Data Sheet
June 1999
ORCA Series 3C and 3T FPGAs
with half-logic ripple connections shown as dashed
lines.
Programmable Logic Cells (continued)
Half-Logic Mode
The result output and ripple output are calculated by
using generate/propagate circuitry. In ripple mode, the
two operands are input into KZ[1] and KZ[0] of each
LUT. The result bits, one per LUT, are F[7:0]/F[3:0] (see
Figure 6). The ripple output from LUT K7/K3 can be
routed on dedicated carry circuitry into any of four adja-
cent PLCs, and it can be placed on the PFU COUT/
FCOUT outputs. This allows the PLCs to be cascaded
in the ripple mode so that nibble-wide ripple functions
can be expanded easily to any length.
Series 3 FPGAs are based upon a twin-quad architec-
ture in the PFUs. The byte-wide nature (eight LUTs,
eight latches/FFs) may just as easily be viewed as two
nibbles (two sets of four LUTs, four latches/FFs). The
two nibbles of the PFU are organized so that any nib-
ble-wide feature (excluding some softwired LUT topolo-
gies) can be swapped with any other nibble-wide
feature in another PFU. This provides for very flexible
use of logic and for extremely flexible routing. The half-
logic mode of the PFU takes advantage of the twin-
quad architecture and allows half of a PFU, K[7:4] and
associated latches/FFs, to be used in logic mode while
the other half of the PFU, K[3:0] and associated latches/
FFs, is used in ripple mode. In half-logic mode, the
ninth FF may be used as a general-purpose FF or as a
register in the ripple mode carry chain.
Result outputs and the carry-out may optionally be reg-
istered within the PFU. The capability to register the
ripple results, including the carry output, provides for
improved counter performance and simplified pipelin-
ing in arithmetic functions.
Ripple Mode
REGCOUT
D
Q
C
C
The PFU LUTs can be combined to do byte-wide ripple
functions with high-speed carry logic. Each LUT has a
dedicated carry-out net to route the carry to/from any
adjacent LUT. Using the internal carry circuits, fast
arithmetic, counter, and comparison functions can be
implemented in one PFU. Similarly, each PFU has
carry-in (CIN, FCIN) and carry-out (COUT, FCOUT)
ports for fast-carry routing between adjacent PFUs.
FCOUT
COUT
F7
K7[1]
K7[0]
D
D
D
D
D
D
D
D
K7
K6
K5
K4
K3
K2
K1
K0
Q7
Q
Q
Q
Q
Q
Q
Q
Q
F6
K6[1]
K6[0]
Q6
The ripple mode is generally used in operations on two
data buses. A single PFU can support an 8-bit ripple
function. Data buses of 4 bits and less can use the
nibble-wide ripple chain that is available in half-logic
mode. This nibble-wide ripple chain is also useful for
longer ripple chains where the length modulo 8 is four
or less. For example, a 12-bit adder (12 modulo 8 = 4)
can be implemented in one PFU in ripple mode (8 bits)
and one PFU in half-logic mode (4 bits), freeing half of
a PFU for general logic mode functions.
F5
K5[1]
K5[0]
Q5
F4
K4[1]
K4[0]
Q4
F3
K3[1]
K3[0]
Q3
F2
K2[1]
K2[0]
Q2
Each LUT has two operands and a ripple (generally
carry) input, and provides a result and ripple (generally
carry) output. A single bit is rippled from the previous
LUT and is used as input into the current LUT. For LUT
K0, the ripple input is from the PFU CIN or FCIN port.
The CIN/FCIN data can come from either the fast-carry
routing (FCIN) or the PFU input (CIN), or it can be tied
to logic 1 or logic 0.
F1
K1[1]
K1[0]
Q1
F0
K0[1]
K0[0]
Q0
CIN/FCIN
5-5755(F)
In the following discussions, the notations LUT K7/K3
and F[7:0]/F[3:0] are used to denote the LUT that pro-
vides the carry-out and the data outputs for full PFU
ripple operation (K7, F[7:0]) and half-logic ripple
operation (K3, F[3:0]), respectively. The ripple mode
diagram in Figure 6 shows full PFU ripple operation,
Figure 6. Ripple Mode
16
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