1. Synchronizers for Low Latency Clock Domain Transfer
Presented by Dmitry Verbitsky

2. Exactly Matched Frequencies
All domains operate from the same clock
Skews may be arbitrary
Skew may vary due to clock jitter, power supply noise, temperature variations, etc.

3. Rationally Related Frequencies
Clocks are derived from a common source
Clock frequencies are rational multiples of each other

4. Closely Matched Frequencies
Clocks are derived from independent sources
Clock are very closely matched in frequencies

5. Arbitrary Frequencies
Clocks are derived from independent sources
Clock can be of any arbitrary frequencies
Assume that clock frequencies are relatively stable – satisfied by nearly all synchronous designs

6. Clock Mismatch Sources
Difference in insertion delays between the two independent clock grids
Reference clock distribution networks
Accumulated phase error between independent PLL sources
Primary clock distribution networks

7. Clock Mismatch Sources(2)
PVT variations
Variation in parameters of the wires
Different sizing of each buffering stage
Presence of adjacent wires and the amount of switching activities between them

8. Interfacing Synchronous and Asynchronous Systems
To achieve a sufficiently small probability of synchronization failure of a single
asynchronous input, all that is required is to allow a sufficiently long time for the
synchronizer to exit the metastable state.

9. Pipelined Synchronization
Instead of transferring W bits every 1/E seconds, one can transfer kW bits every k/E seconds in order to allow k times as much time for synchronization.

10. STARI (Self Timed At Receiver’s Input)
Transmitter and receiver are mesochronous
If the FIFO is initialized to be roughly half full, then throughout operation, the capacity of the FIFO remains roughly half full
The need to check overflow and underflow is avoided
Doesn’t require the absolute synchronization of purely synchronous methods
Doesn’t require the explicit flow control mechanism of purely asynchronous methods

11. MinSTARI FIFO reduces to latch latch-X and a latch controller
Irrespective of the phase relations between FT and FR, FX can always be generated in such a way as to reliably transfer data from input to output

12. Latch Controller State Diagram
Initially starts at 0
Goes to state TR only when has seen both FT and FR events
2 possible cycles:

13. The Latch Controller Circuit

14. Transmitter Clock Event

15. Receiver Clock Event

16. Generate FX

17. Reset aT and aR

18. Reset c and FX

19. Description of the Solution
Low latency, high-speed interface through the integration of three major components:
Data rate matching FIFO
Pointer tracking circuit
Digital filter

20. Data rate synchronization FIFO
Implemented as a circular queue of a given depth
Read and write pointers are expected to exist on different clock grids
FIFO is acting as a buffer between the two domains

21. Data rate synchronization FIFO(2)
For mesochronous systems no need to track if FIFO full or empty
No additional logic is required to ensure the FIFO pointers are running at similar frequences since both clocks will be derived from the same reference

22. Data rate synchronization FIFO(3)
For heterochronous systems whose clocks are ratios of one another, a control circuit is required to reduce the frequency of the faster clock and ensure both pointers are running at the same average data rate

23. Data rate synchronization FIFO(4)
For plesiochronous systems the allowable frequency mismatch is limited by the tracking response time of the final design implementation
In all clocking topologies, any differences between read and write pointers clock rates must be controlled to ensure they do not exceed tracking bandwidth of the final design

24. Pointer Tracking Circuit
By minimizing the number of unread entries in FIFO the latency is reduced
Slow clock drift assumption relaxes the response time requirement and permits to remove the latency of the tracking circuit from the data path

25. Pointer Tracking Circuit(2)
Possible simplifications:
No need to evaluate pointer separation on every clock
One can choose to evaluate pointers at a convenient time to remove ambiguity as they wrap around the FIFO structure
Pointer information, which is delayed while being locally synchronized, can be treated as the current state of the pointer in the other domain

26. Pointer Tracking Circuit(3)
Signal for pointer tracking is the MSB of the pointer
Ensures that the signal will be safely captured through a simple synchronizer chain of flops in the other clock domain
By detecting the falling edge of the MSB, one has a clear indication of when the pointer has wrapped to entry 0 of the FIFO

