaround, fix it for future boards and analyze the long term effects of THEIR ERROR.

     

    Don't think anybody will be open to a massive recall, but the right thing to do is TO FIX THE ERROR.

     

    My .02

    -J

  • in reply to jamodio

    For $38 I don't expect a great deal of support, but I do expect that for a first revision board shipped naked without accessories or a case in limited numbers that it was never intended for the classroom in this form. More likely their target was developers and hackers to help mature the product, find bugs and develop software. I would hope that there will be future revisions, debugged, upgraded, shipped with a case and basic accessories, but I'm happy to tinker about with what I have; that is, after all, what I bought it for.