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Data Sheet
June 1999
ORCA Series 2 FPGAs
6
Lucent Technologies Inc.
Programmable Logic Cells
(continued))
Key: C = controlled by configuration RAM.
Figure 3. Simplified PFU Diagram
5-4573(F)
A4
A3
A2
A1
A4
A3
A2
A1
QLUT3
A0
CARRY
CARRY
A3
A2
A1
A0
QLUT2
B4
B3
B2
B1
B4
B3
B2
B1
QLUT1
B0
CARRY
CARRY
B3
B2
B1
B0
QLUT0
CIN
C0
LSR
GSR
WD[3:0]
CK
CKEN
TRI
PFU_XOR
B4
A4
PFU_NAND
PFU_MUX
C
C
C
C
WD3
WD2
WD1
WD0
C
C
C
T
T
T
T
REG3
SR
EN
REG2
SR
EN
REG1
SR
EN
REG0
SR
EN
O4
O3
O2
O1
O0
F3
C
C
COUT
F2
F1
F0
D0
D1
D2
D3
Q0
Q1
Q2
Q3
C
T
T
T
T
C
Figure 2 and Figure 3 show high-level and detailed
views of the ports in the PFU, respectively. The ports
are referenced with a two- to four-character suffix to a
PFU’s location. As mentioned, there are two 5-bit input
data buses (A[4:0] and B[4:0]) to the LUT, one 4-bit
input data bus (WD[3:0]) to the latches/FFs, and an
output data bus (O[4:0]).
Figure 3 shows the four latches/FFs (REG[3:0]) and the
64-bit look-up table (QLUT[3:0]) in the PFU. The PFU
does combinatorial logic in the LUT and sequential
logic in the latches/FFs. The LUT is static random
access memory (SRAM) and can be used for read/
write or read-only memory. The eight 3-state buffers
found in each PLC are also shown, although they actu-
ally reside external to the PFU.
Each latch/FF can accept data from the LUT. Alterna-
tively, the latches/FFs can accept direct data from
WD[3:0], eliminating the LUT delay if no combinatorial
function is needed. The LUT outputs can bypass the
latches/FFs, which reduces the delay out of the PFU. It
is possible to use the LUT and latches/FFs more or
less independently. For example, the latches/FFs can
be used as a 4-bit shift register, and the LUT can be
used to detect when a register has a particular pattern
in it.