Three Flop Synchronizer for High Frequency

Clock domain crossing into high frequency domains requires careful Mean Time Between Failures (MTBF) management. A standard two stage synchronizer is insufficient if the clock period is shorter than the resolution time required to settle metastability. In these extreme frequency environments, additional synchronization stages are required to maintain system reliability.

The module receives an asynchronous single bit signal and synchronizes it to the destination clock domain. To support reuse across different clock domains, the number of synchronization stages must dynamically adjust based on the provided TARGET_FREQ_MHZ parameter. If the target frequency is 400 MHz or less, the design requires 2 stages. If the frequency is strictly greater than 400 MHz and less than or equal to 800 MHz, it requires 3 stages. If the frequency is strictly greater than 800 MHz, it requires 4 stages.

• Clock edge: posedge clk
• Reset type: Asynchronous active low (rst_n)
• Output values on reset: All synchronizer stages, including sync_out, must go to 0 when rst_n is 0
• Priority rules: Asynchronous reset has priority over all other inputs and immediately clears the synchronization chain

Cycle by cycle trace for the default 3 stage configuration (TARGET_FREQ_MHZ = 500): Cycle 1: rst_n=0, async_in=0 → internal=[0,0,0], sync_out=0 Cycle 2: rst_n=1, async_in=1 → internal=[1,0,0], sync_out=0 Cycle 3: rst_n=1, async_in=1 → internal=[1,1,0], sync_out=0 Cycle 4: rst_n=1, async_in=1 → internal=[1,1,1], sync_out=1 Cycle 5: rst_n=1, async_in=0 → internal=[0,1,1], sync_out=1 Cycle 6: rst_n=1, async_in=0 → internal=[0,0,1], sync_out=1 Cycle 7: rst_n=1, async_in=0 → internal=[0,0,0], sync_out=0

{ "signal": [
  { "name": "clk", "wave": "p......" },
  { "name": "rst_n", "wave": "01....." },
  { "name": "async_in", "wave": "01..0.." },
  { "name": "sync_out", "wave": "0...1.0" }
], "head": { "text": "Three stage synchronization of an asynchronous pulse." } }

| Parameter | Default | Description | |-----------|---------|-------------| | TARGET_FREQ_MHZ | 500 | Target clock frequency in MHz used to calculate stages |

| Signal | Direction | Width | Description | |--------|-----------|-------|-------------| | clk | input | 1 | Destination clock, positive edge triggered | | rst_n | input | 1 | Asynchronous active low reset | | async_in | input | 1 | Asynchronous input signal | | sync_out | output | 1 | Synchronized output signal |

Constraints

• Output sync_out must be driven by the final flip-flop in the synchronization chain; do not use combinational logic for the output
• All flip-flops must be reset asynchronously to 0 when rst_n is 0
• The number of stages must be exactly 2 for TARGET_FREQ_MHZ <= 400, 3 for 400 < TARGET_FREQ_MHZ <= 800, and 4 for TARGET_FREQ_MHZ > 800

Topics

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