Design and Timing Closure Techniques for Managing Wide Semiconductor Timing Variations in Space Applications

1. Design and Timing Closure Techniques for Managing Wide Semiconductor Timing Variations in Space Applications Michael Cuviello Swales Aerospace Incorporated Alexander Osovets Orbital Sciences Corporation September 7, 2005

2. The Design Problem: • Long term exposure to radiation in space • Causes Component degradation resulting in Integrated Circuit specifications with wide timing variations. • Problems guaranteeing timing margin of FPGA based digital systems.

3. Example System Micro-controller Aeroflex RH80C196KDS Memory FPGA ACTEL RT54SX72S Peripherals

4. Example:Aeroflex RH80C196KDS Micro-controller Timing tolerances 40-50% of system clock period Timing tolerance 40-50% of system clock Timing tolerance 40-50% of system clock Signal Timing relative to output clock Signal Timing relative to address latch Worst case timing doesn’t add up!

5. Re-Sync Clock Domain Clock Domain Re-Sync Re-Sync Clock Domain Delay Delay Delay System Clock Time for an Updated Approach:A Multiple Clock System

6. New Issues! • Need for glitch free handshaking logic. • Re-synchronization of cross-clock domain signals. • A more sophisticated static timing analysis tool flow with multi-clock multi-mode capability.

7. Issue 1: Glitch Free Handshaking What Causes FPGA Cells To Glitch? • ASIC combinatorial cells do not glitch, since they are custom designed for a specific function. • In FPGAs all combinatorial functions are created from components available in the slice, i.e. C-Cell. • Actel devices have multiplexer based structures. • This results in all gates being built from muxes. • That in turn implies possibility of output glitches on the input signal transitions, even in the cases when other inputs are stable.

8. Issue 1:Glitch Free LogicWhy do we need glitch free components? • To avoid tri-state bus contention during the enable/disable boundaries of multiple drivers. • To create glitch free clocks and clock like signals i.e. ram writes, interrupts … • Clock gating. • Power dissipation reduction. • EMI reduction.

9. Issue 1:Glitch Free LogicOption 1: Register all desired ports • Hard macro Flip-Flops in Actel R-Cell provide glitch free operation. • However, 54SX-S device has Triple Module Redundancy (TMR) combinatorial voting circuit on the output of the registers. • Fortunately, glitch free operation is verified through simulation by NASA TMR designers. • This solution is not always feasible due to design speed, asynchronous driving modules or other limitations.

10. Issue 1:Glitch Free LogicOption 2 : OR-Gate or Multiplexer Macros • 4 OR-gate macros with glitch free operation • MX2 macro with glitch free operation • Only on S input transition • When A and B inputs = 0 • For Both • The only way to ensure glitch free operation is to keep inputs transitions far apart from each other (few gate delays). • Indicated by ACTEL Engineers; but not guaranteed. • Use syn_keep and alspreserve attributes for instantiation in VHDL. • Significantly reduces output glitch possibility.

11. Issue 1:Glitch Free LogicOption 3 : Asynchronous RS-Latch • Asynchronous RS-Latch may provide glitch free output transition • However, most of the Actel Flop and Latch macros are built from combinatorial logic • DFPCB (presented on the previous slide) is one of few Actel macros comprised out of pure sequential logic. • In addition CLR input has a precedence over PRE input, avoiding an uncertain condition. • Actel confirmed glitch free operation of this device, but again, as long as inputs transitions are kept far enough from each other • The hard macro approach is less susceptible to the affects of routing delays, synthesis and placement variations. • This option shows the most promise. • Good Luck!

12. Issue 2: Synchronizing Clock Domains Out of Phase Correlated Clock Domains - Metastability e-(tco-tmet)/t Tapperture = To * Clock thold+tsetup – metastability window Input Metastable Output tmet – output sample time Normal Output tco – normal output delay

13. Issue 2: Synchronizing Clock Domains Out of Phase Correlated Clock Domains - Metastability • Metastability • The output of the flip-flop becomes indeterminate for an extended period of time beyond the normal specified output delay. • Caused by flip-flop input setup or hold time violations. • tmet denotes the extended duration before the output settles. • tmet is related to a time window about the clock edge in which the input data transitions. • taperture. denotes a time window about the clock edge in which the input data transitions. • As tmet increases, taperture decreases. • The aperture need not be exactly centered on the clock edge.

14. Issue 2: Synchronizing Clock Domains Out of Phase Correlated Clock Domains - Metastability • To reduce the likelihood of propagating an indeterminate value into the system, we double sample data that might transition within the taperture window at a period beyond what tmet is likely to be. • The Worst Case – Correlated Clocks (from the same source) • Presents the possibility that the nominal data transition is always centered exactly in the middle of the aperture. • Typically, only the case of random occurrence is analyzed. • A Latent Fault - Because of timing changes due to radiation and correlated clock domains, this could occur repetitively at sometime during the life of a mission.

15. Issue 2: Synchronizing Clock Domains Out of Phase Correlated Clock Domains - Metastability • To analyze • Rely only on system noise to cause the actual data transition to occur sometime off nominal that is outsidetaperture. • To eliminate any possibility that correlation between the system noise and the input clock might work against us, we consider only thermal noise that we know is truly random. • This thermal noise could be considered to be in the input signal or actually noise on an internal node of the latch in the flip-flop. It is caused by channel resistance of the transistor. • If we make our sampling period large enough, taperture reduces to the point that it is insignificant compared to the timing jitter caused by thermal noise. • Thus, the failure rate because of tmet exceeding our sampling period also becomes very small.

