How to Synchronize a Pausible Clock to a Reference. Robert Najvirt, Andreas Steininger

Transcription

1 How to Synchronize a Pausible Clock to a Reference Robert Najvirt, Andreas Steininger

2 GALS Communication The communication between two (locally) synchronous modules has inevitable potential for metastable upsets Except for smartly designed systems utilizing assumptions on the clock sources Two ways of handling metastability: Time safe vs. value safe ASYNC, Robert Najvirt 2

3 Time safe solution Allow for fixed resolution time Use result no matter what Allows to use free running clock sources A non-zero probability of failure always remains ASYNC, Robert Najvirt 3

4 Value safe solution Wait for metastability to resolve Use result when ready Allows to design circuits free of metastable upsets Requires stretching of clock period ASYNC, Robert Najvirt 4

5 Ideal solution Use free running clock sources Communicate without risk of metastable upsets Unfortunately impossible Synchronization impossibility Pausing and starting a free running clock impossibility Compromise solution: Use ring oscillator referenced with a free running clock ASYNC, Robert Najvirt 5

6 The standard pausible clock Two loops Timing loop guarantees minimum pulse widths Switching loop handles pausing and restarting by delaying a transition ASYNC, Robert Najvirt 6

7 Adding a reference to a pausible clock Calibration of the delay line (ext. reference) Calibration ASYNC, Robert Najvirt 7

8 Adding a reference to a pausible clock Our approach: Add another loop for direct synchronization ASYNC, Robert Najvirt 8

9 Analysis Pausing Behavior when being paused not affected by third loop: The mutex delays the switching loop while paused Metastability is correctly handled in a value safe manner ASYNC, Robert Najvirt 9

10 Analysis Starting First clock transition at an arbitrary phase The next aligned by synchronization loop to coincide with corresponding ones from the reference However, minimum pulse widths still dictated by timing loop Inevitable metastability of C2 handled by Schmitt-Trigger in a value safe manner ASYNC, Robert Najvirt 10

11 Analysis The missing case When, after a metastabe period, the mutex decides against a pause request Clock Pause Ack ASYNC, Robert Najvirt 11

12 Analysis The missing case When, after a metastabe period, the mutex decides against a pause request Clock Pause Ack Equivalent to two successive pause requests ASYNC, Robert Najvirt 12

13 Analysis Synchronization loop delay 1) Direct propagation Ref Clk Sync ASYNC, Robert Najvirt 13

14 Analysis Synchronization loop delay 2) Synchronized propagation Ref Clk Sync ASYNC, Robert Najvirt 14

15 Analysis Synchronization loop delay 3) Metastability Ref Clk Sync ASYNC, Robert Najvirt 15

16 Analysis The three loops together Timing loop adds lower bound Recursion to describe synchronization phase Stable fixed point Unstable fixed point ASYNC, Robert Najvirt 16

17 Observations When being paused and during resynchronization, the circuit behaves just like the standard pausible clock generator In steady state operation, its clock output is synchronous with the reference Phase of generated clock vs. reference clock unrestricted in synchronization phase, synchronization time unbounded Decoupling and other measures can make desynchronization arbitrarily improbable ASYNC, Robert Najvirt 17

18 The disadvantages Requires both a delay line and a clock source Both clock sources must be active The guaranteed minimum clock period is determined by the delay line circuit must tolerate it ASYNC, Robert Najvirt 18

19 Conclusion The circuit is Simple Value safe When stabilized generating a synchronous clock to the reference Providing the same guarantees as a standard pausible clock Thank you! ASYNC, Robert Najvirt 19