HARD_SYNC - HARD_SYNC - 2025.2 English - Primitive: Metastability Hardened Registers

Primitive: Metastability Hardened Registers

PRIMITIVE_GROUP: REGISTER
PRIMITIVE_SUBGROUP: METASTABILITY

Introduction

Metastability hardened registers are generally used for asynchronous domain crossings to synchronize signals, which can incur set up or hold time violations. The LATENCY attribute can be set to configure a dual or triple register synchronizer configuration. The HARD_SYNC must be manually placed using the XDC LOC constraint.

Port Descriptions

Port	Direction	Width	Function
CLK	Input	1	Clock input
DIN	Input	1	Data input
DOUT	Output	1	Data output

Design Entry Method

Instantiation	Yes
Inference	No
IP and IP Integrator Catalog	No

Available Attributes

Attribute	Type	Allowed Values	Default	Description
INIT	BINARY	1'b0, 1'b1	1'b0	Specifies the initial values of the HARD_SYNC output upon completion of configuration and release of GSR.
IS_CLK_INVERTED	BINARY	1'b0 to 1'b1	1'b0	Specifies to use the programmable inversion on the CLK input to change the clock from active-High to active-Low.
LATENCY	DECIMAL	2, 3	2	Specifies whether to use a two stage or three stage synchronizer. Three stage will exhibit better MTBF characteristics however have an additional clock cycle of latency.

VHDL Instantiation Template

Unless they already exist, copy the following two statements and paste them before the entity declaration.

Library UNISIM;
use UNISIM.vcomponents.all;


-- HARD_SYNC: Metastability Hardened Registers
--            Spartan UltraScale+
-- Xilinx HDL Language Template, version 2025.2

HARD_SYNC_inst : HARD_SYNC
generic map (
   INIT => '0',            -- Initial values, '0', '1'
   IS_CLK_INVERTED => '0', -- Programmable inversion on CLK input
   LATENCY => 2            -- 2-3
)
port map (
   DOUT => DOUT, -- 1-bit output: Data
   CLK => CLK,   -- 1-bit input: Clock
   DIN => DIN    -- 1-bit input: Data
);

-- End of HARD_SYNC_inst instantiation

Verilog Instantiation Template


// HARD_SYNC: Metastability Hardened Registers
//            Spartan UltraScale+
// Xilinx HDL Language Template, version 2025.2

HARD_SYNC #(
   .INIT(1'b0),            // Initial values, 1'b0, 1'b1
   .IS_CLK_INVERTED(1'b0), // Programmable inversion on CLK input
   .LATENCY(2)             // 2-3
)
HARD_SYNC_inst (
   .DOUT(DOUT), // 1-bit output: Data
   .CLK(CLK),   // 1-bit input: Clock
   .DIN(DIN)    // 1-bit input: Data
);

// End of HARD_SYNC_inst instantiation

Related Information

UltraScale Architecture Configurable Logic Block User Guide (UG574)

HARD_SYNC - HARD_SYNC - 2025.2 English - Primitive: Metastability Hardened Registers - UG1704