TinyTinyTPU

A minimal 2×2 systolic-array TPU-style matrix-multiply unit, implemented in SystemVerilog and deployed on FPGA.

This project implements a complete TPU architecture including:

• 2×2 systolic array (4 processing elements)
• Full post-MAC pipeline (accumulator, activation, normalization, quantization)
• UART-based host interface
• Multi-layer MLP inference capability
• FPGA deployment on Basys3 (Xilinx Artix-7)

Resource Usage (Basys3 XC7A35T):

• LUTs: ~1,000 (5% utilization)
• Flip-Flops: ~1,000 (3% utilization)
• DSP48E1: 8 slices
• BRAM: ~10-15 blocks
• Estimated Gate Count: ~25,000 gates

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Table of Contents

1. Project Overview
2. Quick Start
3. Simulation & Testing
4. FPGA Build & Deployment
5. Running Inference
6. Project Structure
7. Architecture Details
8. Open Source Tooling (Yosys/nextpnr)

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Project Overview

TinyTinyTPU is an educational implementation of Google's TPU architecture, scaled down to a 2×2 systolic array. It demonstrates:

• Systolic Array Architecture: Data flows horizontally (activations) and vertically (partial sums)
• Diagonal Wavefront Weight Loading: Staggered weight capture for proper systolic timing
• Full MLP Pipeline: Weight FIFO → MMU → Accumulator → Activation → Normalization → Quantization
• Multi-Layer Inference: Supports sequential layer processing with double-buffered activations

Design Philosophy

This is a minimal, educational-scale TPU designed for:

• Learning TPU architecture principles
• Understanding systolic array dataflow
• FPGA prototyping and experimentation
• Small-scale ML inference (2×2 matrices)

For production workloads, scale up the array size (e.g., 256×256 like Google TPU v1).

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Quick Start

Prerequisites

For Simulation:

• Verilator 5.022 or later
• Python 3.8+
• cocotb
• GTKWave or Surfer (for waveform viewing)

For FPGA Build:

• Xilinx Vivado 2020.1 or later (for Basys3)
• OR Yosys + nextpnr (open source alternative, see Open Source Tooling)

For Running Inference:

• Basys3 FPGA board
• USB cable for programming
• Python 3.8+ with pyserial

Installation

# Clone the repository
git clone <repository-url>
cd tinytinyTPU-co

# Set up simulation environment
cd sim
python3 -m venv venv
source venv/bin/activate  # On Windows: venv\Scripts\activate
pip install -r requirements.txt

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Simulation & Testing

Running Tests

All simulation commands must be run from the sim/ directory:

cd sim

# Run all tests
make test

# Run all tests with waveform generation
make test WAVES=1

# Run specific module tests
make test_pe
make test_mmu
make test_mlp
make test_uart
make test_tpu_system

# Run with waveforms
make test_pe WAVES=1

Test Coverage

Test File	Module	Coverage
test_pe.py	Processing Element	Reset, MAC operations, weight capture
test_mmu.py	2×2 Systolic Array	Weight loading, matrix multiply
test_weight_fifo.py	Weight FIFO	Push/pop, wraparound
test_dual_weight_fifo.py	Dual Weight FIFO	Column independence, skew timing
test_accumulator.py	Accumulator	Alignment, buffering, accumulate/overwrite modes
test_activation_func.py	Activation Function	ReLU positive/negative/zero cases
test_normalizer.py	Normalizer	Gain, bias, shift operations
test_activation_pipeline.py	Activation Pipeline	Full pipeline, saturation handling
test_mlp_integration.py	MLP Top	Multi-layer MLP inference
test_uart_controller.py	UART Controller	Command parsing, response generation
test_tpu_system.py	TPU Top	End-to-end system integration

Viewing Waveforms

# List available waveforms
make waves

# Open specific waveform
make waves MODULE=pe
make waves MODULE=mmu
make waves MODULE=mlp_top

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FPGA Build & Deployment

Hardware Connections

Basys3 Pinout:

• UART RX (B18): Receives commands from PC
• UART TX (A18): Sends responses to PC
• Clock: 100 MHz (onboard oscillator)
• Reset: Center button (BTNC, U18)
• LEDs: Status display (see fpga/README.md for LED modes)

UART Settings:

