Lucent Technologies Inc.
11
Data Sheet
June 1999
ORCA Series 2 FPGAs
Programmable Logic Cells
(continued)
The second submode is the
counter submode
(see
Figure 10). The present count is supplied to input
A[3:0], and then output F[3:0] will either be incre-
mented by one for an up counter or decremented by
one for a down counter. If an up counter or down
counter is needed, the control signal to select the direc-
tion (up or down) is input on A4. Generally, the latches/
FFs in the same PFU are used to hold the present
count value.
5-4643(F).r1
Figure 10. Counter Submode with Flip-Flops
In the third submode,
multiplier submode
, a single
PFU can affect a 4 x 1-bit multiply and sum with a par-
tial product (see Figure 11). The multiplier bit is input at
A4, and the multiplicand bits are input at B[3:0], where
B3 is the most significant bit (MSB). A[3:0] contains the
partial product (or other input to be summed) from a
previous stage. If A4 is logical 1, the multiplicand is
added to the partial product. If A4 is logical zero, zero is
added to the partial product, which is the same as
passing the partial product. CIN can hold the carry-in
from the less significant PFUs if the multiplicand is
wider than 4 bits, and COUT holds any carry-out from
the addition, which may then be used as part of the
product or routed to another PFU in multiplier mode for
multiplicand width expansion.
Figure 11. Multiplier Submode
Ripple mode’s fourth submode features
equality
comparators
, where one 4-bit bus is input on B[3:0],
another 4-bit bus is input on B[3:0], and the carry-in is
tied to 0 inside the PFU. The carry-out
(|)
signal will be
0 if A = B or will be 1 if A | B. If larger than 4 bits, the
carry-out (|)
signal can be cascaded using fast-carry
logic to the carry-in of any adjacent PFU. Comparators
for greater than or equal or less than (>, =, <) continue
to be supported using the ripple mode subtractor. The
use of this submode could be shown using Figure 9
with CIN tied to 0.
D Q
COUT
QLUT3
LUT
A3
F3
Q3
COUT
D Q
A2
QLUT2
F2
Q2
D Q
A1
QLUT1
F1
Q1
D Q
A0
QLUT0
CIN
F0
Q0
CIN
+
1 0
A3
B3
0
A4
COUT
F3
+
A2
B2
F2
+
A1
B1
F1
+
A0
B0
F0
CIN
1
0
0
1
0
0
1 0
0
5-4620(F)