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How to Communicate Between CPLD and FPGA?

Communication between a CPLD (Complex Programmable Logic Device) and an FPGA (Field-Programmable Gate Array) depends on the required speed, complexity, and available I/O resources. Below are common methods, along with their advantages and trade-offs.

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1. Parallel Bus (Fast & Simple)
Method:
Use multiple I/O pins for data transfer (e.g., 8-bit, 16-bit, or 32-bit parallel bus).

Add control signals like:

  • CLK (synchronization clock)
  • WR (write enable)
  • RD (read enable)
  • CS (chip select)
  • ACK (acknowledge handshake, if needed).

Pros:
✅ High-speed data transfer (good for burst transfers).
✅ Simple to implement (no complex protocols).

Cons:
❌ Requires many I/O pins (not ideal for pin-limited designs).
❌ Susceptible to noise (if long traces are used).

Example Connection:

FPGA (Master) CPLD (Slave) ------------------------- DATA[7:0] <-------> DATA[7:0] ADDR[3:0] --------> ADDR[3:0] WR --------> WR RD --------> RD CS --------> CS CLK --------> CLK 
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2. Serial Communication (Saves Pins)
Methods:
A. SPI (Serial Peripheral Interface)

  • 4-wire protocol (SCLK, MOSI, MISO, SS).
  • Supports full-duplex communication.
  • Typical speeds: 1–100 MHz.

B. I²C (Inter-Integrated Circuit)

  • 2-wire protocol (SCL, SDA).
  • Supports multi-master mode.
  • Slower (~400 kHz – 3.4 MHz).

C. UART (Asynchronous Serial)

  • 2-wire (TX, RX) or 1-wire (half-duplex).
  • No clock needed (baud rate must match).
  • Simple but slower (typical speeds: 9600–115200 bps).

Pros:
✅ Fewer pins required (good for small CPLDs).
✅ Standardized protocols (easy to interface with MCUs).

Cons:
❌ Slower than parallel bus.
❌ SPI/I²C require clock synchronization.

Example (SPI Connection):

FPGA (Master) CPLD (Slave) ------------------------- SCLK --------> SCLK MOSI --------> SDI MISO <------- SDO SS --------> CS 
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3. Dual-Port RAM / FIFO (High-Speed Buffering)
Method:

  • Use a shared memory block (if FPGA has embedded RAM).
  • FPGA writes data, CPLD reads (or vice versa).
  • Handshake signals (FULL, EMPTY, WR_EN, RD_EN) synchronize access.

Pros:
✅ High throughput (good for streaming data).
✅ Reduces real-time synchronization issues.

Cons:
❌ Requires RAM resources (may not be available in small CPLDs).

Example (FPGA-to-CPLD FIFO):

verilog // FPGA writes data always @(posedge clk) begin if (wr_en && !fifo_full) begin fifo_data <= data_to_send; end end // CPLD reads data always @(posedge clk) begin if (rd_en && !fifo_empty) begin received_data <= fifo_data; end end 
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4. LVDS / Differential Signaling (Noise Immunity)
Method:

  • Use LVDS (Low-Voltage Differential Signaling) for high-speed serial links.
  • Example: Xilinx SelectIO, Intel LVDS.
  • Requires serializer/deserializer (SerDes) if using high speeds (>1 Gbps).

Pros:
✅ High-speed & noise-resistant (good for long PCB traces).
✅ Fewer pins than parallel bus.

Cons:
❌ More complex to implement.
❌ CPLD may not support LVDS (check datasheet).

Example (LVDS Connection):

FPGA (TX) CPLD (RX) ------------------------- TX_P --------> RX_P TX_N --------> RX_N 
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5. Custom Protocol (Flexible but Complex)
Method:
Define a custom handshake protocol (e.g., source-synchronous).

Example:

  • FPGA sends data + strobe signal.
  • CPLD latches data on strobe edge.

Pros:
✅ Optimized for specific needs.

Cons:
❌ Requires careful timing analysis.

Example (Custom Strobe-Based Transfer):

verilog // FPGA sends data always @(posedge clk) begin if (send_data) begin data_bus <= data; strobe <= ~strobe; // Toggle strobe end end // CPLD receives data always @(posedge strobe) begin received_data <= data_bus; end 
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Comparison of Methods

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How to Choose?

  1. Need high speed? → Parallel bus or LVDS.
  2. Limited pins? → SPI, I²C, or UART.
  3. Buffered data transfer? → Dual-port RAM/FIFO.
  4. Noise immunity needed? → LVDS.

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