27. Pointer Tracking Circuit(4)
Designed to maintain the pointers at a specific, user programmable separation
Tracking accuracy is a function of ratios between the clock domains, the digital filter and the pointer sampling rate
If the design is failing in a particular configuration, the pointer separation can be increased to achieve functional operation

28. Pointer tracking circuit(5)
Relevant equations:
F = Number of FIFO entries
S = Desired/Programmed separation
E = Expected local pointer location
A = Actual local pointer location
D = Pointer comparison result
If the local domain is read pointer:
E = F – S and D = A – E
If the local domain is write pointer:
E = S and D = E – A

29. Tracking Logic in RdPtr Domain
In this example, E = 6 and, when the Eval signal asserts, A = 5
Thus D = 5-6=-1 and the pointers are detected as being too far apart.

30. Timing Diagram Detecting Pointers Are Drifting Apart

31. Pointer Adjustment One clock is nominally faster than another
Ptrs too close(D>0) - suppress one Fast Clock
Ptrs too far (D<0) - allow one extra Fast Clock
Neither clock is nominally faster
Ptrs too close(D>0) - suppress one clock on the Write Pointer
Ptrs too far (D<0) - suppress one clock on the Read Pointer

32. Digital Filter Reasons: Example:
Tracking logic is susceptible to metastability on the synchronizer chain
Data rate matching circuit may produce non-uniform clock patterns
Example:
Make adjustment only if in m samples, there were n detections of the pointers too far, (or conversely too close) where n is an integer

33. Sampling Uncertainties
By design any missed event is guaranteed to be capture on the very next clock
This translates to one FIFO entry of uncertainty
The other main contributor to uncertainty is the irregular duty cycle of the throttled clock

34. Uncertainty Due to Sample Jitter

35. Tracking Response Time
F = Number of FIFO entries
S = Number of samples required by the digital filter
l = FIFO throttled data rate, typically the clock period of the slow domain
f = Maximum clock edge mismatch. The degree of phase mismatch between the throttled clock and the data-rate clock
y = Maximum percentage of allowable clock mismatch

36. Tracking Response Time(2)
y=l/((F*S)+(F-1))*l+f)*100
F*S – total sample time
(F-1) – worst case latency to the first sample

37. Tracking Response Time(3)
Simplification : f = l
y = l/((F*S)+(F-1)+1)*l)*100 =
=1/(F*(S+1))*100
By pipelining the throttled clock pattern which controls the faster domains’ pointer, the equation is modified to:
y = 1/(F*(S+1)+P)*100

38. Tracking Response Time(4)
Example:
8 Entry FIFO (F)
3 Sample Filter (S)
1 Clock Uncertainty (f)
8 Clock Pipeline (P)
y = 2.5% or PPM

39. Further Refinements
Looking at the pointer separation slightly earlier in time can predict a pointer collision before it actually occurs. For example, invert the clock on the synchronizer chain
Optimization of digital filter by more accurate tracking of pointers drift to avoid pointer collision when reducing their separation

40. Conclusions
Design effectively reduces the latency across two clock domains in systems where the clock drift is slow but unbounded in duration
The digital nature of design allows the implementation to scale in frequency without the potential risk of self-timed circuits
The only true constraint on its use is that the domain clock frequencies must be known prior to activating the FIFO to ensure that pointers are advancing within the bandwidth of the tracking logic

41. A Predictive Synchronizer for Periodic Clock Domains

42. Synchronizer Architecture

43. Synchronizer Overview
Receives the two clocks and manages safe data transfer both ways
Produces SEND and RECV control outputs to both domains, indicating when it is safe to receive and send new data on both sides, avoiding data misses and duplicates due to mismatched clock frequencies

44. Clock Conflicts Prediction
Can be predicted in advance due to periodic nature of the two clocks
Let’s assume we have a conflict at time zero
The next conflict occurs when there exist some N and K such that:
N*TLOCAL=K*TEXT

