How to Use Lower Clock Speed FPGA?

FPGAs are often associated with high-speed designs, but many applications (e.g., embedded control, sensor interfacing, or low-power systems) don’t require high clock speeds. Using an FPGA at lower clock frequencies can save power, reduce complexity, and improve signal integrity. Here’s how to optimize an FPGA design for lower clock speeds:

1. Choose the Right FPGA

• Low-power FPGA families (e.g., Lattice iCE40, GOWIN GW1N, Microsemi IGLOO2, Xilinx Spartan-7) are optimized for lower speeds and reduced power consumption.

• Avoid high-performance FPGAs (e.g., Xilinx Virtex or Intel Stratix) unless necessary—they consume more static power even at low clock speeds.

2. Clock Management

Use Internal Clock Dividers

Most FPGAs have PLLs (Phase-Locked Loops) or DCMs (Digital Clock Managers) that can generate lower clock frequencies from a high-speed oscillator.

// Example: Xilinx MMCM/PLL to generate 1 MHz from 100 MHzclk_wiz_0 clk_gen (
  .clk_in1(100MHz),
  .clk_out1(1MHz),
  .reset(reset));

• If no PLL is available, use a counter-based clock divider (but avoid gated clocks to prevent glitches).

Avoid Global Clock Buffers for Slow Signals

• Global clock networks have high fanout but consume extra power. For very slow clocks (<1 MHz), consider using regular routing.

3. Power Optimization

Dynamic Power Reduction

• At lower clock speeds, dynamic power (P = C·V²·f) decreases linearly with frequency.

• Clock gating (disabling unused logic) helps, but modern FPGAs have built-in power-saving modes.

Static Power Reduction

• Use FPGAs with low static power (e.g., flash-based FPGAs like Microsemi ProASIC3).

• Lower the core voltage if supported (e.g., 1.2V instead of 1.8V).

4. Timing Constraints & Relaxed Requirements

Set Correct Timing Constraints

• Even at low clock speeds, define constraints (e.g., create_clock -period 1000ns -name clk_slow for 1 kHz).

• Relaxed constraints allow the tools to optimize for area/power instead of speed.

Avoid Over-optimization

• No need for pipelining or high-frequency optimizations (e.g., retiming).

• Use larger logic blocks (e.g., 16-bit adders instead of serialized ones).

5. I/O Considerations

Slow Slew Rates

Configure I/O pins for low slew rate (reduces noise and power at the cost of speed).

(* IO_STANDARD = "LVCMOS33", SLEW = "SLOW" *) output my_signal;

Schmitt-Trigger Inputs

• For slow, noisy signals (e.g., buttons), enable Schmitt-trigger inputs to improve noise immunity.

6. Use Sleep Modes (If Available)

• Some FPGAs (e.g., Lattice MachXO3) support sleep modes where unused logic is powered down.

• Example: Wake up on an external interrupt.

7. HDL Coding for Low Speed

Avoid high-speed constructs (e.g., DDR registers).

Use polling instead of interrupts for simple control logic (reduces complexity).

Example: A slow state machine for a sensor interface:

always @(posedge clk_1MHz) begin
  case(state)
    IDLE: if (sensor_ready) state <= READ;
    READ: begin
      data <= sensor_value;
      state <= IDLE;
    end
  endcaseend

8. Verification & Testing

• Simulate at the target clock speed to catch issues (e.g., metastability in asynchronous crossings).

• Use a low-speed clock in hardware tests (e.g., 1 Hz for LED debugging).

When to Use a Low-Clock-Speed FPGA?

• Battery-powered devices (IoT, wearables).

• Legacy interfaces (e.g., SPI at 100 kHz, UART at 9600 baud).

• Control-heavy (not compute-heavy) designs.

Alternative: Use a Microcontroller

If the design is very slow (<1 MHz) and simple, a low-cost MCU (e.g., STM32, PIC) might be more efficient than an FPGA.

Conclusion

Using an FPGA at lower clock speeds involves:

1. Selecting a power-optimized FPGA.

2. Generating clean, divided clocks.

3. Relaxing timing constraints.

4. Optimizing I/O and power settings.

5. Writing efficient (but not speed-optimized) HDL.

This approach saves power, reduces cost, and simplifies design while maintaining flexibility.