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Chapter 2: Logic Elements and Logic Array Blocks in the Cyclone III Device Family

Logic Array Blocks

Cyclone III Device Handbook

December 2011

Altera Corporation

Volume 1

Arithmetic Mode

Arithmetic mode is ideal for implementing adders, counters, accumulators, and

comparators. An LE in arithmetic mode implements a 2-bit full adder and basic carry

chain (Figure 2–3). LEs in arithmetic mode can drive out registered and unregistered

versions of the LUT output. Register feedback and register packing are supported

when LEs are used in arithmetic mode.

Figure 2–3 shows LEs in arithmetic mode.

The Quartus II Compiler automatically creates carry chain logic during design

processing. You can also manually create the carry chain logic during design entry.

Parameterized functions, such as LPM functions, automatically take advantage of

carry chains for the appropriate functions.

The Quartus II Compiler creates carry chains longer than 16 LEs by automatically

linking LABs in the same column. For enhanced fitting, a long carry chain runs

vertically, which allows fast horizontal connections to M9K memory blocks or

embedded multipliers through direct link interconnects. For example, if a design has a

long carry chain in a LAB column next to a column of M9K memory blocks, any LE

output can feed an adjacent M9K memory block through the direct link interconnect.

If the carry chains run horizontally, any LAB which is not next to the column of M9K

memory blocks uses other row or column interconnects to drive a M9K memory

block. A carry chain continues as far as a full column.

Logic Array Blocks

Logic array blocks (LABs) contain groups of LEs.

Topology

Each LAB consists of the following features:

■

16 LEs

Figure 2–3. Cyclone III Device Family LEs in Arithmetic Mode

clock (LAB Wide)

ena (LAB Wide)

aclr (LAB Wide)

CLRN

ENA

sclear

(LAB Wide)

sload

(LAB Wide)

Chain Output

Row, Column, and

Direct link routing

Row, Column, and

Direct link routing

Local Routing

Three-Input

LUT

Three-Input

LUT

cin (from cout

of previous LE)

data2

data1

cout

data4

data3

Connection

Packed Register Input

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