EP3C25 nStatus 1V

[Altera_Forum]

I have five EP3C25 borad, 3 of them are working well. But the rest 2 will sometimes boot fail. The working mode is AS or fast AS from EPCS4. There're 10K pull up on nStatus, nConfig, Conf_done, MSELs are 010 or 101, tied to GND or 3V. 

 Fail boards are in common that the nStatus pin goes from 0V to about 1.1V after power up or after the nConfig is tied to ground then released up, no DCLK or other signals from EP3C25. On the success boot, it can load configuration from EPCS4, while the nStatus is about 1.5V and will slowly move down to 1.2V.  Do you have any idea on this situation? Thx.

[Altera_Forum]

Could you look at those signals with a scope instead of a voltmeter? 

Which package is it? If it's a EP3C25E144 you must check that the chip's exposed bottom pad is properly soldered to ground.

[Altera_Forum]

Thank you for your reply. 

 Actually, I read the voltage from the measurement of a scope, not from multimeter. The nStatus will go to 0V as soon as I press the nConfig to ground, then it will rise to 1.1V after release of nConfig.   The package I'm using is QFP240, which has no gound pad in bottom.

[Altera_Forum]

In that case 1.1V is really strange.... what pull-up to you have on nStatus, and to which power supply is it connected to?

[Altera_Forum]

I connect the nStatus via a 10K resistor to 3V power. This 3V is not the same net as VCCIO or VCCA which is connected to EP3C25, but the voltage is the same. Will this cause the problem?

[Altera_Forum]

nStatus is open drain so the actual voltage doesn't matter that much, as long as the absolute maximum ratings are applied. It looks like a hardware failure. When you say same voltage, do you mean that you supply VCCA with 3V? It is supposed to be 2.5V (and it is the same for the MSEL pins by the way).

[Altera_Forum]

The EP3C25's VCCA pin is connected to a seperate 2.5V. However, the MSEL pins are connected to 3V VCCIO. Is this OK or not?

[Altera_Forum]

The manual says 2.5V so connecting them to 3V isn't ok. That said I'm not sure it would have any real impact on the system. The only two issues I see would be with clamping diodes (but then, it's only 0.5V...) and a problem due to power supply ordering. Although in the latter case maybe the por circuitry prevents the FPGA from reading the MSEL pins before all the supplies are at their nominal level. 

Do the FPGA's on the failing boards answer properly through JTAG?

[Altera_Forum]

Yes. The identifaction and programming using JTAG cable is always OK, never failed.  

I think the JTAG mode can be used on any combination of MSEL pins because this mode is superior than other modes, right?

[Altera_Forum]

Yes you are right, and it means the FPGA is not defective. Your problem could come from the fact that the MSEL pins are misread during POR. Unfortunately I guess there isn't an easy way to connect them to VCCA instead of VCCIO to test that hypothesis.

[Altera_Forum]

There's still one question. Will the MSEL pins be read after each nConfig toggle? If so, the problem may quite probably on the MSEL pins. Because I found that there were more failure occured on nConfig release than on POR. For several times the loading after POR was OK but then reset nConfig failed.

[Altera_Forum]

I'm not sure... the manual doesn't say anything about being able to change the MSEL pins before a reconfiguration, and even insists on the fact that they should only be directry connected to ground or VCCA and not controlled by an external component. This could indicate that the MSEL pins are only read once, at the beginning of POR (or that they aren't registered at all...)