EP2S15 ALTERA [Altera Corporation], EP2S15 Datasheet - Page 27

• Manufacturer Part Number: EP2S15
• Description: Stratix II Device Family
• Manufacturer: ALTERA [Altera Corporation]

Figure 2–14. Example of a 3-bit Add Utilizing Shared Arithmetic Mode
Altera Corporation
May 2007
3-Bit Add Example
implemented in adders.
implemented in LUTs.
+
2nd stage add is
1st stage add is
1 1 0 1
1 1 0
+
Binary Add
1 1 0
1 0 1
0 1 0
0 0 1
+
C2 C1 C0
R3 R2 R1 R0
Shared Arithmetic Chain
In addition to the dedicated carry chain routing, the shared arithmetic
chain available in shared arithmetic mode allows the ALM to implement
a three-input add. This significantly reduces the resources necessary to
implement large adder trees or correlator functions.
The shared arithmetic chains can begin in either the first or fifth ALM in
an LAB. The Quartus II Compiler creates shared arithmetic chains longer
than 16 (8 ALMs in arithmetic or shared arithmetic mode) by linking
LABs together automatically. For enhanced fitting, a long shared
+
+
Equivalents
Decimal
X2 X1 X0
S2 S1 S0
2 x 6
Y2 Y1 Y0
Z2 Z1 Z0
+
13
6
5
2
1
X0
Y0
Z0
X1
Y1
Z1
X2
Y2
Z2
ALM Implementation
ALM 1
ALM 2
3-Input
3-Input
3-Input
3-Input
3-Input
3-Input
3-Input
3-Input
LUT
LUT
LUT
LUT
LUT
LUT
LUT
LUT
Stratix II Device Handbook, Volume 1
S0
C0
S1
C1
S2
C2
shared_arith_in = '0'
'0'
carry_in = '0'
Stratix II Architecture
R0
R1
R2
R3
2–19