I’ve soldered a LPC1756 on a schmartboard, tested every pin, added oscillators, capacitors… As I found nothing about capacitors requirements in the NXP documentation, I had a look at what was done on the Olimex 1766 board:http://www.olimex.com/dev/pdf/ARM/LPC/L … TK-SCH.pdf. (& I translated the pin number from LQFP100 to LQFP80).
I was trying to bring some power to my blank/unprogrammed LPC1756, but it seems there’s something wrong with it : if I plug the #77 pin (VDD) to 3.3V, after a very short delay (250ms?), my regulator output voltage falls from 3.3V to about 1.5V, and the regulator is getting very hot. :shock: It’s not a short-circuit, but it’s not far from that !
Leaving the #77 pin floating, I’ve tested the RSTOUTN pin at 1.5V… bringing the RESET pin at 0V or 3.3V doesn’t change anything.
I’m stuck here… I really don’t know what could be happening…
Did some of you encounter such problem ? Have any idea about what coud be wrong ?
a) Without it - temporarily break all 3V3 connections between your regulator and the MCU. (be sure to break “all” of them - even pull up connections)
b) That done - confirm that - by itself the regulator works correctly. Add a 100 ohm R from reg. out to ground - again confirm that the regulator remains @ 3V3 with this ~33mA load. If this checks out - proceed to c.
c) Using a high-impedance meter - measure from your VDD junction (as it is distributed to the MCU) to ground. (haven’t worked w/your MCU but expect that resistance between VDD junction & ground is greater than 1K ohm. (and likely much higher) If your measurement shows lower you likely have a mis-connect - or may have misunderstood the MCUs order of pinout numbering.
If you can find/correct VDD wiring error remember to reconnect 3V3 reg. out to VDD junction of the MCU.
Are you sure that you understand MCU numbering sequence (as you’ve manually installed it on breadboard?) (mean no offense - have seen firms reverse or offset the pinout)
Please be sure that “every” component which ties to VDD is included in your resistance measurement.
Replace the 100 ohm test R between VReg and ground with ~33 ohm. If the VReg holds up under this load (100mA) it is likely ok. (unless oscillating)
Most MCU like this have multiple VDD & VSS pins. You usually have to wire ALL of these - appropriately. Often it is required (at least desirable) to employ small bypass caps close to each power pin pair.
*** It is NOT appropriate to “just” tie pin 77 to VDD - your initial writing suggests this…
I’ve checked all your pins VDD & VSS - I agree with your choice of pinouts.
Am concerned that you’re able to “isolate” pin 77. Normally all such pins are treated as “group” - perhaps in your prototyping you had this pin tied to ground - or floating. It is not correct procedure to “pull” individual power pins - strongly suspect that your MCU is fini.
You also have to observe ESD handling procedures. (drew 1/2" spark to wall switch this morning - high ESD in US midwest today)
This is the weakness in “hand-prototype” assembly. All of your work is at risk - and will have to be undone to repair. I can’t see anything wrong in your schematic…
For the ESD, before doing anything, I’m touching the ground part of a USB cable plugged into my computer… I saw that somewhere, that’s a simple and effective way !
Before you pull the MCU - remeasure the resistance between pin 77 and ground. Have you a hairline solder bridge or similar?
How did you ever come to establish that pin 77 caused current increase? Did you systematically “lift” each power pin? And did you start this process after you noted the VReg output drop?
Quite frankly I’ve never lost an MCU and I’ve been really hard on them. I really suspect there’s just something wrong, like your pin numbering is incorrect. Have you checked with a multimeter to see if you are shorting neighboring pins? Also check to make sure the output pads actually match the leads that you think they match to.
I’m looking hard at that picture. It looks like the chips pin 1 indicator is on the bottom left of the picture and the pin 1 indicator on the board is on the top left. Are you sure the chip is oriented correctly?
I really think the chip is just rotated. The diagonal soldermask line on the top left is the pin 1 indication on the board. The smaller circle on the chips bottom left is the pin 1 indicator on the chip. The larger circles are just from the injection molding.
