I'm using the MSP430F6638 and encountering mysterious power supply current spikes. I have a 1 ohm shunt resistor in series with power supplied to the microcontroller. All DVCC and AVCC pins are bypassed with 0.1uF capacitors and connected to the resistor (I'm not attempting any precision analog functions). I also have a 10uF ceramic capacitor on the uC side of the resistor. I'm measuring the differential voltage across the 1 ohm resistor with a National Instruments DAQ device. The spikes are 4-6mV in amplitude and about 15us in duration. They occur periodically at intervals ranging from less than 1ms to almost 30ms. They occur more often when the processor is awake than when it is asleep, and they still occur while the processor is reset.
I'm very familiar with the problems caused by conflicting outputs and floating inputs. This sounds like a classic case of floating inputs, but I have ruled that out step by step. Eventually I wound up building a board with just the microcontroller, a 4MHz crystal, and USB interface. We wrote software that sets all of the I/O pins to low-level outputs and immediately goes to sleep. I matched the TI USB demo board (MSP-TS430PZ100USB) except for pin 29. The data sheet explicitly states multiple times to ground pin 29 (LCDCAP) if not used, but the demo board leaves it open. I started out with pin 29 grounded. I still saw the spikes. I lifted pin 29 and the spikes went away. To be thorough, I grounded the pin again, but the spikes did not return. Now my minimal experimental board will not produce the spikes no matter what I do. My other boards continue to produce the spikes regardless of whether pin 29 is grounded or open.
I have experimented with grounding and measurement techniques to make sure the spikes are not a measurement artifact. I went so far as to rig a differential amplifier (INA113) and confirm the measurements with an oscilloscope. I put a 1 ohm resistor in series with a 3.3V power supply to the demo board and do not see the spikes. When the spikes are present on a board they don't go away when USB is connected or disconnected, asleep or awake, or even when the processor is reset.
I am an experienced designer and troubleshooter and I have exhausted my knowledge and bag of tricks on this problem. The project is at risk because we can't move forward with this unexplained phenomenon. I'm looking for someone who a) has seen this problem and can tell me how to resolve it, b) has fresh ideas for trying to figure it out, or c) who can provide consulting with payment contingent upon resolution of the problem.
