22 Altera Corporation
ACEX 1K Programmable Logic Device Family Data Sheet
Normal Mode
The normal mode is suitable for general logic applications and wide
deco ding f unc tions that can t ak e a d van tag e of a casca d e cha i n. In norm a l
mode, four data inputs from the LAB local interconnect and the carry-in
are inputs to a 4-input LUT. The compiler automatically selects the carry-
in or the DATA3 signal as one of the inputs to the LUT. The LUT output
can be combined with the cascade-in signal to for m a cascad e ch ain
throu gh the casca de-out signa l. Eith er th e regis ter or the L UT can be us ed
to drive both the local interconnect and the FastTrack Interconnect routing
structure at the same time.
The LUT and the register in the LE can be used independently (register
pack ing). To support regi ster p acking , the LE ha s two ou tputs; one d rives
the local interconnect, and the other drives the FastTrack Interconnect
routin g struc tur e. The DATA4 signal can drive the register directly,
allowing the LUT to compute a function that is independent of the
registered signal; a 3-input function can be computed in the LUT, and a
fourth independent signal can be registered. Alternatively, a 4-input
function can be generated, and one of the inputs to this function can be
used to drive the register. The register in a packed LE can still use the clock
enab le, clear , and pres et sign als in th e LE . In a pa cked L E, the reg ister can
drive the FastTrack Interconnect routing structure while the LUT drives
the local inter con nect, or vice versa.
Arithmetic Mode
The arithmetic mode offers two 3-input LUTs that are ideal for
implementi ng ad d ers, accumulat ors, and co mparato r s. One L UT
computes a 3-input function; the other generates a carry output. As shown
in Figure 11, the first LUT uses the carry-in signal and two data inputs
from the LAB loca l in te rconn ect to g ene rat e a c ombi nat orial or regis tere d
output. For example, in an adder, this output is the sum of three signals:
a, b, and car ry-in. The secon d LUT uses th e same th ree signal s to generat e
a carry-out signal, thereby creating a carry chain. The arithmetic mode
also supports simultaneous use of the cascade chain.
Up/Down Counter Mode
The up/down counter mode offers counter enable, clock enable,
synchronous up/down control, and data loading options. These control
signa ls a re ge ne rate d by th e d at a in puts from the LAB lo cal inter conn ect ,
the car ry-in sig nal, and outp ut feedb ack from the p rogrammable re gister .
Two 3- input L UTs ar e us ed; one gene rat es the c ount er da ta, a nd th e othe r
generates the fast carry bit. A 2-to-1 multiplexer provides synchronous
loading. Data can also be loa ded asynchro nously with t he clear and preset
register control signals without using the LUT resources.