Answers Database

M1 map: What are the rules for merging FFs into an IOB with the MAP -pr b switch?


Record #2207

Product Family:  Software

Product Line:  FPGA Implementation

Problem Title:
M1 map: What are the rules for merging FFs into an IOB with the MAP -pr
b switch?



Problem Description:
Keywords: map, -pr b, IOB, FFs, flip-flops

Urgency: Standard

with the -pr b switch to pack registers into the IOB, move
registers into the IOBs?


Solution 1:

Under the following circumstances, a CLB FF will get merged
into an IOB when map is invoked with the -pr b option :

1.  The D pin of the FF must be connected to the output of an
    IBUF or the Q pin must be connected to the input of an
    OBUF or an OBUFT.  In the case of the FF connected to the
    input of an OBUF or OBUFT, there must be only ONE load on
    this signal.

    The output of the CLB FF can only drive a single output
    net.  For example, if the output feeds back into the input
    of the CLB FF, the FF will not be merged, because this
    counts as an additional load.

    For the 4000EX/XL this applies to
    non-I/O latches as well flip-flops.

2.  The IOBs must have input or output flip-flops.  The XC5200
    architecture does NOT have IOB flip-flops.

3.  There is no CLB external net attribute (X flag) attached
    to the signal between the flip-flop and the I/O buffer.

4.  The flip flop does not use an asynchronous set or reset
    signal.

5.  For all 4K devices, a flop/latch will not be added to an
    IOB if it is part of a macro.

6.  For all 4K and 3K devices, a flop/latch will not be added
    to an IOB if it has a BLKNM or LOC conflict with the IOB.

7.  For 4K devices, a flop/latch will not be added to an IOB
    if its control signals (clock or clock enable) are not
    compatible with those already defined in the IOB.  This is
    for those cases where

    - a flop (latch) is already in the IOB, and a latch (flop)
      needs to be added, or

    - the 4kEX/XL Output MUX is being used, or

    - a flop already exists in the IOB that is clocked by a
      different clock signal.

8.  In XC4000EX, if a constant 0 or 1 is being driven on the
    IOPAD, a flop/latch with a CE will not be added to the
    input side of the IOB.



End of Record #2207