LFSC datasheet(9/237 Pages) LATTICE

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2-5

Architecture

Lattice Semiconductor

LatticeSC/M Family Data Sheet

Modes of Operation

Each Slice is capable of four modes of operation: Logic, Ripple, RAM and ROM. Table 2-2 lists the modes and the

capability of the Slice blocks.

Table 2-2. Slice Modes

Logic Mode

In this mode, the LUTs in each Slice are conﬁgured as combinatorial lookup tables. A LUT4 can have 16 possible

input combinations. Any logic function with four inputs can be generated by programming this lookup table. Since

there are two LUT4s per Slice, a LUT5 can be constructed within one Slice. Larger lookup tables such as LUT6,

LUT7 and LUT8 can be constructed by concatenating other Slices in the PFU.

Ripple Mode

Ripple mode allows the efﬁcient implementation of small arithmetic functions. In ripple mode, the following func-

tions can be implemented by each Slice:

• Addition 2-bit

• Subtraction 2-bit

• Up counter 2-bit

• Down counter 2-bit

• Comparator functions of A and B inputs

- A greater-than-or-equal-to B

- A not-equal-to B

- A less-than-or-equal-to B

Ripple Mode includes an optional conﬁguration that performs arithmetic using fast carry chain methods. In this con-

ﬁguration (also referred to as CCU2 mode) two additional signals, Carry Generate and Carry Propagate, are gener-

ated on a per slice basis to allow fast arithmetic functions to be constructed by concatenating Slices.

RAM Mode

In this mode, distributed RAM can be constructed using each LUT block as a 16x1-bit memory. Through the combi-

nation of LUTs and Slices, a variety of different memories can be constructed.

The Lattice design tools support the creation of a variety of different size memories. Where appropriate, the soft-

ware will construct these using distributed memory primitives that represent the capabilities of the Slice. Table 2-3

shows the number of Slices required to implement different distributed RAM primitives. Dual port memories involve

the pairing of two Slices, one Slice functions as the read-write port. The other companion Slice supports the read-

only port. For more information on RAM mode, please see details of additional technical documentation at the end

of this data sheet.

Table 2-3. Number of Slices Required For Implementing Distributed RAM

ROM Mode

The ROM mode uses the same principal as the RAM modes, but without the Write port. Pre-loading is accom-

plished through the programming interface during conﬁguration.

Logic

Ripple

RAM

ROM

PFU Slice

LUT 4x2 or LUT 5x1

2-bit Arithmetic Unit

SPR 16x2

DPR 16x2

ROM 16x2

SPR16x2

DPR16x2

Number of Slices

Note: SPR = Single Port RAM, DPR = Dual Port RAM

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