Electronic – NXP LPC GPIO Glitches on every power up. How to solve

There are a few voltage levels you need to maintain.

• VCC(Start) or V(Min), the lowest that clock operations can happen. This is the voltage that the Power On Reset circuit operates at. It's 0.7 * V(B_IT-), typically 1 Volt. The MSP430 should be in POR reset at that time.

• V(B_IT-), the Brown Out Reset circuit trigger voltage. Typically 1.35 Volts. Any quick drops below this voltage triggers the BOR reset.

• V(B_IT+) or Vhys(B_IT-), the BOR circuit hysterics value. Minimum voltage after power drops to the BOR trigger, in which the BOR timer starts. It will be held in reset up to 2 milliseconds after it rises past this. Typically 150mV above V(B_IT-). V(B_IT-) + Vhys(B_IT-) should be ≤ 1.8V Max.

• V(Min), the Minimum Operating Voltage. This is 1.8V. Anything below this is undefined/bad. Between V(Min) and V(B_IT-), you have no idea what is happening.

• Vcc(MhzMin) for Your Clock speed. Not all clock speeds are supported at all VCC voltages. For example, 3.3V is the minimum for 16 mHz. If VCC drops below this, undefined/bad, again.

In practice, the MSP will keep running until somewhere in the 1.35 to 1.7 volt section, badly, until the BOR threshold is passed. You can't rely on any data at this point. But if your clock is too high, then the Vcc(MhzMin) is the real threshold.

Then you have to consider the same voltage ranges for the DAC you are using. It will have its own screwy thresholds for proper operation, and for undefined/bad but still operating ranges.

Sadly this setup won't work. If you examine the datasheet carefully it states that the MOSFET has a threshold voltage which is guaranteed to be between 1.5V and 2.5V, with 1.8V typical.

Even assuming you are lucky and you've got a specimen whose threshold is at 1.5V (best case for you), that doesn't mean that the MOSFET magically turns ON when its Vgs voltage reaches that value. That's the minimum voltage needed to make the MOSFET just barely conduct: in that line of the datasheet you can notice that the threshold voltage is specified at scant 250μA of Id. That level of current is insufficient to operate a common relay reliably.

Note: (as pointed out by @SpehroPefhany in a comment) these are the values at 25°C. If the ambient temperature is lower (e.g. winter, cold climate, circuit placed in cold rooms) the current at that level of Vgs will be even smaller until the MOSFET warms up!

To use a MOSFET as a closed switch you must drive it into the ON region, and specifically in the ohmic region, i.e. that part of the output characteristics where it behaves as a (small value) resistance:

As you can see, the curves shown correspond to higher values of Vgs (~2.8V or higher). You can better appreciate the problem looking at the Rds(on) graph, i.e. "the resistance of the switch":

From the graph on the right you may see that Rds(on) doesn't vary much with current, but the graph on the left tells another story: if you lower Vgs under ~4V you get a steep increase in resistance.

To summarize: this MOSFET cannot be turned on with a mere 1.8V. At least you should provide enough Vgs to make it conduct in the worst case, i.e. Vgs(TH)=2.5V. And this is confirmed by your experiment at 3.3V.