Contents
How does clock domain crossing work?
Clock Domain Crossing (CDC) Techniques A dual flip flop synchronizer is a circuit where two Flip Flops are connected back to back in the destination clock domain. Once the destination domain receives the request, it sends an “ack” signal to the source domain which uses 2-FF synchronizer to synchronize it.
What are different ways to synchronize between two clock domains?
Common methods for synchronizing data between clock domains are:
- Using m-FF based synchronizers.
- Using MUX based synchronizers.
- Using Handshake signals.
- Using FIFOs (First In First Out memories).
- Using Toggle synchronizers.
- Using Xilinx specific clock domain crossing (CDC) tools.
What is clock domain crossing in FPGA?
Clock Domain Crossing (CDC) is a common occurrence in a multiple clock design. CDC occurs whenever data is transferred from a flip-flop driven by one clock to a flip-flop driven by another clock. CDC issues could cause significant amount of failures in both ASIC and FPGA devices.
Why is it necessary to gating a clock?
Clock gating is a popular technique used in many synchronous circuits for reducing dynamic power dissipation, by removing the clock signal when the circuit is not in use. Clock gating saves power by pruning the clock tree, at the cost of adding more logic to a circuit.
How does two flop synchronizer work?
A two flip-flop synchronizer As said earlier, the two flop synchronizer converts a signal from source clock domain to destination clock domain. The input to the first stage is asynchronous to the destination clock. So, the output of first stage (Q1) might go metastable from time to time.
What causes Metastability?
Metastability can occur when signals are transferred between circuitry in unrelated or asynchronous clock domains. The mean time between metastability failures is related to the device process technology, design specifications, and timing slack in the synchronization logic.
What is double flopping?
A metastable state is one in which the output of a Flip-Flop inside of your FPGA is unknown, or non-deterministic. When a metastable condition occurs, there is no way to tell if the output of your Flip-Flop is going to be a 1 or a 0. A metastable condition occurs when setup or hold times are violated.
Why is gating the clock bad?
Do not use an AND or OR gate (or any more complex combinatory term) to derive a clock signal from another clock signal. The reason for that rule is that race conditions among the multiple inputs of the combinatory term may cause multiple clock edges (glitches) where you expect only one clock edge.
How do you implement clock gating?
What is the proper way to implement clock gating in RTL? The clock gating signal should only toggle when the latch is closed, otherwise there is a chance for glitches and metastability issues. For an active high latch, the gating signal should toggle on the falling edge of the clock. Rising edge for active low latches.
What does it mean to check clock domain crossings?
Checking that asynchronous signals cross clock domains correctly means identifying the data and control paths, and ensuring that the receive clock domain data flow is controlled by a multiplexer with a select line fed by a correctly synchronized control line.
Can you cross clock domains in an FPGA?
Crossing clock domains inside of an FPGA is a common task, but it is one that many digital designers have trouble with. Problems can occur if the digital designer does not understand all of the details involved in crossing from one clock domain into another. A single clock domain entails all of the Flip-Flops that are driven by one clock.
How to pass signals from one clock domain to another?
A single clock domain entails all of the Flip-Flops that are driven by one clock. In this article, the first two sections describe how to pass individual signals from one clock domain to another. The last section goes into detail about how to use a FIFO to send large amounts of data between two clock domains.
Can a clock have more than one clock domain?
Indeed, all of the registers set within an entire design on the same edge of a same clock form a single clock domain. Combinatorial logic based upon this register set is also within this same clock domain. As an example, Fig 1 shows four separate clock domains within a design.