EP20K400CF672C7 Altera, EP20K400CF672C7 Datasheet - Page 15

• Manufacturer Part Number: EP20K400CF672C7
• Description: APEX 20KC
• Manufacturer: Altera

Altera Corporation
Figure 7. APEX 20KC Cascade Chain
AND Cascade Chain
d[(4 n – 1)..(4 n – 4)]
d[3..0]
d[7..4]
LUT
LUT
LUT
Cascade Chain
With the cascade chain, the APEX 20KC architecture can implement
functions with a very wide fan-in. Adjacent LUTs can compute portions
of a function in parallel; the cascade chain serially connects the
intermediate values. The cascade chain can use a logical AND or logical OR
(via De Morgan’s inversion) to connect the outputs of adjacent LEs. Each
additional LE provides four more inputs to the effective width of a
function, with a short cascade delay. Cascade chain logic can be created
automatically by the Quartus II Compiler during design processing, or
manually by the designer during design entry.
Cascade chains longer than ten LEs are implemented automatically by
linking LABs together. For enhanced fitting, a long cascade chain skips
alternate LABs in a MegaLAB structure. A cascade chain longer than one
LAB skips either from an even-numbered LAB to the next even-numbered
LAB, or from an odd-numbered LAB to the next odd-numbered LAB. For
example, the last LE of the first LAB in the upper-left MegaLAB structure
carries to the first LE of the third LAB in the MegaLAB structure.
shows how the cascade function can connect adjacent LEs to form
functions with a wide fan-in.
LE n
LE1
LE2
APEX 20KC Programmable Logic Device Data Sheet
d[(4 n – 1)..(4 n – 4)]
OR Cascade Chain
d[3..0]
d[7..4]
LUT
LUT
LUT
Figure 7
LE n
LE1
LE2
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