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
Lucent Technologies Inc.
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