How to Implement FPGA Matrix Multiplier Assignments with Single and Multi-MAC Designs

Dissecting the Assignment Requirements

Let’s take the attached assignment as a representative example. Here’s what it expects:

• Multiply two 4×4 matrices A and B to produce C.
• Use 8-bit unsigned integers for inputs and a 20-bit result.
• Start with a single MAC processor: one multiplication and accumulation per clock cycle.
• Upgrade to four MAC units for parallel processing.
• Store inputs and outputs in RAM (not testbench-loaded).
• Build a data generator for two specific test cases.
• Compute an XOR checksum of the result.
• Track and display performance in clock cycles.
• Control outputs using switches and 7-segment displays on the DE1-SoC board.

This isn’t just a programming assignment. It’s a hardware systems design problem, and approaching it as such is the key to success.

Tools You’ll Need

To complete this assignment, you should have:

• Verilog HDL proficiency
• Familiarity with DE1-SoC FPGA board
• Quartus Prime software for compilation
• ModelSim/Verilog simulator (optional but useful)
• A basic understanding of FSMs, RAM blocks, 7-segment display logic, and timing

Project Structuring and Documentation

Don't underestimate the value of proper documentation and file organization. Create separate modules for:

• mac_unit.v – your core multiplier-accumulator.
• fsm_controller.v – controls loading, computing, and checksum.
• ram_a.v, ram_b.v, ram_c.v – memory blocks.
• checksum.v – calculates XOR.
• display_driver.v – handles 7-segment logic.
• top_module.v – integrates all pieces.

This modular structure not only helps during development but also makes your project easier to debug, present, and upgrade later (like moving from 1 MAC to 4 MACs).

Designing the Core Logic: From FSMs to MAC Units

The core logic lies in your control structure and how effectively you pipeline the data between different stages. Let's now zoom into the core parts of the system design.

Matrix Multiplication with One MAC

Using one MAC unit means your system will operate serially—just one multiplication and one accumulation per cycle. That’s 16 (rows) × 4 (cols) × 4 (partial products) = 256 cycles for full computation.

Key considerations:

• Multiply A[i][k] * B[k][j] and accumulate in a register.
• Loop through rows and columns using internal counters.
• Carefully manage memory access to prevent read-after-write hazards.

Sample Verilog outline:

Scaling to Four MAC Units

When you move to four MACs, your datapath becomes parallel:

• Use 4 multipliers in parallel, each handling one set of A[i][k] * B[k][j].
• Accumulate partial sums simultaneously.
• Requires wider RAM read widths (e.g., 32-bit to fetch 4 bytes).

This part drastically improves performance but increases complexity—especially in memory interfacing, control signals, and result alignment.

Memory and ROM Design

You’ll use:

• RAM A for matrix A – 16 words × 8 bits.
• RAM B for matrix B – 16 words × 8 bits.
• RAM C for output – 16 words × 20 bits.

ROM-based input generator must test edge cases:

• 0 × 0 (tests zeroing logic),
• 128 × 128 (midpoint case),
• 255 × 255 (max multiplication value).

You may encode these using hardcoded ROM modules or .mif files if supported in Quartus.

Building Intelligent Control: FSMs, Checksums, and Performance Counters

Once you’ve got your MAC unit(s) and RAM interface in place, it’s time to build the brains of your design—the FSM.

FSM Stages in Detail

Here’s a suggested FSM sequence:

1. Idle State – Wait for the start signal.
2. Load A and B – Load matrices into RAM from ROM.
3. Multiply – Perform the MAC operations.
4. Write to RAM C – Store results.
5. Checksum – Read C, compute XOR.
6. Done – Output result.

Make sure your FSM transitions only on clock edges, and include state indicators in your Verilog for debugging (e.g., LEDs).

Checksum Calculation Logic

Checksum logic is straightforward but needs precision:

• XOR all 16 20-bit values from RAM C.
• Use a loop counter and a cumulative XOR register.
• Output result on 7-segment displays based on switch state.

Make sure this occurs after the computation phase, not during.

User Interface and Optimization: Switches, Displays, and Pipelining

No FPGA assignment is complete without user interaction—and this one leverages switches and 7-segment displays to show both checksum and performance.

Switch-Controlled Outputs

Use the SW inputs on the DE1-SoC board to toggle modes:

• SW[0] – Display XOR checksum
• SW[1] – Display performance counter
• SW[2] – Choose test input A/B pair

Implement this via simple case logic in Verilog:

Performance Counter Logic

Start counting only during the MAC stage. Use a separate register:

if (in_mac_stage) cycle_counter <= cycle_counter + 1;

When SW is set, this value is shown on the display in hexadecimal format.

Optimizations and Extra Credit Ideas

If your instructor allows extensions or you want to stand out:

• Add pipelining to reduce cycle time per MAC.
• Use handshaking signals for RAM communication (e.g., valid/ready).
• Introduce a debug LED array that reflects FSM states in real time.
• Add input multiplexing to expand to larger matrices like 8×8 or 16×16.

Wrapping It All Together: Testing, Debugging, and Deployment

Finally, once the Verilog is written, simulated, and synthesized—it's time to test your design on the actual FPGA hardware. This is where many students stumble. Here's how to avoid common pitfalls:

• Simulate extensively before deployment. Use ModelSim to test FSM transitions, MAC logic, memory reads/writes.
• Debounce switch inputs to prevent glitches.
• Use LEDs for internal state monitoring during early testing.
• Compare hardware results with a known software reference (e.g., MATLAB or Python) for checksum verification.
• Make use of incremental compilation in Quartus to speed up rebuilds.

Conclusion

Solving matrix multiplier processor assignments on FPGA is no small feat. It requires you to:

• Architect a reliable datapath.
• Design FSMs that are functionally correct and cycle-accurate.
• Manage multiple RAMs with precise timing.
• Integrate MAC units (serial or parallel) efficiently.
• Handle display logic for user interaction.

When approached the right way, these assignments become a rewarding experience that takes your digital design skills to the next level. They're not about just "getting the right output," but about engineering a system that functions correctly in a hardware environment—one clock cycle at a time.

So the next time you get an assignment like this, remember: it’s not just a project—it’s a miniature processor design challenge. And now, you’re equipped to solve it like a pro.