ORCA Series 2 FPGAs

Data Sheet

June 1999

Programmable Logic Cells (continued)

C0

A4

A4

A3

A3 QLUT3

A2

A2

F3

A1

A1 QLUT2

A0

A0

F1

B4

B4

B3

B3 QLUT1

B2

B2

F0

B1

B1 QLUT0

B0

B0

two operands are input into A[3:0] and B[3:0]. The four

result bits, one per QLUT, are F[3:0] (see Figure 9).

The ripple output from QLUT3 can be routed to dedi-

cated carry-out circuitry into any of four adjacent PLCs,

or it can be placed on the O4 PFU output, or both. This

allows the PLCs to be cascaded in the ripple mode so

that nibble-wide ripple functions can be expanded eas-

ily to any length.

COUT

B3

A3

B3 COUT

A3 QLUT3

F3

B2

B2

F2

A2

A2 QLUT2

5-2751(F).r3

Figure 8. F5M Modeâ€”One Six-Input Variable

Function

F5M Modeâ€”One Six-Input Variable Function

The LUT can be used to implement any function of six-

input variables. As shown in Figure 8, five input signals

(A[4:0]) are routed into both the A[4:0] and B[4:0] ports,

and the C0 port is used for the sixth input. The output

port is F1.

Ripple Mode

The LUT can do nibble-wide ripple functions with high-

speed carry logic. Each QLUT has a dedicated carry-

out net to route the carry to/from the adjacent QLUT.

Using the internal carry circuits, fast arithmetic and

counter functions can be implemented in one PFU.

Similarly, each PFU has carry-in (CIN) and carry-out

(COUT) ports for fast-carry routing between adjacent

PFUs.

The ripple mode is generally used in operations on two

4-bit buses. Each QLUT 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 QLUT and is used as input into the current

QLUT. For QLUT0, the ripple input is from the PFU CIN

port. The CIN data can come from either the fast-carry

routing or the PFU input B4, or it can be tied to logic 1

or logic 0.

The resulting output and ripple output are calculated by

using generate/propagate circuitry. In ripple mode, the

B1

B1

F1

A1

A1 QLUT1

B0

B0 QLUT0

F0

A0

A0

CIN

CIN

Figure 9. Ripple Mode

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The ripple mode can be used in one of four submodes.

The first of these is adder/subtractor mode. In this

mode, each QLUT generates two separate outputs.

One of the two outputs selects whether the carry-in is

to be propagated to the carry-out of the current QLUT

or if the carry-out needs to be generated. The result of

this selection is placed on the carry-out signal, which is

connected to the next QLUT or the COUT signal, if it is

the last QLUT (QLUT3).

The other QLUT output creates the result bit for each

QLUT that is connected to F[3:0]. If an adder/subtractor

is needed, the control signal to select addition or sub-

traction is input on A4. The result bit is created in one-

half of the QLUT from a single bit from each input bus,

along with the ripple input bit. These inputs are also

used to create the programmable propagate.

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