• Baud Rate: 115200
• Data Bits: 8
• Parity: None
• Stop Bits: 1

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Running Inference

Python Host Interface

The project includes a Python driver for communicating with the FPGA:

cd host

# Basic inference demo
python3 inference_demo.py

# Gesture recognition demo (requires trained model)
python3 gesture_demo.py

# Interactive test
python3 test_tpu_driver.py

Inference Demo

The inference_demo.py script demonstrates:

1. Loading weights into the TPU
2. Loading input activations
3. Executing inference
4. Reading results

Example Usage:

from tpu_driver import TPUDriver

# Connect to FPGA (adjust port as needed)
tpu = TPUDriver('/dev/ttyUSB0')  # Linux
# tpu = TPUDriver('COM3')         # Windows

# Load 2×2 weight matrix
weights = [[1, 2], [3, 4]]
tpu.write_weights(weights)

# Load 2×2 activation matrix
activations = [[5, 6], [7, 8]]
tpu.write_activations(activations)

# Execute inference
tpu.execute()

# Read results
result = tpu.read_result()
print(f"Result: {result}")

Gesture Recognition Demo

The gesture_demo.py script implements a simple gesture classifier:

• Trains a 2-layer MLP on mouse movement data
• Classifies gestures as "Horizontal" or "Vertical"
• Real-time inference on FPGA

Running the Demo:

cd host
python3 gesture_demo.py

Model Training:

cd model
python3 train.py
# Generates: gesture_model.json

UART Protocol

The TPU uses a simple byte-based UART protocol:

Commands:

• 0x01: Write Weight (4 bytes: W00, W01, W10, W11)
• 0x02: Write Activation (4 bytes: A00, A01, A10, A11)
• 0x03: Execute (start inference)
• 0x04: Read Result (returns 4 bytes: acc0[31:0])
• 0x05: Read Result Column 1 (returns 4 bytes: acc1[31:0])
• 0x06: Read Status (returns 1 byte: state[3:0] | cycle_cnt[3:0])

See host/tpu_driver.py for full protocol implementation.

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Project Structure

tinytinyTPU-co/
├── rtl/                          # SystemVerilog RTL source files
│   ├── pe.sv                     # Processing Element (MAC unit)
│   ├── mmu.sv                    # 2×2 Matrix Multiply Unit (systolic array)
│   ├── weight_fifo.sv            # Single-column weight FIFO
│   ├── dual_weight_fifo.sv       # Dual-column weight FIFO with skew
│   ├── accumulator.sv            # Top-level accumulator
│   ├── accumulator_align.sv      # Column alignment logic
│   ├── accumulator_mem.sv        # Double-buffered accumulator memory
│   ├── activation_func.sv        # ReLU/ReLU6 activation
│   ├── normalizer.sv             # Gain/bias/shift normalization
│   ├── loss_block.sv             # L1 loss computation
│   ├── activation_pipeline.sv    # Full post-accumulator pipeline
│   ├── unified_buffer.sv          # Ready/valid output FIFO
│   ├── mlp_top.sv                # Top-level MLP integration
│   ├── tpu_bridge.sv              # UART-to-MLP bridge
│   ├── uart_controller.sv         # UART command processor
│   ├── uart_rx.sv                # UART receiver
│   ├── uart_tx.sv                # UART transmitter
│   └── tpu_top.sv                # Complete TPU system
│
├── sim/                          # Simulation environment
│   ├── Makefile                  # Build and test automation
│   ├── requirements.txt          # Python dependencies
│   ├── tests/                    # cocotb Python testbenches
│   │   ├── test_pe.py
│   │   ├── test_mmu.py
│   │   ├── test_weight_fifo.py
│   │   ├── test_dual_weight_fifo.py
│   │   ├── test_accumulator.py
│   │   ├── test_activation_func.py
│   │   ├── test_normalizer.py
│   │   ├── test_activation_pipeline.py
│   │   ├── test_mlp_integration.py
│   │   ├── test_uart_controller.py
│   │   └── test_tpu_system.py
│   └── waves/                    # Generated VCD waveforms
│
├── fpga/                         # FPGA deployment files
│   ├── basys3_top.sv             # Top-level FPGA wrapper
│   ├── basys3.xdc                # Pin constraints
│   ├── build_vivado.tcl          # Automated build script
│   ├── basys3_top.bit            # Generated bitstream
│   └── README.md                 # FPGA-specific documentation
│
├── host/                         # Python host interface
│   ├── tpu_driver.py             # TPU communication driver
│   ├── tpu_compiler.py           # Model compilation utilities
│   ├── inference_demo.py          # Basic inference demo
│   ├── gesture_demo.py           # Gesture recognition demo
│   └── test_tpu_driver.py        # Driver unit tests
│
├── model/                        # ML model training
│   ├── train.py                  # Model training script
│   └── gesture_model.json        # Trained model (JSON format)
│
└── README.md                     # This file

