Data Sheet
June 1999
ORCA Series 2 FPGAs
Programmable Logic Cells (continued)
F5M and F5X Modes—Special Function Modes
C0
F3
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.
A4
A3
A2
A1
A0
A4
A3
A2
A1
A0
QLUT3
QLUT2
F2
F3
B4
B3
B2
B1
B0
B4
B3
B2
B1
B0
QLUT1
QLUT0
F1
F0
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.
F0
5-2754(F).r3
Figure 6. F5M Mode—Multiplexed Function of Two
Independent Five-Input Variable
Functions
C0
F3
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.
A4
A3
A2
A1
A0
A4
A3
A2
A1
A0
HLUTA
F3
F2
F1
The output of the MUX is:
B4
B3
B2
B1
B0
B4
B3
B2
B1
B0
HLUTB
F1 = (HLUTA & C0) + (HLUTB & C0)
F1 = (F3 & C0) + (F0 & C0)
F0
The output of the exclusive OR is:
F1 = HLUTA HLUTB C0
F1 = F3 F0 C0
F0
5-2755(F).r2
The output of the NAND is:
Figure 7. F5X Mode—Exclusive OR Function of Two
Independent Five-Input Variable
Functions
F2 = HLUTA & HLUTB & C0
F2 = F3 & F0 & C0
Lucent Technologies Inc.
9
ETC [ ETC ]
