1. Penn ESE370 Fall2011 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 24: November 4, 2011 Synchronous Circuits

2. Today Clocking –Latches –Registers Timing discipline Penn ESE370 Fall2011 -- DeHon 2

3. Clocking Latches, Registers Penn ESE370 Fall2011 -- DeHon 3

4. Why Clocked Circuits? Synchronize external events Reuse logic –FSM –Pipelining Synchronize internal use of logic Penn ESE370 Fall2011 -- DeHon 4

5. Challenge Logic paths have different delays –E.g. different output bits in an adder Delay of signal data dependent –E.g. length of carry Delay is chip dependent –E.g. Threshold Variation Delay is environment dependent –E.g. Temperature Penn ESE370 Fall2011 -- DeHon 5

6. Challenge Logic paths have different delays Delay of signal data dependent Delay is chip dependent Delay is environment dependent Proper behavior depends on inputs being coordinated –Match the inputs that should interact Penn ESE370 Fall2011 -- DeHon 6

7. Discipline Add circuit elements to –hold values –and change at coordinated point Control when changes seen by circuit Only have to make sure to wait long enough for all results Decouple –timing of signal change –from timing of signal usage Penn ESE370 Fall2011 -- DeHon 7

8. Synchronous Discipline Add state elements (registers, latches) Compute –From state elements –Through combinational logic –To new values for state elements Penn ESE370 Fall2011 -- DeHon 8

9. What does this do? Penn ESE370 Fall2011 -- DeHon 9

10. Latch  =0  Out=In  =1  Out=Out  transitions 0  1 Out holds value Penn ESE370 Fall2011 -- DeHon 10

11. Latch In pass-through mode (  =0), –acts like buffer In latch mode (  =1), –holds last value given Penn ESE370 Fall2011 -- DeHon 11

12. Latch In pass-through mode (  =0), –acts like buffer In latch mode (  =1), –holds last value given Timing Requirements? Penn ESE370 Fall2011 -- DeHon 12

13. Latch Timing Must present input value sufficiently before the  transitions 0  1 –Must have time to propagate and charge Out –About how long is that in this case? Setup Time (t su ) – must setup latch input prior to pass  hold transition Penn ESE370 Fall2011 -- DeHon 13

14. Latch Timing Must not change input before switched over to hold state –How long in this case? –Takes time for inverter to charge before hold path enabled. Penn ESE370 Fall2011 -- DeHon 14

15. Latch Timing Must not change input before switched over to hold state Hold Time (t hold )– must hold data input until pass  hold transition complete Penn ESE370 Fall2011 -- DeHon 15

16. What happens here? Penn ESE370 Fall2011 -- DeHon 16

17. Observe Latch alone –In flow-through mode half of cycle –Can still get flow-through, combinational cycles Penn ESE370 Fall2011 -- DeHon 17

18. Multiple Latch Discipline Open latches at disjoint times At all times one latch on every path is closed Penn ESE370 Fall2011 -- DeHon 18

19. Register Two back-to-back latches –Open one latch at a time –Having one of each on every cycle breaks up combinational cycle Penn ESE370 Fall2011 -- DeHon 19

20. Register Pass  hold on input latch samples value Hold  pass on output latch presents stored value to circuit Penn ESE370 Fall2011 -- DeHon 20 Master and Slave latches

21. Register How long from  0 rise to output? How long from  1 fall to output –Part of clk  output (t clk-q ) Penn ESE370 Fall2011 -- DeHon 21

22. Clock Signal Can we use a single signal for clock? Penn ESE370 Fall2011 -- DeHon 22

23. Clock Issues Possible failure modes? –Flow through during transition? –Loading on clock phases –Delay in compute  1 ? Penn ESE370 Fall2011 -- DeHon 23

24. What if it’s early?  0 early Penn ESE370 Fall2010 -- DeHon 24

25. What if it’s late?  0 late Penn ESE370 Fall2011 -- DeHon 25

26. What does this do? Penn ESE370 Fall2010 -- DeHon 26

27. What does this do? Outputs when –Input 0? –Input 1? Can ever both be on? What does output waveform look like? Penn ESE370 Fall2010 -- DeHon 27

28. Clocking Discipline Penn ESE370 Fall2011 -- DeHon 28 Identify: setup, hold, clk->Q, logic evaluation

29. Clocking Discipline Penn ESE370 Fall2011 -- DeHon 29

30. Clocking Discipline Follow discipline of combinational logic broken by registers Compute –From state elements –Through combinational logic –To new values for state elements As long as clock cycle long enough, –Will get correct behavior Penn ESE370 Fall2011 -- DeHon 30

31. Next Week Monday: guest lecturer (andre away) Tuesday: Andre away (no office hour) Wednesday: Midterm –No lecture –Midterm 7-9pm in Towne 303 –New Project out Thursday: (read project) Friday: Class Penn ESE370 Fall2011 -- DeHon 31

32. Ideas Synchronize circuits –to external events –disciplined reuse of circuitry Leads to clocked circuit discipline –Uses state holding element –Prevents Combinational loops Timing assumptions (More) complex reasoning about all possible timings Penn ESE370 Fall2011 -- DeHon 32