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The destination clock domain latches the data as soon as the request signal is received and sends an acknowledgment signal back to the source clock domain.The source clock domain updated the data in the bus as soon as the acknowledgment signal is received. the request signal is passed through an m-ff synchronizer to the destination clock domain. The source clock latches updated data onto the source FF, then raise a request command to the destination clock. Handshake signals is just an extension of mux based synchronization with an additional acknowledgment line. clock domain crossing (CDC) is on a data bus.source clock domain is slower than destination clock domain.We can extend the idea of MUX based synchronizers to get Handshake based synchronizers. With this method we can be sure that the data that reaches the destination clock domain is stable and is not skewed in time. A mux synchronizer eliminates this problem by performing CDC on only a control signal which indicates to the destination clock domain that there is new data available on the bus. This will lead the destination flops to register incorrect data. Data is stable for at least m+1 clock cycles.Ĭlock domain crossing (CDC)on multiple bits using m-ff synchronizers can lead to data being skewed in time.Clock domain crossing (CDC)is on a data bus.The source clock domain is slower than destination clock domain.This technique is used when the following criteria is satisfied. A grey encoded control signal has a hamming distance of one and hence any error due to CDC will keep the control in the previous state rather than moving to an illegal state This is because, CDC only ensures that you are latching a stable signal at the destination clock domain, it does not ensure that you are latching the correct data. This attribute places the destination flops as close as possible.įor multi bit clock domain crossing (CDC) using m-FF synchronizer, it is preferable to grey encode the control data. In Xilinx devices you can also add the attribute:Īttribute ASYNC_REG of destff1 : signal is ”TRUE” Īttribute ASYNC_REG of destff2 : signal is ”TRUE” Here the conditions to be satisfied is that Input Data of the m-flip-flop Synchronizer should be stable for at least m+1 clocks of the destination clock. This idea can be further extended to get m-FF synchronizers. The idea here is that by keeping 2 FF in the destination clock domain, the signal has enough time for the meta-stability to settle down. clock domain crossing (CDC) is on a control signal (either single bit or multibit).īy satisfying this criteria we can be sure that the destination clock captures any possible changes in signal without loss.the source clock domain is slower than destination clock domain.This Technique can be used if the following criteria is satisfied. Using Xilinx specific clock domain crossing (CDC) tools.Using FIFOs (First In First Out memories).Here a meta-stable condition could cause the design to activate wrong data-paths or move the design to an invalid state.Ĭommon methods for synchronizing data between clock domains are: In FPGA latching a meta-stable signal can cause entire designs to come to halt.įor example let us consider a case statement If a signal is passed between two clock domains then there is a chance that the signal could go meta stable, i.e a state where the signal is has not settled to either a high or low (1 or 0). But, 100MHz and 59MHz clocks (whose phase relationship changes over time) are in two separate clock domains. For example, a clock and its derived clock (via a BUFR, Divider) are in the same clock domain because they have a constant phase relationship. This Section covers the following topics:Ī clock domain is a part of a design that has a clock that operates on a single clock and is often asynchronous to, or has a variable phase relationship with, another clock in the design. CDC Synchronization Techniques – Discusses the synchronization techniquesįPGA Clock Domain Crossing (CDC) Background.Background – Prepares the background required for further discussions.This document is organized into the following sections: Improper clock domain crossing can cause the design to stop working at random times and hence can be disastrous if left unchecked. This document provides an overview of the importance of clock domain crossing (CDC) and introduces the reader to methods and techniques for taking care of clock domain crossing so that the design meets all functional requirements for a stable design.