PCB #2 had the capacitors installed originally when I tested it. I took them off to see if they were causing the short, and I just haven’t put them back on.
I have the same uC in a DIP package on a breadboard. I have it connected the same way, with all the same valued capacitors. I do not pull the unused pins high or low, and I have never had a problem with that circuit. The datasheet does not differentiate between the DIP and SSOP packages in any way, so I am utterly confused about why I’m having so much trouble with this.
All I can think of is the “wires” on the breadboard are 1/2 an inch long, ie 5 holes including the chip pin, but your PC board has tracks much longer that may well be acting as antennas for signals into the unused inputs.
Before you try another IC on the board, I’d recommend soldering some resistors to earth from the inputs.
Regarding the soldering iron, connect an ohm meter between the tip and the earth pin…should be a short circuit.
I’ve mentioned the iron because of personal experience some years ago. It was a Weller PT that managed to have a broken earth in the handle. It also just so happened to have a faulty heater element that put about 20 VAC onto the barrel and tip. Lots of WTF moments and general bad language could be heard for the several days it took to realise the problemS. Soldering irons should NOT be permitted to have two faults simultaneously.
ps 22K should do unless someone suggests different.
I’ve been away from this site for a few days, sorry.
In the beginning of this thread, everyone was chasing a design error that resulted in reverse polarity to the device. Based on the observations of device #3, I don’t believe that is the case.
See quote below. I think that RRP has the correct diagnosis. The device is correctly powered, but the floating inputs go into a linear region and current consumption begins to climb until thermal runaway occurs.
A way to test this is to use a new device and short ALL default inputs together to Gnd. Then apply power. <This is a generalization. I’ve NOT read the datasheet to see if there are exceptions>
I suspect if you look at the dev board you had been using previously, you will see some termination built in. Can you post a schematic or link to that? it would be helpful.
Counter argument of OP was that it worked in a breadboard.
If open inputs oscillate, then the breadboard capacitance may lower the frequency enough for the chip to not blow itself up.
On the other hand. If this realy is the cause, I would trow all those chips in the garbage bin and use a decent uC, that does not self destruct so easily.
I understand increased current consumption because of floating input, but suicidal IC’s are a bridge too far for me.
My breadboard circuit is just the “recommended minimum connection” on page 20 of the datasheet. I have a few random components connected too. I do not have each IO terminated to ground or anything, and it works fine.
Not much left here. The DIP package may simply be more fault tolerant on startup or you may be damaging the chip when you solder it. The start up oscillation seems reasonable and using pull downs on startup couldn’t hurt as a test. Always a chance of a bad batch of chips. It would be painful but if you have a DIP socket you could try wiring that on and try the DIP package.
Maybe so, but then why wouldn’t Microchip mention this difference between DIP and SSOP in their datasheet?
I was wondering if the chips might be bad, but I had the same issue on 2 dspic33ev256gm002 and 1 dspic33ev256gm102. I wouldn’t think they would manufacture different parts in the same batch. Maybe I’m very unlucky and received 2 separate bad batches of chips…unlikely.
I received my protoboard in the mail today and tested my SSOP uC on a breadboard. I only built the “minimum recommended” circuit in the datasheet, plus the programmer circuit.
When I first applied power, my power supply’s displayed voltage jumped around quite a bit between 3 and 5 volts before settling at 5V. Excessive current was not drawn. I was able to identify the target device and program it successfully.
So I think that means my PCB has an error somewhere. Or maybe the bypass capacitors are bad. I don’t know, I feel as if a dozen different people have reviewed my schematic/layout, and no one has found anything that resembles an error.
I used several dsPic’s a decade ago but all were THT. Never had a problem after discovering it was very picky about capacitance; Especially with respect to the XTAL. Also, unwieldy capacitance at power supply and LCD. Needed to dial it in differently on the PCB versus the Breadboard.
Perhaps of no help but. Screenshot of schem section…
Very interesting. I’m not using an external crystal on mine. I think there’s just something wrong with my PCB. Maybe it’s my clearances between contacts on the SSOP footprint.
I didn’t want to do this but I had no choice. I soldered 30 AWG wire to my 0603 and 1206 capacitors, then I soldered the other end to some male headers and plugged the caps into my uC #4 breadboard. They appear to work just fine (as in they aren’t getting destroyed when they receive power). So that’s another thing to eliminate from the list of potential suspects.
I think you answered in your original post, but I want to make certain: Have you powered up a bare board with no parts mounted?
Use a current limited supply. This can be done with a good bench supply or
a cheap supply and a power resistor,
a cheap supply with an e-load in series to limit the current.
As a practical matter, I find that 1 Amp and 5V are good settings. So if you use a power resistor, use something like 5 ohms and 10 watts rating to limit the current into a short at 1 Amp.
If your resistor heats too much, its value (and the resulting current) can drift as it heats and make this process more difficult to compare readings as you work.
With the board powered up, use a good digital multimeter (set to a sensitive voltage scale) to probe your traces and vias and find the millivolt level voltage drops. The polarity of these voltage drops indicate current flow through pcb resistance and should point to the direction of current flow.
This way of measuring is much better than simply using an ohm meter. That can work in theory but is very difficult to measure a few milliohms.
This method might work with a populated board but I think it is much better on a bare one. I have used this method successfully more than once.
With an LM317 an a 1 Ohm resistor, you can make an 1.25A current source or sink, depending on how you connect it.
If you use a 1.25 Ohm resistor and calibrate it to be very near to 1A, then it is a simply yet useful addition to your toolbox for measuring small resistances. Each mV measured is a mOhm, and because your current sources is a separate circuit it’s also very easy to make 4-terminal measurements.
Calibration can be done with your DMM in current mode and adding blobs of solder on a piece of resistance wire. Just tinker a bit until you have 1.00A.
Related to finding shorts:
Small audio amp (Such as TDA2050) and a function generator to make a relatively high current AC current, which you can pick up with an inductor (or cassette tape head), amplify a bit and either measure or use headphones to follow the (short) circuit current through your PCB.
It also seems to be getting time to not exclude the “weird” stuff, but there is too much to make some list, and it’s all suggestions.
Could it be that your power supply has overshoot during trurn on?
How careful are you generally with ESD precautions?
That is a great idea, but be careful to heat sink the LM317. With a few volts differential and 1 Amp, several watts of heat will cook even a TO-220 version LM317 unless it is on a good heatsink.
My other comment is that for locating voltage drops (not precise resistance measurements) to locate a short, I want to have a stable current but a precise current is less important.
Never mind about my last post with the capacitors, they aren’t working fine. After leaving power applied for a few minutes because I forgot to shut off the supply, the limited current was reached and the voltage dropped low. Also, I experience the same thing when I have the through hole caps plugged in, I just didn’t realize it yesterday.
Now my 4th uC, which is on a protoboard mounted to a breadboard, is also toasted. I saw pins 27/28 glow up and now my programmer can’t identify the target device anymore.
So there’s another clue, I don’t think it’s my PCB’s fault anymore. I only have 2 uCs remaining, maybe 3 if #3 is still good.
