Stratix II: 8-Input Fracturable LUT in the ALM

The new and innovative logic structure of Stratix® II devices delivers unprecedented performance and logic efficiency. This logic structure is built from basic logic units known as adaptive logic modules (ALMs). Each ALM contains a variety of (look-up table) LUT-based resources, two full adders, carry-chain segments, two flipflops, and many additional logic enhancements that can be flexibly divided into two adaptive LUTs (ALUTs). Logical functions with up to 7 inputs and complex logic-arithmetic functions can be implemented in one ALM.

Research has shown that look-up tables (LUTs) with a higher input count provide better performance in FPGA designs. At the same time, logic fabrics that use arrays of 4 or less input LUTs provide better area efficiency. The new Stratix II logic structure achieves the best of both worlds—its performance is comparable to 7-input LUT based FPGAs and its logic efficiency is better than programmable devices using a 4-input LUT. Figure 1 shows the different LUT configurations that a single ALM can support and Table 1 describes each ALM configuration.

Figure 1. Stratix II ALM Configurations

Figure 1. Stratix II ALM Configurations

Table 1. Stratix II ALM Configurations

Example

Description

The logic structure in Stratix II devices can implement two independent 4-input (or smaller) LUTs (4-LUT) per ALM. This configuration is “backward-compatible” and ideal for migrating a design that is optimized for traditional 4-input LUT FPGAs to the Stratix II device family.

Stratix II ALMs can implement a 5-input LUT (5-LUT) and a 3-input LUT (3-LUT) per ALM. The inputs to the two LUTs are independent of each other. The 3-input LUT can be used to implement any logic function that has three or fewer inputs. Therefore, a 5-input LUT and 2-input LUT are also allowed.

The ALMs within the Stratix II architecture can be configured to implement a 5-input LUT and a 4-input LUT per ALM. One of the inputs must be shared between the two LUTs. The 5-input LUT has up to four independent inputs. The 4-input LUT has up to three independent inputs. The sharing of inputs between LUTs is very common in FPGA designs, and Quartus® II software will automatically seek logic functions that are structured in this manner.

Stratix II ALMs can implement two 5-input LUTs per ALM. In this case, two of the inputs between the LUTs are common and up to three independent inputs are allowed for each 5-input LUT.

6-LUT graphic

Stratix II ALMs support any 6-input logic function per ALM. If there are two 6-input functions that have the same logic operation and four shared inputs, then these two 6-input functions can be implemented in one Stratix II ALM.

For example, a 4x2 crossbar switch that has four data input lines and two sets of unique select signals requires 4 LEs in a Stratix device. In a Stratix II device, this same function only consumes one ALM. In another example, a single Stratix II ALM can implement two 6-input AND gates that have four common inputs. The same function in a Stratix device requires three LEs.

In the extended mode, the Stratix II logic structure can perform certain logical functions with up to 7 inputs per ALM. Quartus II software can automatically recognize the applicable 7-input function and fit it into an ALM. For detailed information about the types of 7-input functions that can be implemented within an ALM, refer to the Stratix II Device Handbook.

ALM Performance Advantages

When implementing a logic function with a large number of inputs, the logic function is decomposed into smaller cascading logic blocks that are restricted by the size (number of inputs) of the look-up table (LUT) in the FPGA. Each cascading LUT is considered a logic level. The number of logic levels and the programmable routing segments on the critical path dictate the performance of the system. More logic levels and routing segments mean longer logic propagation delay and slower system performance.

The Stratix II ALM goes beyond the simple 4-input-LUT structure and extends the logic capacity to efficiently construct any logic functions with 5 or 6 inputs. See Figure 2 for a comparison of a generic 4-input LUT logic structure verses an ALM-based logic structure. When ALMs are configured in the extended LUT mode, many 7-input functions can be implemented by each ALM. With ALM’s ability to implement functions with higher input counts, the Stratix II logic structure offers an average performance improvement of 50 percent as compared to Stratix by:

  • Reducing the number of logic levels required for the overall combinatorial logic
  • Reducing the extra programmable routing needed in the 4-input-LUT implementation
  • Reducing the stress on the demand for general routing resources

Figure 2. Stratix II ALM’s Native 6-Input-LUT Support Reduces Logic Levels and Programmable Routing Delays

ALM Logic Efficiency Advantages

The ALM’s flexible logic structure gives Stratix II FPGAs an average 25 percent more efficient use of logic over prior FPGA families. A key innovation lies in the ability of Stratix II ALMs to implement two LUTs of the same or different sizes to exactly match a design’s combinational logic that is decomposed into smaller and different-sized logic blocks.

Quartus II software can automatically utilize the full potential of the Stratix II ALM to implement LUTs of different sizes. As shown in Figure 3, when implementing a function of three variables in the traditional 4-input LUT structure, the fourth, unused port is wasted. However, for a Stratix II ALM, after implementing a 3-variable function, it can still be used to implement a 5-input function.

Figure 3. Stratix II Logic Efficiency Example

The Stratix II logic structure further improves logic efficiency, allowing the implementation of two 6-input LUTs that perform the same logic operation in one ALM. As shown in Figure 4, a 4x2 crossbar switch takes four LUTs in a generic 4-input LUT architecture; however, the same design will fit in one Stratix II ALM.

Figure 4. Generic 4-Input LUT Logic Structure vs. Stratix II 4x2 Crossbar Switch Logic Structure

Figure 4. Generic 4-Input LUT Logic Structure vs. Stratix II 4x2 Crossbar Switch Logic Structure

The superior performance achieved by the ability to implement wide-input LUTs in one Stratix II ALM combined with the exceptional logic efficiency make the Stratix II device family the ideal choice for high-performance and high-density designs.