Data Sheet

June 1999

ORCA Series 2 FPGAs

Programmable Logic Cells (continued)

F5M and F5X Modesâ€”Special Function Modes

The PFU contains logic to implement two special func-

tion modes which are variations on the F5 mode. As

with the F5 mode, the LUT implements two indepen-

dent five-input functions. Figure 6 and Figure 7 show

the schematics for F5M and F5X modes, respectively.

The F5X and F5M functions differ from the basic F5A/

F5B functions in that there are three logic gates which

have inputs from the two 5-input LUT outputs. In some

cases, this can be used for faster and/or wider logic

functions.

As can be seen, two of the three inputs into the NAND,

XOR, and MUX gates, F0 and F3, are from the LUT.

The third input is from the C0 input into PFU. Since the

C0 input bypasses the LUTs, it has a much smaller

delay through the PFU than for all other inputs into the

special PFU gates. This allows multiple PFUs to be

cascaded together while reducing the delay of the criti-

cal path through the PFUs. The output of the first spe-

cial function (either XOR or MUX) is F1. Since the XOR

and MUX share the F1 output, the F5X and F5M

modes are mutually exclusive. The output of the NAND

PFU gate is F2 and is always available in either mode.

To use either the F5M or F5X functions, the LUT must

be in the F5A/F5B mode; i.e., only 5-input LUTs

allowed. In both the F5X and F5M functions, the out-

puts of the five-input combinatorial functions, F0 and

F3, are also usable simultaneously with the special

PFU gate outputs.

The output of the MUX is:

F1 = (HLUTA & C0) + (HLUTB & C0)

F1 = (F3 & C0) + (F0 & C0)

The output of the exclusive OR is:

F1 = HLUTA âŠ• HLUTB âŠ• C0

F1 = F3 âŠ• F0 âŠ• C0

The output of the NAND is:

F2 = HLUTA & HLUTB & C0

F2 = F3 & F0 & C0

C0

F3

A4

A4

A3

A3 QLUT3

A2

A2

F3

F2

A1

A1 QLUT2

A0

A0

B4

B4

B3

B3 QLUT1

B2

B2

F0

F1

B1

B1 QLUT0

B0

B0

F0

5-2754(F).r3

Figure 6. F5M Modeâ€”Multiplexed Function of Two

Independent Five-Input Variable

Functions

C0

F3

A4

A4

HLUTA

A3

A3

A2

A2

A1

A1

F3

F2

A0

A0

B4

B4

HLUTB

B3

B3

F1

B2

B2

F0

B1

B1

B0

B0

F0

5-2755(F).r2

Figure 7. F5X Modeâ€”Exclusive OR Function of Two

Independent Five-Input Variable

Functions

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