Hello everyone. First of all I might have fried my mbed, it didn't let out any magic smoke or anything. It just suddenly stopped working. The problem happend when I connected it to a breakout board, which has a 5V regulator that is connected to VU (pin 2 on the right side), while the USB cable was plugin in to my laptop. Everything working normal before, when I only had it connected to my laptop with the USB cable, but as soon I plugged in my cable for the regulator the LED's went off, so I quickly pulled out the cable for the regulator. Now I won't start up (LED not turned on and it doesn't gets mounted as a flash drive on my computer).

My question is: can anything happen if you connect 5V to VU while also having a USB cable plugged in, as that seems to be the only reasonable explanation why it might have fried. Is one even aloud to connect 5V directly to VU? The reason I did that, was because I wanted USB host to work, without having to connect the USB cable.

I have also tried connecting 5V to VIN, but it's still doesn't turns on. I think I might fried the FPF2123 (IC3), but I can't figure out why it might be fried, because the datasheets says that it has "reverse current blocking" (http://www.fairchildsemi.com/ds/FP/FPF2123.pdf). But if I measure the output pin (pin 5) the voltage is only about 100 millivolts.

Any helped would be much appreciated, as I would really like it to be up and running again :(

I guess If ICs are getting hotter and your pc doesnt recognize it, your mbed is more fried than KFC. Source: My mbed is dead.

Hermes Trismegisto:

It doesn't get hot at all or starts smelling, so I'm not sure whats wrong. Like your KFC joke btw :D

Christian Lerche:

I have tried boh of them, and nothing is working. I also tried powering it directly from an Arduino's 5V supply directly to VIN, but then the Arduino just restarts. It look like there's some kind of short. I powered it from a 7805 with filtering caps, so it shouldn't be a problem. I have probed it with my multimeter and everything seems okay.

- Lauszus

Here is the schematic:

I see multiple problems in this schematic. Some of those problems could easily kill your mbed (and probably have....)

Power Jack J1: there is no protective diode that would prevent damage when you connect a powersupply that has negative on central pin. In that case you destroy the 7805. The mbed should survive since it has a diode on Vin. There is also no protection against an overvoltage (>14V).

IC1 7805: there is no reverse protection diode between Vout and Vin. That can lead to problems (ie dead 7805) when you have large capacitors on Vout that keep voltage up when you disconnect the external powersupply. Probably not a big problem here since you have only 100n on Vout.

IC1 7805: the 5V output is connected to mbed VU. The VU pin is a 5V output derived from the USB socket on mbed. It is not a good idea to force external 5V voltage from IC1 on that VU. It is also not allowed to force the 5V VU from mbed onto the Vout of the 7805 even when that is not active and you have no powersupply on J1. That will destroy the 7805. You have to cut that connection.

X2 USB Host: Vcc is connected to IC1 Vcc through jumper JP2. The IC1 7805 will receive only 5V on its input when you have jumper JP2 inserted and you connect mbed as USB device through X2. The 7805 will not work in this case. It needs about 3V difference between Vin and Vout. Note that you will destroy the USB host Vcc when you connect any external Powersupply on J1 and forget to remove the jumper JP2.

X1 and X2 have D+ and D- connected in parallel. That could destroy USB device, mbed and/or host should you ever connect both X1 and X2. Better to provide jumpers on D-, D+ that activate only one of them.

JP1 disconnects R1 and R2 from ground at the same time. The Rs are needed when using mbed as USB host. However, when the single jumper JP1 is removed in USB device mode it means that you then have D- and D+ connected through R1 and R2 in series. That is not correct. Use two separate jumpers or even better, use jumpers to fully disconnect either X1 or X2 as mentioned above.

Wim

Dear Wim.

Thank you for your time and valuable response. I will just go thru your list and make a response for each.

Power Jack J1: Thank you for noticing. Yes, there should definitely be a protection diode, though we aren't going to make overvoltage protection as the user would have to take care of this by himself - this board is only a periphiral board, so the user should know the mbed limitations!

IC1 7805: Sure, we should have this too!