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Architecture Details

Systolic Array Dataflow

PE00 -> PE01    Activations flow horizontally (right)
  |       |     
PE10 -> PE11    Partial sums flow vertically (down)
  |       |
acc0    acc1    Outputs to accumulator

Weight Loading (Diagonal Wavefront):

• Cycle 0: W10 → col0, no capture
• Cycle 1: W00 → col0 (capture), W11 → col1 (no capture)
• Cycle 2: W01 → col1 (capture)

Activation Flow:

• Row 0: A00 → PE00 → PE01
• Row 1: A10 → PE10 → PE11 (with 1-cycle skew)

Pipeline Stages

1. Weight FIFO: Stores weights, outputs with column skew
2. MMU (Systolic Array): Matrix multiply-accumulate
3. Accumulator: Aligns columns, double-buffered storage
4. Activation Pipeline:
   • Activation function (ReLU/ReLU6)
   • Normalization (gain × bias + shift)
   • Quantization (int8 with saturation)
5. Unified Buffer: Output FIFO with ready/valid handshaking

Multi-Layer MLP

The MLP controller manages sequential layer processing:

State Machine:
IDLE → LOAD_WEIGHT → LOAD_ACT → COMPUTE → DRAIN → TRANSFER → NEXT_LAYER → WAIT_WEIGHTS → ...

• Double Buffering: Activations ping-pong between buffers for layer-to-layer transfer
• Weight Loading: Weights loaded per layer via UART
• Pipeline Overlap: While layer N drains, layer N+1 weights can be loaded

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Open Source Tooling (Yosys/nextpnr)

Overview

While Vivado is the standard toolchain for Xilinx FPGAs, open-source alternatives exist:

• Yosys: Synthesis (RTL → netlist)
• nextpnr: Place & Route (netlist → bitstream)

Setup

Installation (Ubuntu/Debian):

# Install Yosys
sudo apt-get install yosys

# Install nextpnr (for Xilinx 7-series)
# Requires building from source - see nextpnr documentation
git clone https://github.com/YosysHQ/nextpnr.git
cd nextpnr
cmake . -DARCH=xilinx
make -j$(nproc)
sudo make install

Installation (macOS):

brew install yosys
# nextpnr requires manual build

Building with Yosys/nextpnr

Step 1: Synthesis (Yosys)

cd fpga

# Create synthesis script
cat > synth.ys << 'EOF'
# Read RTL files
read_verilog -sv ../rtl/pe.sv
read_verilog -sv ../rtl/mmu.sv
read_verilog -sv ../rtl/weight_fifo.sv
read_verilog -sv ../rtl/dual_weight_fifo.sv
read_verilog -sv ../rtl/accumulator_align.sv
read_verilog -sv ../rtl/accumulator_mem.sv
read_verilog -sv ../rtl/accumulator.sv
read_verilog -sv ../rtl/activation_func.sv
read_verilog -sv ../rtl/normalizer.sv
read_verilog -sv ../rtl/loss_block.sv
read_verilog -sv ../rtl/activation_pipeline.sv
read_verilog -sv ../rtl/unified_buffer.sv
read_verilog -sv ../rtl/mlp_top.sv
read_verilog -sv ../rtl/uart_rx.sv
read_verilog -sv ../rtl/uart_tx.sv
read_verilog -sv ../rtl/uart_controller.sv
read_verilog -sv ../rtl/tpu_bridge.sv
read_verilog -sv ../rtl/tpu_top.sv
read_verilog -sv basys3_top.sv

# Set top module
hierarchy -top basys3_top

# Synthesize
synth_xilinx -top basys3_top -family xc7

# Write netlist
write_verilog basys3_top_synth.v
write_json basys3_top.json
EOF

# Run synthesis
yosys synth.ys

Step 2: Place & Route (nextpnr)

