F1/F2 polyfuses gone

For those of you wanting the executive overview:

jbeale wrote:I just received my 2nd R-Pi, this one from stock (yay MCM Electronics) which took just a week to ship from Ohio to California. I was pleased, but surprised to see that the USB fuses F1, F2 are now just 0-ohm jumpers (photo below). I was able to confirm they measure less than 0.1 ohms. Is this the new standard going forward?

by Burngate

You've obviously recieved a Chinese pirate copy - it won't have all the features a proper Pi would have, such as not working with some keyboards, and the like.

by eben » 25 Aug 2012 10:34

Yes. After seeing how many people have decided to modify their boards with zero-ohm links, and having done some safety and reliability testing, we've decided that the best course of action is to remove the USB fuses from the design. Right now they're linked out, but on a subsequent board revision the pads will be removed entirely.

by liz » 25 Aug 2012 10:37

And I'm impressed someone noticed so quickly!

From Me Peg: Unlike on this Forum, I do not read every little thing on the org forum. . . BUT . . . is it just me or does this seem kind of sneaky? Or to put it another way. . . what happened to the "Open" in the open software/hardware world? Or did I miss the instructions on what to do with the two pi's I now own to make them more stable? No suggestions on buying the beagleboard Morgaine!

Thanks for the heads up on the 0-ohm change.  Someone at my hackerspace Pi meetup last night told me he got his Pi from MCM last week, so I asked I could see it... and it was indeed "ohmless":

I'm very interested to see at a future meeting what will happen when we swap his Pi into someone's setup who is having trouble with USB devices that might be power related.  With the USB host port polyfuses gone, I'm wondering if it is still be necessary to recommend using a self-powered hub for a WiFi adapter.  (the GWU625 I've been using lists "Transmit : < 380mA. ; Receive: < 250mA").

I was tempted to replace the fuses by a pair of 0 ohm resistors but I only had 1206 size and didn't like how they landed on the pads,

casually I had some 500mA polyfuses of the same size, so I used those.

After the change I didn't have any problems to have an old Digitus Dongle with the Realtek 8187 chipset working directly from the

one of the R-Pi USB ports, this dongle says on the ConfiguratorDescriptor.MaxPower: 500mA

Ddin't do much to test if there is any packet loss issues with WiFi.

-J

For the polyfuse on the micro usb power input, I see on the eLinux wiki it is 1.1A with a hold current of 700mA.  Does that mean the Pi can only draw 700mA before the polyfuse starts to gain resistance and decrease the 5V input? 

I'm guessing the downside to 0-ohm or bigger 500mA polyfuse is that the USB devices can now draw enough current to make the power input polyfuse react.  I wonder if this would effect the functioning off the rest of the Pi (which I think it using 3.3V and 1.8V rails, right?).  Hookup an external harddisk directly and the Pi reboots?

> Hookup an external harddisk directly and the Pi reboots?

We don't really know what the future plans are for power distribution. Morgaine has advocated that the USB power shouldn't go through F3. Some people have suggested 1 ohm instead of zero, or larger capacitors, in order to prevent voltage sags from hot plugged USB devices. Anyway, this change seems like an improvement, even if it doesn't solve all the power issues.

> Hookup an external harddisk directly and the Pi reboots?

We don't really know what the future plans are for power distribution. Morgaine has advocated that the USB power shouldn't go through F3. Some people have suggested 1 ohm instead of zero, or larger capacitors, in order to prevent voltage sags from hot plugged USB devices. Anyway, this change seems like an improvement, even if it doesn't solve all the power issues.

I can't remember if there is an specific value required/recommended by the USB 2.0 standard, I've been usnig 150µF and the reference design schematics and eval board for the SMSC LAN9512 show that value as well for each port.

The R-Pi has a single 47µF shared by both ports.

PS. I"m still waiting to see what else is on "Pete's list", this "silent" fix by RPF replacing the polyfuses by a 0 ohm resistor is another engineering oddity and it does not sound professional that they decided to do it (and apparently testing it) because we were doing it, they should have a more reasonable explanation.

-J