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AR# 30949

Spartan II - WARNING:Pack:1542 The IOB does not support FDS unless INITSTATE equals HIGH

Description

Spartan II - The IOB does not support FDS unless INITSTATE equals HIGH

WARNING:Pack:1542 - The register XXX has the property IOB=TRUE, but was not packed into the output side of an I/O component. Register XXX has INITSTATE equal to LOW. The IOB does not support FDS unless INITSTATE equals HIGH.

Solution

The IOB FF in Spartan-II device is a FD, not a FDS.

If a register does not require a REV port (set/reset function), its INITSTATE can be either High or Low. But when a register needs both INIT and REV function, it can be packed into IOB only when INIT = High.

You can learn the reason by reading the truth table of FDS:

Inputs Outputs

S D C Q

1 X 1

0 1 1

0 0 0

When S = 0, it functions like normal FD.

When S = 1, it sets Q to 1. This equals to INIT = 1. It is impossible to let Q=0 while S = 1.

AR# 30949
Date Created 05/16/2008
Last Updated 12/15/2012
Status Active
Type General Article