Hook open drain "power good" to nSTATUS or nCONFIG?

Connect PWRGD to nConfig. Do not let nConfig go high until all supplies are within tolerance. (IOW, pull nConfig low if any supply is out of tolerance.)

From the StratixIIGX data sheet:

"(4) VCCPD must ramp-up from 0 V to 3.3 V within 100 us to 100 ms. If VCCPD is not ramped up within this specified time, the Stratix II GX device will not configure successfully. If the system does not allow for a VCCPD ramp-up time of 100 ms or less, hold nCONFIG low until all power supplies are reliable."

Since in general you will not be able to control brownouts (i.e. you cannot guarantee any particular risetime), you really do need a precision supply monitor.

Regards, Allan

Somehow I managed to edit out the part where I explained that brownout detectors made on linear chips (e.g. your regulator) are probably much more accurate than the ones made on digital chips (e.g. your FPGA). They also won't suffer from risetime limitations, etc.

Regards, Allan

Another note from the StratixIIGX data sheet:

"(3) Maximum VCC rise time is 100 ms, and VCC must rise monotonically from ground to VCC."

To me, this implies that you need a precision (i.e. external) monitor on all rails. The last time I designed a board with an FPGA (last month), I used the PWRGD outputs on several different regulators in conjunction with an LTC2900 so that every rail on the board was monitored. The resultant (active low) signal was ANDed into the nCONFIG signal.

The LTC2900 is expensive (~$3.63 Digikey), but flexible. There are hundreds of similar parts to choose from.

Regards, Allan

I think it is best to use a voltage supervisor circuit that accurately monitors Vcc and provides a clean and long reset signal. It's also useful for brown-out conditions and orderly reset when the power fails.

Eli