45. Clock Conflicts Prediction(2)
Find the smallest D such that:
TLOCAL+ D = M* TEXT
(N-1)*TLOCAL = K*TEXT – TLOCAL
(N-1)*TLOCAL = (K-M) *TEXT + D
Conflict prediction is achieved by creating a Predictive Clock which is a version of the external clock delayed by D

46. Clock Conflicts Prediction(3)
Predicted and Local clocks conflict one TLOCAL cycle before the imminent conflict of the External and Local clocks
Sampling the input (which is affected by RxCK) is delayed by a keep-out time TKO, where TKO>dZ

47. Conflict Detector
FF1 and FF2 effectively sample Clk2 d time after and d time before the rising edge of Clk1, respectively
Either FF may become metastable
One half cycle of Clk1 is allotted for metastability resolution
If Clk2 has risen during the 2d detection period, the top AND gate is enabled and Conflict output is generated

48. Computing Clock Cycle Time
Circuit starts with minimal delay and increases (or decreases) delay until it is equal to a full cycle
The clock divider and flip-flop provide a loop delay (of two local clock cycles)
Time resolution of Conflict detector must be larger than adjustment step
Once the lower delay line has converted to TLOCAL, its programming code is copied to the upper delay line

49. Computing Clock Cycle Time(2)
The TLOCAL unit safely computes cycle time with precision dL
DLL convergence time is:

50. Clock Predictor
“Predicted clock” output provides a copy of external clock, delayed by D, one local cycle time in advance
Loop delay must be the maximum of the two clock cycles

51. Rate Reducer
The delay introduced by Rate Reducer between successive adjustments is 4TLOCAL+4TEXT
Total tuning time of Programmable Delay 1 is:

52. Clock Predictor Precision
Clock Predictor safely generates a delayed version of the external clock that periodically precedes its original version by TLOCAL with precision

53. Conflict Prevention Circuit
The dC-conflict detector produces the Keep-Out signal upon a dC conflict of the local and predicted clocks
The Clock Select circuit produces RxCK depending on Keep-Out
RxCK is either the original local clock (when there is no predicted conflict) or the TKO delayed local clock (when a conflict is predicted)

54. Prediction Timing Diagram

55. TKO constraint Definition: Theorem: R: The rising edge event of RxCK
D: Event R when Keep-Out = 1
Theorem:
If L1 and P occur within dC time of each other, then D and E are safely separated by at least dZ of each other

56. TKO constraint(2)
Proof:
Need to confirm that:
By definition:

57. Avoiding Misses and Duplicates

58. Duplicate and Miss Control Circuit

59. Duplicate and Miss Control Circuit(2)

60. Conclusions
Synchronizer takes advantage of periodic nature of clocks in order to predict potential conflicts in advance, and to conditionally employ an input sampling delay to avoid such conflicts
Adjusts automatically to wide range of clock frequencies
Avoids sampling duplicate data or missing any input

61. References
Wade L. Williams, Philip E. Madrid, Scott C. Johnson, "Low Latency Clock Domain Transfer for Simultaneously Mesochronous, Plesiochronous and Heterochronous Interfaces," async, pp , 13th IEEE International Symposium on Asynchronous Circuits and Systems (ASYNC'07), 2007
J.N. Seizovic, “Pipeline Synchronization”, Proceedings of the 1st International Symposium on Advanced Research in Asynchronous Circuits and Systems, pp , 1994.
A. Chakraborty and M.R. Greenstreet, “Efficient Self-Timed Interfaces for Crossing Clock Domains,” Proceedings 9th IEEE International Symposium on Asynchronous Circuits and Systems (ASYNC’03), pp , 2003.
U. Frank, T. Kapschitz and R. Ginosar, “A Predictive Synchronizer for Periodic Clock Domains,” J. Formal Methods in System Design (special issue on Formal Methods for Globally Asynchronous Locally Synchronous Design), 28(2): , 2006