UART Implementation Methods
FPGA engineers implementing UART face three choices — each suited to a different project context and level of control. Choosing the wrong approach wastes time: a vendor IP that cannot be customized, or a full processor SoC for a task that needs only a 100-line state machine. This episode provides the decision framework and maps the course structure to each approach.
Three Approaches to UART on FPGA
┌─────────────────────────────────────────────────────────┐
│ UART on FPGA — 3 Methods │
├─────────────────┬──────────────────┬────────────────────┤
│ Method 1 │ Method 2 │ Method 3 │
│ Vendor IP Core │ Soft Processor │ Custom RTL │
│ │ + UART IP │ Design │
├─────────────────┼──────────────────┼────────────────────┤
│ Easy & fast │ Software-driven │ Full control │
│ AXI-compatible │ Highly scalable │ Best performance │
│ Limited flex │ High resources │ Longer dev time │
└─────────────────┴──────────────────┴────────────────────┘
Method 1: Vendor IP Core
Use a verified UART IP from Xilinx/Intel — add via IP Catalog with a few clicks
Pros
- Quick integration: drag-and-drop from IP Catalog
- AXI-compatible: connects directly to MicroBlaze, Zynq ARM via AXI4-Lite
- Pre-verified: stable, vendor-tested IP
- Interrupt support: built-in hardware interrupt
- Low resource usage: optimized implementation
Cons
- Limited flexibility: hard to modify baud rate or interface behavior
- No software stack: requires a processor for control logic
- Vendor lock-in: Xilinx/Intel specific
Method 2: Soft Processor + UART Peripheral
Connect UART IP to a soft-core CPU (MicroBlaze / Nios II) via AXI bus
Pros
- Software-driven control: implement complex logic in C code
- OS support: FreeRTOS or bare-metal
- Scalable: connect multiple peripherals via AXI interconnect
Cons
- High resource usage: CPU + BRAM + peripherals
- Latency: software scheduling adds delay
- Toolchain complexity: requires both Vivado and Vitis
MicroBlaze ──AXI4-Lite──► AXI UART Lite ──► TX/RX
Method 3: Custom RTL Design
Implement UART TX/RX directly in Verilog/SystemVerilog
Pros
- Full control: customize timing, bit width, parity, and more
- Lightweight: only the logic you need → minimal resource usage
- Best performance: cycle-accurate operation
- Portable: works on any FPGA platform (Xilinx, Intel, Lattice…)
- Educational: builds deep understanding of FSM and timing design
Cons
- Development time: requires design, simulation, and verification
- Debug complexity: needs ILA or oscilloscope to troubleshoot
Full Comparison
| Feature | Vendor IP | Soft Processor + IP | Custom RTL |
|---|---|---|---|
| Ease of Use | Very high (drag-and-drop) | Medium (HW+SW integration) | Low (requires expertise) |
| Customization | Limited | High (via software) | Complete control |
| Performance | High, deterministic | Depends on SW efficiency | Highest, cycle-accurate |
| Resource Usage | Low | High (CPU + memory) | Low to moderate |
| AXI Integration | Native | Native via processor | Manual |
| Learning Value | Low | Medium | High |
Quick prototype → Vendor IP Software control needed → Soft Processor Learning / optimization / portability → Custom RTL
Decision Guide
| Requirement | Best Method |
|---|---|
| Working in 30 minutes | Vendor IP (Method 1) |
| Multi-peripheral SoC | Soft Processor (Method 2) |
| Understand UART deeply | Custom RTL (Method 3) |
| Vendor-agnostic, portable | Custom RTL (Method 3) |
| Strict latency or resource limit | Custom RTL (Method 3) |
Lecture Structure
Episodes 5–7: Custom RTL Design
(Baud Generator, TX FSM, RX FSM, Testbench, FPGA implementation)
Episodes 9–13: MicroBlaze + AXI UART Lite IP
(Block Design, Vitis C Code, Co-Simulation)
Episode 15: Final comparison and recap
Episode 4 Summary
- Method 1 (Vendor IP): fast, pre-verified, native AXI support
- Method 2 (Soft Processor): C code control, highly extensible
- Method 3 (Custom RTL): full control, best performance, highest learning value
- This lecture covers both RTL design and MicroBlaze approaches
Next Episode
Episode 5: Custom RTL Design Requirements → TX FSM → RX FSM → Verilog implementation
Key Takeaways
- Vendor IP (Method 1): fastest path to a working UART, but opaque and tied to the vendor’s synthesis tool
- Soft Processor + UART IP (Method 2): enables C software control and AXI peripheral integration, but requires significant BRAM/LUT resources for the CPU
- Custom RTL (Method 3): complete control, smallest footprint, vendor-agnostic — and the only method that builds a real understanding of UART timing and FSM design
- This course covers both approaches (RTL in EP05–08, MicroBlaze in EP09–14) — comparing them on the same hardware is the most instructive outcome
Code and References
- RTL source repository: https://github.com/Easy-FPGA/easyfpga-uart-core-sv.git
- YouTube: https://www.youtube.com/@easy-fpga