16. Issue 2: Synchronizing Clock Domains Out of Phase Correlated Clock Domains - Metastability Tc Clock Uniform Input Distribution Corelated Input Distribution Td Tjitter Apperture Window Tapperture

17. Issue 2: Synchronizing Clock Domains Out of Phase Correlated Clock Domains - Metastability MTBF is expected value of the time between failures For uniform failure distribution MTBF =E(t)=T/Ne Where T is operation time, Ne is number of faults Ne= fd*T*Pfail= fd*T* Tapperure/Tjitter e-(tco-tmet)/t MTBF =1/fjitter*fd*To*

18. Tjitter Vnoise Issue 2: Synchronizing Clock Domains Out of Phase Correlated Clock Domains Input Jitter Estimate MTBF Estimate for Synchronizer in Actel RT54SX72S Input slew dU/dt=1V/ns MTBF = (Tjitter * Td / C1) * e(C2 * tmet) For Actel RT54SX72S C1 = To*e-(tco/ t) 7E-10 C2=1/ t 1e10 tmet = Tc-tsetup-tpd-tco tpd = propagation delay tsetup = flip-flop setup time. tco = flip-flop output delay tco+tpd+tsetup  1.5 ns Tc = 1/32 MHz Td = 1/8 MHz MTBF  2.62 x 10110 years Thermal noise V2noise=KT/C For T=303K(30C); C= 5E-12 pF k=1.38E-23 J/K Vnoise~=30μV translates into Tjitter~=.03pS This is a conservative noise estimate disregarding system noise

19. Issue 3: Static Timing Analysisfor Multi-Clock Domains Multimode Systems • Multiple Pseudo Asynchronous clocks - phase shifted synchronous clocks with slow drifting undefined phases. • Synchronization to master clock leads to messy manual analysis with reduced operating frequency and high probability of mistakes. Clock uncertainty is on the order of full cycle. • Sequential multi-cycle CPU operation and external DMA block require multi-mode analysis. • Board level cross chip analysis is required to meet ICs specification. • CLOCK DOMAINS ENCAPSULATION AND BOUNDARY RE-SYNCHRONIZATION IS A BETTER SOLUTION. • PRIMETIME IS AN ASIC TOOL, WHICH CAN BE USED FOR FPGA ANALYSIS TO MEET TIMING REQUIREMENTS.

20. Issue 3: Static Timing Analysisfor Multi-Clock Domains Multimode Systems Processor BlockDiagram Addr Data Wr_n wrln Clko OE ALE CS RAM CPU DMA FPGA Sys_clk

21. Setup Time Ts : (1) “Address Latch” valid to (2) “Read Not” valid to (3) Data Required on “AD” bus - wr (4) “Address Latch” detected to (5) “SRAM Read Not” valid to (6) SRAM Access time to (7) D bus to AD bus delay. 31.25ns Typical SYS_CLK T1min=2*Clk-10 T2min=2*Clk-5 T3max=2*Clk-26 ADV_RD 1 2 3 ALE RD_N Add/Data Address Required Data Valid T5max= Tp_rd3 T7max= Tp_d_ad 7 T4max=3*Clk + Ts_ale T6max =Tsram ALE_1 T1min=2*Clk-10 ALE_2 ALE_SYNC 4 Address CSsram 5 RD_n_3v 6 Data Issue 3: Static Timing AnalysisExample of Manual Analysis Required with full resynchronization to master clock SRAMBusRead

22. Issue 3: Static Timing AnalysisMulti Mode, Multi Clock Analysis SYS_CLK Read margin Address margin Clocks Clocks Clocks Clocks Clocks Write margin 80C196BusTiming

23. Issue 3: Static Timing AnalysisCross chip Data Mode Analysis Latch Ram Address 80C196BusTimingData Read margins from Ram CS to CPU read strobe RAM data out propagation delay CPU data read

24. Issue 3: Static Timing AnalysisPrimeTime Advantages for FPGA design • Industry standard timing analysis tool • Tcl scripting capabilities • Case analysis capabilities • Allows you to perform board level timing analysis • Advanced timing analysis features: • exceptions handling • multiple clocks and frequencies • transparent latch and time borrowing • mode analysis …

25. Issue 3: Static Timing AnalysisActel FPGA Static Timing AnalysisResults w/ PrimeTime • Analyzed modes for setup and hold margins for best and worst case corners: • CPU address to RAM write • CPU address to IO write • CPU data to RAM write • CPU data to IO write • RAM data to CPU read • Timing analysis results: • Setup timing margins were improved from negative slack to greater than 20% positive slack. • Some tight hold margins were identified for buffer insertion. • CPU address latch • Clkout register to register • Sys_clk register to register • DMA RAM read/write

26. Summary • Radiation environments present unique challenges for timing closure. • Relaxed Timing Definition • Long term timing Drift • To address these challenges, a single clock synchronous design was changed to a multi-clock architecture. • To implement the new architecture, new issues had to be addressed. • Guaranteeing glitch free handshaking signals from FPGA logic blocks. • Careful analysis of metastability in the cross-domain re-synchronization circuits. • Multi-mode, multi-clock Static Timing Analysis • The new design successfully improved overall system performance, timing margin, and quality of design.

27. References [1] Actel, “Metastability Characterization Report for Actel Antifuse FPGAs,” Application Note, May 2004 [2] Actel, “Antifuse Macro Library Guide for Software 6.0,” May 2004 [3] Paul R. Gray and Robert G. Meyer. Analysis and Design of Analog Integrated Circuits. Wiley, New York, 1993. [4] Charles Dike and Edward (Ted) Burton, “Miller and Noise Effects in a Synchronizing Flip-Flop,” IEEE J. Solid State Circuits, vol. 34, No. 6, June 1999.