IP Core Reset Interface

When you generate an IP core for a design under test (DUT), HDL Coder™ creates internal reset‑synchronization logic that generates a global reset to reset both the DUT and the interface logic. HDL Coder generates this synchronization logic and configures it based on the model reset settings. The sources driving the global reset synchronizer depend on whether the IP core includes a register interface. When a register interface is present, HDL Coder combines the relevant reset inputs before passing them to the synchronizer. When no register interface is present, the synchronizer uses only the external IP core reset. The IP core reset interface is closely related to the IP core clock interface. For more information, see IP Core Clock Interface.

Reset Signals in IP Core With Register Interfaces

The register interface consists of logic for:

  • Communication with external IP by using the AXI4-Lite or AXI4 protocol

  • Address decoding and registers accessible through an AXI4 or AXI4-Lite interface

The AXI4 Protocol module is reset using AXI4_ARESETN, while the DUT and IP core registers are reset by a global reset signal. This global reset combines three sources: IPCORE_RSTN, AXI4_ARESETN, and a soft reset triggered by writing to the Reset Register at address 0x0. These reset signals can be synchronous or asynchronous. Asynchronous resets may cause metastability when de-asserted during the clock latching window. To prevent this, HDL Coder automatically inserts reset synchronization logic, shown as the Reset Sync block, which aligns the global reset signal to the IP core clock domain.

The diagram below illustrates the clock and reset connections for the register interface and DUT, along with the DUT enable and soft reset registers.

Reset Signals in IP Core Without Register Interfaces

When your application does not require an AXI4 register interface, set the Generate default register interface parameter to off. For more information on how to generate an IP core without register interface, see Generate Board-Independent HDL IP Core from Simulink Model.

In this configuration, the IP core reset signal is the sole reset source and serves as the input to the internal reset synchronizer that resets the DUT logic. This figure shows the structure of an HDL IP core generated without a register interface.

Reset Synchronization Logic

The reset signals can be either synchronous or asynchronous. Using asynchronous reset signals can result in potential metastability issues in flip-flops when the reset de-asserts in the latching window of the clock. The reset synchronization logic prevents generation of possible metastable values when combining the reset signals.

The reset synchronization logic synchronizes the global reset signal to the IP core clock domain. When you generate an IP core, the IP core contains a Reset Sync block that represents the reset synchronization logic. The Reset Sync block contains two back-to-back flip-flops that are synchronous to the IPCore_CLK signal. The flip-flops de-asserts the reset signal two clock cycles after the IPCore_CLK signal becomes high. This synchronous de-assertion avoids generation of a global reset signal that has possible metastable values.

Synchronous or Asynchronous Resets

The reset synchronization logic works differently depending on whether you specify the Reset type model configuration parameter as Synchronous or Asynchronous on the model. This table shows the differences between the generated HDL code for the reset synchronization logic when you generate the IP core with synchronous and asynchronous reset signals.

  • If your Reset type is Asynchronous, the synchronization logic asserts the reset signal asynchronously and de-asserts the reset signal synchronously.

  • If your Reset type is Synchronous, the synchronization logic asserts and de-asserts the reset signal synchronously.

Reset type is Asynchronous

Reset type is Synchronous

... 
...
reg_reset_pipe_process : PROCESS (clk, reset_in)
  BEGIN
    IF reset_in = '1' THEN
      reset_pipe <= '1';
    ELSIF clk'EVENT AND clk = '1' THEN
      IF enb = '1' THEN
        reset_pipe <= const_0;
      END IF;
    END IF;
  END PROCESS reg_reset_pipe_process;

  reg_reset_delay_process : PROCESS (clk, reset_in)
  BEGIN
    IF reset_in = '1' THEN
      reset_out <= '1';
    ELSIF clk'EVENT AND clk = '1' THEN
      IF enb = '1' THEN
        reset_out <= reset_pipe;
      END IF;
    END IF;
  END PROCESS reg_reset_delay_process;
END rtl;

...
...
reg_reset_pipe_process : PROCESS (clk)
  BEGIN
    IF clk'EVENT AND clk = '1' THEN
      IF reset_in = '1' THEN
        reset_pipe <= '1';
      ELSIF enb = '1' THEN
        reset_pipe <= const_0;
      END IF;
    END IF;
  END PROCESS reg_reset_pipe_process;

  reg_reset_delay_process : PROCESS (clk)
  BEGIN
    IF clk'EVENT AND clk = '1' THEN
      IF reset_in = '1' THEN
        reset_out <= '1';
      ELSIF enb = '1' THEN
        reset_out <= reset_pipe;
      END IF;
    END IF;
  END PROCESS reg_reset_delay_process;
END rtl;

Active-High or Active-Low Resets

HDL Coder generates different reset synchronization logic depending on whether you set the Reset asserted level model configuration parameter to Active-high or Active-low. The table shows the synchronization logic and the corresponding HDL code for both reset levels.

Reset asserted level is Active-high

Reset asserted level is Active-low

Reset synchronization logic for Active-high reset

Reset synchronization logic for Active-low reset

...
...
reg_reset_pipe_process : PROCESS (clk)
  BEGIN
    IF clk'EVENT AND clk = '1' THEN
      IF reset_in = '1' THEN
        reset_pipe <= '1';
      ELSIF enb = '1' THEN
        reset_pipe <= const_0;
      END IF;
    END IF;
  END PROCESS reg_reset_pipe_process;

  reg_reset_delay_process : PROCESS (clk)
  BEGIN
    IF clk'EVENT AND clk = '1' THEN
      IF reset_in = '1' THEN
        reset_out <= '1';
      ELSIF enb = '1' THEN
        reset_out <= reset_pipe;
      END IF;
    END IF;
  END PROCESS reg_reset_delay_process;
 
...
...
reg_reset_pipe_process : PROCESS (clk_in)
  BEGIN
    IF clk_in'EVENT AND clk_in = '1' THEN
      IF reset_in = '0' THEN
        reset_pipe <= '0';
      ELSIF enb = '1' THEN
        reset_pipe <= const_1;
      END IF;
    END IF;
  END PROCESS reg_reset_pipe_process;

  reg_reset_delay_process : PROCESS (clk_in)
  BEGIN
    IF clk_in'EVENT AND clk_in = '1' THEN
      IF reset_in = '0' THEN
        reset_out <= '0';
      ELSIF enb = '1' THEN
        reset_out <= reset_pipe;
      END IF;
    END IF;
  END PROCESS reg_reset_delay_process;

HDL Coder computes the Reset_Before_Sync signal depending on whether you generate IP core with register interface or without register interface:

  • If you generate an IP core without a register interface, the Reset_Before_Sync is the same as the IP core reset signal.

  • If you generate an IP core with a register interface, HDL Coder determines the Reset_Before_Sync signal by using the IP core reset, AXI4 interconnect reset, and soft reset signals.

See Also

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