Effect of nCE (Chip Enable)

Hello FvM,

Thank you for the reply. The original intention of this thread was to clarify the effect of nCE=High during i) Configuration and ii) User Mode. Do any pins become high-Z if not already so? If so, which pins? Any other effects on pin functions, impedance and internal configuration?

We have had (only) two observations up to now: 1) Intel FPGA will not lose its configuration contents if nCE is pulled high during User Mode, and 2) The JTAG will not work during both Configuration and User Modes if nCE is pulled high.

It seems that - more than this - no-one really knows more precisely what the effect of pulling nCE high might be.

We wanted to understand the effect of nCE=high because we had the following design concept:

1) use FPP to configure the FPGA via a Host processor's parallel bus

2) reuse this parallel bus during User Mode for Host to FPGA communication

3) however, this parallel bus is also shared with a Flash device to allow the Host to initially boot

So we were investigating the possible use of nCE to ensure that the FPP pins are inactive and not clocking any internal configuration function during 2) and 3) and on occasion when the Host's boot flash mentioned in 3) should be updated.

The Note under C10GX51003 2017.11.10 says in respect of AS configuration pins: "If you wish to gain control of the EPCQ-L pins, hold the nCONFIG pin low and pull the nCE pin high. This causes the device to reset and tri-state the AS configuration pins."

It would be great to have confirmation of this for the FPP-related pins too, since they differ from the AS pins. This would confirm the feasibility of 3) above. We could hold nCE high in this case.

And for 2) above, it would be great to see that, for example, if nCE is pulled high, only Configuration (and JTAG) functions are made inactive but that User Mode pin functions (including those shared on the Configuration pins) continue to be unaffected.

It seems logical to expect that Configuration pins should become inactive upon entering User Mode and that they may therefore be toggled at will with no undue User Mode effect, which would greatly obviate the need for the nCE considerations above.

Yes, buffers would be a way of decoupling the affected dedicated configuration pins, but it would be great to KNOW that a buffer is definitely necessary of that it definitely is not necessary in order to prevent unexpected User Mode behaviour. Or that nCE may else be useful in some way.