Stratix IV FPGA vs. Virtex-5 Logic Efficiency
Stratix® IV FPGAs use the adaptive logic module (ALM) to implement logic functions, which is extremely efficient because of the ALM's fracturability. When compared to the LUT-FF pair of Virtex-5 devices, the ALM fracturability results in the following advantages for Stratix IV FPGAs:
Figures 1 and 2 show the basic building blocks of Stratix IV (ALM) FPGAs and Virtex-5 (LUT-FF pair) devices, respectively. Table 1 shows a feature comparison of the logic structures.
Figure 1. Stratix IV FPGA ALM
Figure 2. Virtex-5 LUT-FF Pair
| Table 1. Stratix IV ALM vs. Virtex-5 LUT-FF Pair Feature Comparison |
| Feature |
ALM |
LUT-FF Pair |
| Number of LUT Inputs |
8 |
6 |
| Fracturable LUT |
Yes |
No |
| Register Count per LUT |
2 |
1 |
| Dedicated Full Adders |
2 |
0 |
When implementing logic, the ALM can implement a select set of 7-input functions, all 6-input logic functions, and two independent functions consisting of smaller LUT sizes, such as two independent 4-input LUTs. The ALM requires very few inputs to be shared, as shown in Table 2, resulting in less wasted logic while maximizing performance and the amount of available logic.
Table 2 shows just a few combinations of functions and compares the number of inputs shared for Stratix IV FPGAs and Virtex-5 devices.
| Table 2. Stratix IV ALM vs. Virtex-5 LUT-FF Pair Flexibility |
| Output 1 |
Output 2 |
Number of Shared Inputs (Minimum) for
Stratix IV ALM |
Number of Shared Inputs (Minimum) for
Virtex-5 LUT |
| 5-LUT |
5-LUT |
2 |
5 |
| 5-LUT |
4-LUT |
1 |
4 |
| 5-LUT |
3-LUT |
0 |
3 |
| 4-LUT |
4-LUT |
0 |
3 |
| 4-LUT |
3-LUT |
0 |
2 |
| 3-LUT |
3-LUT |
0 |
1 |
For example, implementing a 5-input and a 3-input function, the ALM implements the two independent functions without sharing any inputs, while the LUT-FF pair must either share inputs or, if there are no common inputs, use two 6-LUTs, resulting in wasted logic.
Figure 3. A 5-Input and a 3-Input Function Implementation in the Stratix IV ALM and the Virtex-5 LUT-FF Pair
Because of efficient fracturability, the Stratix IV ALM has, on average, a 1.8x advantage over the Virtex-5 LUT-FF pair, and the advantage can go as high as 2.3x on certain designs. This is evidenced by the benchmark analysis conducted on a set of 65 customer designs. In Figure 4, the horizontal black line at the 1 mark indicates a point at which the number of logic elements for Virtex-5 (LUT-FF pair) and Stratix IV (ALMs) are the same.
Figure 4. Logic Efficiency Comparison
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