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### A Survey of Logic Block Architectures

For Digital Signal Processing Applications

Presentation Outline

- Considerations in Logic Block Design
- Computation Requirements
- Why Inefficiencies?
- Representative Logic Block Architectures
- Proposed
- Commercial
- Conclusions: What is suitable Where?

Why DSP??? The Context

- Representative of computationally intensive class of applications datapath oriented and arithmetic oriented
- Increasingly large use of FPGAs for DSP multimedia signal processing, communications, and much more
- To study the “issues” in reconfigurable fabric design for compute intensive applications What is involved in making a fabric to accelerate multimedia reconfigurable computing possible?

Elements of a Reconfigurable Architecture

- Logic Block/Processing Element
- Differing Grains Fine>>Coarse>>ALUs
- Routing
- Dynamic Reconfiguration

So what’s wrong with the typical FPGA?

- Meant to be general purpose lower risks
- Toooo Flexible! Result: Efficiency Gap
- Higher Implementation Cost, Larger Delay, Larger Power Consumption than ASICs
- Performance vs. Flexibility Tradeoff Postponing Mapping and Silicon Re-use

Solution? See how FPGAs are Used?

- FPGAs are being used for “classes” of applications Encryption, DSP, Multimedia etc.
- Here lies the Key Design FPGAs for a class of applications
- Application Domain Characterization Application Domain Tuning

Domain Specialization

COMPUTATION defines ARCHITECTURE

- Target Application Characteristics known beforehand? Yes
- Characterize the application domain
- Determine a balance b/w flexibilty vs efficiency
- Tune the architecture according

Categorizing the “Computation”

- Control Random Logic Implementation
- Datapath Processing of Multi-bit Data
- Conflicting Requirements???

Datapath Element Requirements

- Operates on Word Slices or Bit Slices
- Produces multi-bit outputs
- Requires many smaller elements to produce each bit output i.e. multiple small LUTs

Control Logic Requirements

- Produces a single output from many single bit inputs
- Benefits from large grain LUT as logic levels gets reduced

Logic Block Design: Considerations

- “How much” of “what kinds” of computations to support?
- Tradeoff: Generality vs Specialization

So what do we have to support?

- Datapath functionality, in particular arithmetic, is dominant in DSP.
- The datapath functions have different bit-widths.
- DSP designs heavily use multiplexers of various size. Thus, an efficient mapping of multiplexers should be supported.
- DSP functions do contain random logic. The amount of random logic varies per design.
- Some DSP designs use wide boolean functions.

DSP Building Blocks

- Some techniques widely used to achieve area-speed efficient DSP implementations
- Bit Serial Computations
- Routing Efficient
- Bit Level Pipelining Increases throughput even more
- Digit Serial Computation
- Combining “Area efficiency” of bit-serial and with “Time efficiency” of Bit-parallel

Classes of DSP-optimized FPGA Architectures

- Architectures with Dedicated DSP Logic
- Homogeneous
- Hetrogeneous
- Globally Homogeneous, Locally Heterogenous
- Architectures of Coarser Granularity
- With DSP Specific Improvements (e.g. Carry Chains, Input Sharing, CBS)

Bit-Serial FPGA with SR LUT

- Bit-serial paradigm suites the existing FPGA so why not optimize the FPGA for it!
- Logic block to support efficient implementation of bit-serial data path and bit-level pipelining
- LUTs can be used for combinational logic as well as for Shift Registers

The Proposed Bit Serial Logic Block Architecture

- 4x4-input LUTs and 6 flip-flops.
- The two multiplexers in front of the LUTs are targeted mainly for carry-save operations which are frequently used in bit-serial computations.
- There are 18 signal inputs and 6 signal outputs, plus a clock input.
- Feed-back inputs c2, c3, c4, c5 can be connected to either GND or VDD or to one of the 4 outputs d0, d1, d2, d3. Therefore, each LUT can implement any 4-input functions controlled by inputs a0, a1, a2, a3 or b0, b1, b2, b3.
- Programmable switches connected to inputs a4 and b4 control the functionality of the four multiplexers at the output of LUTs. As a result, 2 LUTs can implement any 5-input functions.
- The final outputs d0, d1, d2, d3 can either be the direct outputs from the multiplexers or the outputs from flip-flops. All bit-serial operators use the outputs from flip-flops; therefore the attached programmable switches are actually unnecessary. They are only present in order to implement any other logic functions other than bit-serial datapath circuits.
- Two flip-flops are added (inputs c0 and c1) to implement shift registers which are frequently used in bit-serial operations.

Digit-Serial Logic Block Architecture

- Digit–Serial Architectures process one digit (N=4 bits) at a time
- They offer area efficiency similar to bit-serial architectures and time-efficiency close to bit-parallel architectures
- N=4 bits can serve as an optimal granularity for processing larger digit sizes (N=8,16 etc)

Digit-Serial Building Blocks

A Pipelined Digit-Serial Unsigned Multiplier For Y=8 bits

Signed Digit-Serial Multiplier

A Digit-Serial Signed Booth’s Pipelined Multiplier with Y=8

Examples of Implementations

N=4 Unsigned

Multiplier

N=4 Signed

Multiplier

Two N=2

Multipliers

Bit-Level

Pipelined

Mixed-Grain Logic Block Architecture

- Exploits the adder inverting property
- Efficiently implements both datapath and random logic in the same logic block design

Adder Inverting Property

Full Adder and Equations Showing

The Inverting Property

An optimal structure derived from

the property

Computation Field Programmable Architecture

- A Heterogeneous architecture with cluster of datapath logic blocks
- Separate LUT Based Logic Blocks for supporting random logic mapping
- Basic Logic Block called a Partial Adder Subtraction Multiplier (PASM) Module

Conclusions

- Traditional general purpose FPGA inefficient for data path mapping
- Logic blocks with DSP specific enhancements seem a promising solution
- Coarse Grained Logic can achieve better application mapping for data path but sacrifice flexibility
- Dedicated Blocks (Multipliers) increase performance but also increases cost significantly

Conclusions

- PDSPs with embedded FPGA can achieve a good balance between performance and power consumption
- So…Which approach is the best? No single best exists

Suitability of Approaches

- Highly computationally intensive applications with large amounts of parallelism can use platform FPGAs where often large resources are required and power consumption is not an issue.
- Here cost/function will be lowest

Suitability of Approaches

- Field Programmable Logic based coprocessors can benefit from coarse grained blocks where most control functions are implemented by the PDSP itself

Suitability of Approaches

- Higher flexibility and lower cost can be achieved with logic blocks with DSP specific enhancements but flexibility to implement control logic in an efficient manner.

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