OR3C55 datasheet(13/210 Pages) AGERE

13 / 210 page

Lucent Technologies Inc.

Data Sheet

June 1999

ORCA Series 3C and 3T FPGAs

Programmable Logic Cells (continued)

Look-Up Table Operating Modes

The operating mode affects the functionality of the PFU input and output ports and internal PFU routing. For exam-

ple, in some operating modes, the DIN[7:0] inputs are direct data inputs to the PFU latches/FFs. In memory mode,

the same DIN[7:0] inputs are used as a 4-bit write data input bus and a 4-bit write address input bus into LUT

memory.

Table 3 lists the basic operating modes of the LUT. Figure 4—Figure 10 show block diagrams of the LUT operating

modes. The accompanying descriptions demonstrate each mode’s use for generating logic.

PFU Control Inputs

Each PFU has five routable control inputs and an active-low, asynchronous global set/reset (GSRN) signal that

affects all latches and FFs in the device. The five control inputs are CLK, LSR, CE, ASWE, and SEL, and their

functionality for each logic mode of the PFU (discussed subsequently) is shown in Table 4. The clock signal to the

PFU is CLK, CE stands for clock enable, which is its primary function. LSR is the local set/reset signal that can be

configured as synchronous or asynchronous. The selection of set or reset is made for each latch/FF and is not a

function of the signal itself. ASWE stands for add/subtract/write enable, which are its functions, along with being an

optional clock enable, and SEL is used to dynamically select between direct PFU input and LUT output data as the

input to the latches/FFs.

All of the control signals can be disabled and/or inverted via the configuration logic. A disabled clock enable indi-

cates that the clock is always enabled. A disabled LSR indicates that the latch/FF never sets/resets (except from

GSRN). A disabled SEL input indicates that DIN[7:0] PFU inputs are routed to the latches/FFs. For logic and ripple

modes of the PFU, the LSR, CE, and ASWE (as a clock enable) inputs can be disabled individually for each nibble

(latch/FF[3:0], latch/FF[7:4]) and for the ninth FF.

Table 3. Look-Up Table Operating Modes

Mode

Function

Logic

4- and 5-input LUTs; softwired LUTs; latches/FFs with direct input or LUT input; CIN as direct input to

ninth FF or as pass through to COUT.

Half Logic/

HalfRipple

Upper four LUTs and latches/FFs in logic mode; lower four LUTs and latches/FFs in ripple mode; CIN

and ninth FF for logic or ripple functions.

Ripple

All LUTs combined to perform ripple-through data functions. Eight LUT registers available for direct-in

use or to register ripple output. Ninth FF dedicated to ripple out, if used. The submodes of ripple mode

are adder/subtractor, counter, multiplier, and comparator.

Memory

All LUTs and latches/FFs used to create a 32 x 4 synchronous dual-port RAM. Can be used as single-

port or as ROM.

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