The simple CPU v1a FPGA is an entry-level soft processor designed for education, rapid prototyping, and lightweight control tasks. It delivers a clear, synthesizable Verilog or VHDL core that runs on low-cost FPGAs while providing a familiar register-transfer interface.
By using a compact datapath and straightforward instruction set, this core keeps resource usage low, making it ideal for hobbyist boards, small form-factor modules, and learning-oriented designs.
Simple CPU v1a Overview and Key Specifications
Quick reference to help you compare versions, judge resource needs, and understand typical use cases at a glance.
| Attribute | Value | Notes |
|---|---|---|
| Core Name | Simple CPU v1a | Soft processor for FPGA |
| Implementation | Synthesizable RTL (Verilog/VHDL) | Ports easily across families |
| Typical LUT Usage | 800–2000 LUTs | 7-series and Cyclone equivalents |
| Clock Frequency | 20–100 MHz device-dependent | Measured on common development kits |
| Interrupts | Non-maskable, timer, external | Basic interrupt controller included |
| Debug Support | UART, JTAG bridge, CSR readback | Enables printf-style diagnostics |
| Typical Use Cases | Education, small controllers, I/O hubs | Not for high-performance workloads |
Design Philosophy and Architecture
The Simple CPU v1a emphasizes clarity and portability rather than peak performance. Its compact 32-bit datapath and classic load-store architecture make the control flow easy to follow for students and new FPGA developers.
Registers, ALU operations, and memory access are exposed through readable modules, which keeps the RTL maintainable and simplifies integration with custom peripherals.
Implementation and Tool Flow
Because the core is delivered as human-readable RTL, you can target multiple FPGA families from Xilinx, Intel, Lattice, and Gowin. A recommended implementation flow includes synthesis, place-and-route, and timing analysis using the vendor-provided or open-source toolchain.
Add appropriate timing constraints for your platform, generate the configuration bitstream, and program the device. Minimal board support is required, typically only a clock, reset, and optional UART for debugging.
Integration with Peripherals and Bus Interface
Simple CPU v1a connects to memories and peripherals through a basic bus interface. You can attach RAM, ROM, and simple input/output modules using a straightforward address and chip-select scheme.
- Wishbone-style bus for lightweight, predictable integration
- Memory-mapped I/O for GPIO, timers, and UART
- Easy to extend with custom accelerators or coprocessors
- Supports basic boot flow from embedded memory or external flash
Performance, Optimization, and Power
Expect single-cycle instructions for the core ALU and simple load-store operations when the design runs at nominal frequency. Pipelining stages are balanced to keep timing closure straightforward on mid-range FPGAs.
Dynamic power scales with frequency and toggling activity, while static power remains low due to the relatively small configuration. You can further reduce power by disabling unused peripheral clocks through control registers.
Getting Started and Best Practices
Begin with the reference platform that matches your target FPGA, and use the provided example projects to verify basic functionality before integrating custom logic.
- Start with the default clock and verify basic register operations on a breadboard or simulator
- Add and test one peripheral at a time to isolate integration issues
- Apply official FPGA timing constraints and run static timing analysis before deployment
- Document custom instruction encoding and bus interfaces for long-term maintainability
FAQ
Reader questions
Can I use Simple CPU v1a in a commercial product?
Yes, you can use Simple CPU v1a in commercial products, subject to the license terms provided with the core and any third-party components you include.
What is the maximum clock frequency on a Spartan-7 device?
On a typical Spartan-7 part, Simple CPU v1a can reach 60–80 MHz after standard synthesis and place-and-route, depending on the specific device speed grade and I/O constraints.
How do I add a custom instruction to the core?
To add a custom instruction, modify the ALU control and decode logic, extend the opcode map, and update the register file interface, then regenerate the core and verify timing for your target device.
Does Simple CPU v1a include a floating-point unit?
No, Simple CPU v1a does not include a hardware floating-point unit; you can implement lightweight software floating-point or connect a dedicated FPU peripheral via the bus if needed.