# Generate bitstream
nextpnr-xilinx \
    --xdc basys3.xdc \
    --json basys3_top.json \
    --write basys3_top_routed.json \
    --fasm basys3_top.fasm

# Generate bitstream (requires Xilinx tools or open-source fasm2bit)
# Note: fasm2bit conversion may require Xilinx tools or open-source alternatives

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Building with Vivado

The project includes a TCL script for automated Vivado builds:

cd fpga

# Build bitstream (synthesis + implementation + bitgen)
vivado -mode batch -source build_vivado.tcl

# Expected build time: 5-10 minutes
# Output: basys3_top.bit

Build Script Details:

• Creates Vivado project: vivado_project/tinytinyTPU_basys3
• Synthesizes all RTL files from ../rtl/
• Implements design with timing constraints
• Generates bitstream: basys3_top.bit
• Creates reports: utilization, timing, DRC

Resource Utilization (Post-Implementation):

• Check vivado_project/tinytinyTPU_basys3.runs/impl_1/utilization_post_impl.rpt
• Check vivado_project/tinytinyTPU_basys3.runs/impl_1/timing_summary_post_impl.rpt

Programming the FPGA

Via Vivado Hardware Manager (GUI):

1. Connect Basys3 board via USB
2. Open Vivado
3. Open Hardware Manager
4. Auto-connect to target
5. Program with basys3_top.bit

Via Command Line:

vivado -mode tcl
open_hw_manager
connect_hw_server
open_hw_target
set_property PROGRAM.FILE {basys3_top.bit} [get_hw_devices xc7a35t_0]
program_hw_devices [get_hw_devices xc7a35t_0]

Via OpenOCD (Alternative):

# If using OpenOCD with Digilent cable
openocd -f interface/ftdi/digilent_jtag_hs3.cfg -f target/xc7a35t.cfg
# Then use GDB or other tools to program

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Limitations & Considerations

Current Status:

• Yosys synthesis works well for most SystemVerilog constructs
• nextpnr supports Xilinx 7-series but may have timing/routing challenges
• Bitstream generation (fasm2bit) may require Xilinx tools or open-source alternatives

Recommendations:

• For development: Use Vivado for reliable builds
• For open-source exploration: Use Yosys for synthesis, verify with Vivado
• For production: Stick with Vivado until open-source toolchain matures

Future Work:

• Create automated Yosys/nextpnr build script
• Document fasm2bit conversion process
• Benchmark open-source vs. Vivado results

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Troubleshooting

Simulation Issues

Verilator Errors:

• Ensure Verilator 5.022+ is installed
• Check SystemVerilog syntax (use make lint)

Test Failures:

• Run with WAVES=1 to generate waveforms for debugging
• Check sim/test_output.log for detailed error messages

FPGA Build Issues

Synthesis Errors:

• Check RTL files are in rtl/ directory
• Verify SystemVerilog syntax (Vivado may be stricter than Verilator)

Timing Violations:

• Check timing_summary_post_impl.rpt
• May need to add pipeline stages or reduce clock frequency

Place & Route Failures:

• Check utilization reports
• Verify constraints in basys3.xdc

Hardware Issues

UART Not Working:

• Verify COM port: ls /dev/ttyUSB* (Linux) or Device Manager (Windows)
• Check baud rate: 115200
• Verify TX/RX pins in constraints file

LEDs Not Responding:

• Check bitstream programmed correctly
• Verify reset button (center button)
• Check switch settings for LED modes (see fpga/README.md)

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Contributing

Contributions welcome! Areas for improvement:

• Additional test coverage
• Performance optimizations
• Documentation improvements
• Open-source toolchain support
• Larger array sizes

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License

MIT License

Copyright (c) 2026 Alan Ma, Abiral Shakya

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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References

• Google TPU Paper
• TPU Architecture Blog
• Systolic Arrays
• Basys3 Reference Manual

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Acknowledgments

• Inspired by Google's TPU architecture (thank you Cliff and Richard for your time!)
• The boys from the TinyTPU team!!
• Edmund and the Yosys / Symbiotic EDA crew
• Stanford FAF for the support, funding, and community!
• Princeton ECE Dept for the Basys 3 to play around with :)