Logic Elements
Figure 2–8 shows the carry-select circuitry in a LAB for a 10-bit full adder.
One portion of the LUT generates the sum of two bits using the input
signals and the appropriate carry-in bit; the sum is routed to the output
of the LE. The register can be bypassed for simple adders or used for
accumulator functions. Another portion of the LUT generates carry-out
bits. A LAB-wide carry-in bit selects which chain is used for the addition
of given inputs. The carry-in signal for each chain, carry-in0or
carry-in1, selects the carry-out to carry forward to the carry-in signal
of the next-higher-order bit. The final carry-out signal is routed to an LE,
where it is fed to local, row, or column interconnects.
Figure 2–8. Carry Select Chain
LAB Carry-In
0
1
LAB Carry-In
Sum1
A1
B1
LE1
LE2
LE3
LE4
LE5
Carry-In0
Carry-In1
Sum2
Sum3
Sum4
Sum5
A2
B2
LUT
LUT
data1
data2
Sum
A3
B3
A4
B4
LUT
LUT
A5
B5
0
1
Carry-Out0
Carry-Out1
Sum6
Sum7
Sum8
Sum9
Sum10
A6
B6
LE6
LE7
LE8
LE9
A7
B7
A8
B8
A9
B9
A10
B10
LE10
LAB Carry-Out
Altera Corporation
May 2008
2–11
Preliminary
ALTERA [ ALTERA CORPORATION ]
