SoC Clock Synchronizers Project

SoC Clock Synchronizers Project Characterization Presentation Elihai Maicas Harel Mechlovitz

Presentation Agenda: • The synchronization problem • Project motivation • Synchronization classifications • Various solutions • Our goals • Timeline

The synchnization problem • Large chips have multiple clock domains because: • Chip interfaces with several unrelated blocks • Chip has inner IPs that require different frequency • Chip size is growing, what makes it hard to design one LARGE single clock • And more…

The synchnization problem • Example: A communication Hub

The synchnization problem • When spreading out the problem, it comes to transfer data from transmitter to receiver: • Given that ckA and ckB are not from the same clock domain, there is a probability that the receiver won’t sample the data correctly • Metastability • ts/th issues • Duplicate / dropped samples

The synchnization problem • What is the probability of this unfortunate situation to occur ? • In general, the probability of synchronization failure can be calculated as follows:P(failure) = P(enter metastable state) · P(still in metastable state after tw)

The synchnization problem • Flip-flop can enter a metastable state, when its data input D changes the state during the aperture time or sampling window of the flip-flop • Probability of an input transition to occur during the sampling window is computed by dividing the apeture time ta by the clock period tcy

Project motivation • Sync problems become more and more frequent in the industry • Common knowledge is quite insufficient • Solutions are not well categorized • Too little do we know about the various solutions • Common synchronization mistakes • Some of the solutions were never looked at closely for proper correctness checking

Synchronization classification • We can classify different synchronization problems to number of groups:

Synchronization classifications • Mesochronous • Phase difference stays constant • We could have a problem if clkB came too fast after clkA (not allowing proper ts), or too slow (not allowing th)

Synchronization classifications • Plesiochronous • Phase difference drifts • ∆f< ε • Other • Every few cycles we might have a sync problem needs to be solved

Synchronization classifications • Periodic • Events are periodic, therefore enables prediction • The sychronizer can detect a conflict enough time a head for the resualt to be ready on time

Synchronization classifications • Asynchronous • Communication between two asynchronic blocks • Sampling asynchronic signals (real-world input devices) for a synchronized block • Synchronization is required when the outputs or output events depend on the order in which input events are received • Asynchronous design is sometimes selected for eliminating the need for synchronization

Various solutions • General solution • The Two-FF synchronizer AKA Brute-Force synchronizer • The first flop samples signal A • AW has a high probability of being in a metastable state • The second flop samples AW after a large waiting time allowing the metastable state to decay

Various solutions • Mesochronous solution • By delaying the clock with the actual phase difference, one of the registers will sample correctly

Various solutions • Plesiochronous solution • Using FIFO synchronizer, we can keep all timing needed for right sample

Various solutions • Periodic solution • Using prediction for shorter latency • Result (unsafe signal) is ready by the time input arrives

Various solutions • Asynchronous solution • Both clocks are aperiodic • Advantages • Lower probability of synchronization failure • Inherent flow-control

Our goals • Our main goal is to compare between various synchronization methods, with the following criteria: • Latency • Area • Power • Simplicity • Plug-n-play • Categorize the various solutions and give certain parameters for the choosing process of a synchronizer

Our goals • In addition, we will check correctness of above circuits with the following circuit:

Timeline

Q & A

SoC Clock Synchronizers Project