Erratic Start Up issue

[jean-riegardtva]

Good day

I am working on the LPC1857FET256. The board that my company has designed houses the chip. Every one out of a hundred times when the chip is powered up it draws a huge amount of current and does not start up . This draws the external power source in current limit mode with currents in excess of 1.5A. I am sure if I remove the current limiting on the power supply the chip will destroy itself. When I short out the Vdd line to Vss for a fraction of as second the chip boots up as normal and everything works great. The chip is fed by a 3.3V linear voltage regulator. I first suspected that this was the culprit but I soldiered it out and gave it 3.3V directly from the power supply and the problem persisted. I tried to add a 10uF cap between the 3.3V output of the regulator and ground to smooth out any potential issues. This made the problem significantly worst and it happens now almost every second time. It is almost as if something inside the chip latches when the power is applied sometimes. I also found that the problem is way more common when you switch on the power supply when it is connected to the board as apposed to when you first switch on the power supply and then connect the circuit. All the Vdd lines(VDDA, VDDREG, VDDIO, USB0_VDDA3V3_DRIVER and USB0_VDDA3V3) are connected together. Similarly all the Vss lines(VSSA,VSS, VSSIO,USB0_VSSA_TERM ,USB0_VSSA_REF) are also connected together. I have tried to put the processor into reset so that it does not start up but this has absolutely no effect telling me that the issue is way before the RESET pin is even checked. I will appreciate any inputs greatly.

Kind Regards 

JR

[soledad]

Hi,

After check your schematic file, I found a resistor between Vbat and 3.3v, we suggest to connect the vdd lines according the following figure.

In addition, I would recommend that RTCX1 be grounded since it is an input. RTCX2 is an output, so it can be left open.

Do you have more than one board? If yes, is this issue present in all boards?

Is it possible to do a X ray test?

Have a great day,
Sol

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