IC1 7805: This is one of our main concerns, as the problem lies in the Vu on the mbed. The Vu only supplies 5V when the mbed is powered by its' on board USB port. But as we need 5V for USB Host applications, we need to be able to provide these 5V whether the mbed is powered thru the on board USB or by an external power supply. So what would you recommend us to do? We would like to avoid too many jumpers!

X2 USB Host: I can't see why this should have any effect on the USB Host supply. Or yes, I can see that the 7805 won't be able to provide 5V, which is used for Host, when Client mode is used and power is derived from there. But yet again, the mbed can't support both USB Host and Client at the same time, so this doesn't matter! The user should just be aware of that he can't use both!

X1 and X2: Well I see the problem. Yet again we have to tell the user that Host and Client can't be used at the same time - but to avoid damaging your computer or mbed we might make the jumper solution.

JP1: The jumper solution also fix's this problem as you describe - though we haven't had any problems with these resistors being connected between D+ and D- (when JP1 is removed). Both USB Host and Client have been working.

Yet again thank you for your valuable response. Hopefully you also have some wise words on the Vu 5V problem as elaborated.

Best Regards

Thomas Jespersen

CEO, TKJ Electronics

Hi Thomas,

Concerning the problem of the powersupply when mbed serves as USB Host:

Assuming that you dont want any more jumpers or additional active hardware, I would recommend that you use 2 Schottky diodes to feed the USB device either from VU or from the 7805. The diodes will also protect both the 7805 and mbed VU from destroying eachother. There is a small disadvantage to the simple diode solution: your USB voltage will be slightly less than 5V. However, the forward voltage drop of Schottky diodes is rather low (< 0V3), that should not be a problem to most USB devices. You can reduce the problem for the 7805 by cranking up the output voltage to 5V3. That may be done my adding a Schottky diode between pin 2 of the 7805 and GND. I have used 1N5818 Schottky diodes in the past. See diagram below.

X2 USB Client: when you connect mbed as client it will get power from its USB host through X2. That 5V input is currently connected to Vcc and the 7805 will not work correctly because its input voltage is too low. In addition, if you accidentally connect your external powersupply (which must be at least 8V) without removing the jumper JP2 then this will result in overloading the 5V input from the USB Host that feeds X2, which will probably damage that host. You might want to use another Schottky diode to prevent that from happening: feed the 5V from the USB Client port X2 to Vin via a diode and do the same for the 5V from the 7805.

Wim

Dear Wim.

Regarding the schottky solution I am unsure if this will work.

As the schottky diodes has a forward voltage of at least 0.45V (not 0.3V) this will make the USB Host 5V voltage drop to about 4.5V. I am unsure if this is too low for all USB devices to work properly!

About your X2 USB Client solution I think this might be the way to go, as the Vin can be as low as 4.5V :-)

Thank you once again.

Best Regards

Thomas Jespersn

CEO, TKJ Electronics

Hi Thomas, the diode voltage drop depends a bit on the current you draw. There is an alternative when you are concerned the drop may be too much for an attached USB device. The solution would be a DC/DC converter that accepts inputs between say 3V5 and 9V and outputs 5V.

• Connect J1 via a diode to the DC/DC input
• Connect X2 USB client 5V input via a diode to the DC/DC input
• Connect mbed VU via a diode to the DC/DC input

This way you have 3 possible sources of powersupply, all protected from eachother. There is no need for the 7805. External DC input is also protected against reverse polarity.

The DC/DC 5V output is connected to mbed Vin and to X1 USB host 5V output. The internal diodes in mbed make sure that everything is fine when you power mbed through its onboard mini USB.

Wim

Dear Wim.

I think we have found a solution.

By using two Schottky diodes together with a jumper for the power supply selection I think we managed to overcome the problems. We will also make use of your other points/comments so thank you very much..

I will upload a new and updated schematic soon.

Best Regards

Thomas Jespersen

If schottky voltage drop is too great its possible to make an active switch using FETs and a control IC. Anyway bear in mind that USB power is limited and the voltage may sag below 5v.