PCB Layout Errors

The following errors were found today during testing:

1) SEPIC regulator library is incorrect.  I must of been looking at the application circuit which shows the pinout for the 6 pin qfn package.  The circuit doesn’t say that the package being used in that diagram is either package explicitly.  Since it shows pin numbers at all it kind of deceiving since most devices with different packages still have the same pinout.

2) Reset switch is always on.  The way it switches is opposite from what the package shows in the board layout. 

I found the errors above because i’m trying to debug what is causing my circuit not to operate properly.  At this point i’ve removed all of the voltage regulator pins and the reset switch.

Problem: When power is applied from the battery, the battery voltage drops from 3.0VDC to ~0.8VDC
Hypothesis: Something is dropping a lot of current causing the battery voltage to sag. 

Analysis steps:

• Measured Resistance between VCC and GND and it’s 2M Ohms.  Because there is a resistance there when power isn’t applied this rules out the possibility that that VCC and GND are shorted. 
• When i tested running the circuit off of USB, VCC was about 1.5V and the PIC got really hot.  I only had it plugged in for a second.  But, the fact that i may have damaged the PIC is a possiblity.  But, if the PIC was damaged in a way that was causing a short we’d expect there to be a resistance between ground and VCC.  Additionally, when a chip gets damaged due to too much current running through it, as far as i know it usually damages it in such a way that it becomes an open circuit.  
• Next, i verified the theory above that when a device is damaged due to too much current that they become open circuits not closed circuits.  For my test i used a general purpose diode which i hooked up to a power supply with 20V across it with no resistor.  Before i tried to damage it the resistance from anode to cathode was 700 Ohms and from cathode to anode it was a lot (didn’t register on meter).  After letting 3A run through the diode for about 20 minutes.
• I haven’t tried hooking up the USB since i have disconnected the Voltage regulator.  Now that the regulator isn’t hooked up we can test the other possible source of trouble which is the PIC’s VUSB connector.  It appeared that a lot current was still going through the device.  That means with nothing connected to VCC there is still a lot of current getting pumped through it.  So, if there is no connection between the VCC and VinUSB then

1) it must be draining current through the other PIC pins connected to the USB connector  - or -

2) Some connection on the USB cable is hooked up wrong. 

• After reviewing the PIC Datasheet i found the following info:

“The regulator is enabled by default and can be disabled through the VREGEN configuration bit. When enabled, the voltage is visible on pin VUSB.”

Well… if the regulator is enabled by default then how are you supposed to even program the device to begin with if having an external regulator hooked up to VUSB when the internal tranceiver is enabled is a no no.

Another piece of information that i don’t understand which is probably a hint of the big problem:

“VDD must be greater than VUSB at all times, even with the regulator disabled.

• Before i search the web to find out why this is a problem i’m just going to desolder VUSB really quick to see if that’s the problem.
• Removing VUSB did not fix the problem. 
• Completely removing the regulator also didn’t fix the problem.

At this point lets just review what is hooked up.  Battery -> Switch -> VCC,  VCC-> 2M Ohm -> GND  (2M Ohms of resistance between ground and VCC), Vin from he USB port isn’t connected to anything so the only thing connected at all from the USB connector is D+, D- and GND. 

Areas to checkout

• Does connecting USB draw a lot of current even with there being no path from Vin USB -> VCC

YES! This means that there could be some issue with the capacitors, or there is some issue with not having pullups on D- and D+. or of course that VDD and VSS are swtiched…

• PROBLEM FOUND!#$@#$@#$ Apparently the eagle library i used has VDD and VSS switched.  All signs were pointing to this being the problem but, it just seemed very unlikely since i didn’t make the library and they were posted on the internet.