Design of watchdog circuit based on CPLD technology

Designing a watchdog circuit using CPLD (Complex Programmable Logic Device) technology involves creating a hardware-based mechanism to monitor the system's health and reset the system if it becomes unresponsive. A watchdog timer (WDT) is commonly used in embedded systems to detect and recover from software hangs or crashes.

Below is a step-by-step guide to designing a watchdog circuit using a CPLD:

1. Watchdog Circuit Overview
Purpose: The watchdog circuit monitors the system's activity. If the system fails to "feed" the watchdog (i.e., send a periodic signal), the watchdog resets the system.

Key Components:

• Timer: Counts down from a predefined value.
• Reset Logic: Generates a reset signal if the timer expires.
• Feed Signal: A periodic signal from the system to reset the timer.

2. CPLD Design Steps
Step 1: Define Inputs and Outputs
Inputs:

• clk: System clock.
• feed: Signal from the system to reset the watchdog timer.
• reset: External reset signal (optional).

Outputs:

system_reset: Reset signal to the system.

Step 2: Design the Timer

• Use a counter to implement the timer.
• The counter decrements on each clock cycle.
• If the counter reaches zero, the watchdog triggers a reset.

Step 3: Implement the Feed Mechanism

• The feed signal resets the counter to its initial value.
• If the feed signal is not received within a specific time, the counter expires, and the system is reset.

Step 4: Reset Logic
Generate the system_reset signal when the counter expires.

3. HDL Implementation (Verilog Example)
Here’s an example of a watchdog timer implemented in Verilog:

verilog

module watchdog_timer (
    input wire clk,          // System clock
    input wire feed,          // Feed signal from the system
    input wire reset,         // External reset signal
    output reg system_reset   // Reset signal to the system
);

    // Parameters
    parameter TIMER_WIDTH = 8; // Width of the timer counter
    parameter TIMER_MAX = 255; // Maximum value of the timer

    // Internal signals
    reg [TIMER_WIDTH-1:0] counter; // Timer counter

    // Timer logic
    always @(posedge clk or posedge reset) begin
        if (reset) begin
            counter <= TIMER_MAX; // Reset the counter
            system_reset <= 0;    // Deactivate reset signal
        end else if (feed) begin
            counter <= TIMER_MAX; // Reset the counter on feed signal
            system_reset <= 0;    // Deactivate reset signal
        end else if (counter == 0) begin
            system_reset <= 1;    // Activate reset signal
        end else begin
            counter <= counter - 1; // Decrement the counter
            system_reset <= 0;      // Deactivate reset signal
        end
    end

endmodule

4. Explanation of the Code
Timer Counter:

• The counter is initialized to TIMER_MAX (e.g., 255).
• It decrements on every clock cycle.

Feed Signal:

When the feed signal is high, the counter is reset to TIMER_MAX.

Reset Logic:

If the counter reaches zero, the system_reset signal is activated.

External Reset:

The reset signal initializes the counter and deactivates the system_reset signal.

5. Simulation and Testing
Use a simulation tool (e.g., ModelSim) to test the watchdog timer.

Test cases:

1. Normal operation: The feed signal is received periodically, and the counter resets.
2. Failure case: The feed signal is not received, and the system_reset signal is triggered.
3. External reset: Verify that the reset signal initializes the counter.

6. Integration with the System

• Connect the system_reset output to the reset pin of the microcontroller or system.
• Ensure the system periodically toggles the feed signal during normal operation.

7. Advantages of Using a CPLD

• Reliability: Hardware-based watchdog timers are more reliable than software-based ones.
• Customizability: The timer duration and behavior can be easily customized in the CPLD.
• Low Latency: The CPLD responds quickly to system failures.

8. Example Application

• Embedded Systems: Use the watchdog timer to recover from software hangs in microcontrollers.
• Industrial Control: Ensure critical systems reset in case of failures.
• Automotive: Monitor the health of electronic control units (ECUs).

By following this design, you can create a robust watchdog circuit using CPLD technology to enhance the reliability of your system.