Understanding Input Stages in CMOS: Transition Times, Termination, and Practices

1. Input Stages

2. Input Stages - Introduction • Most CMOS inputs have rise/fall time limits • Most inputs also have some hysteresis • Typical symbols in specifications tR, tTLH - rise time tF, tTHL - fall time tT - transition time • Waveform measurement • typically from 10% to 90% but not always • sometime parameter measurement method is not specified

3. Input Stages - Practice • Data sheets may list a parameter for information only and not 100% tested • Laboratory devices have shown that not all qualified devices will meet the data sheet • One case was when a part was shrunk • Migration to a faster process • Oscillations observed • Conservative margins recommended

4. Input Stages - Termination • Floating CMOS inputs are, in general, 'bad.' • Totem-pole currents, oscillations, etc. • Some devices offer pull-up/down resistors • SX-S only active during power transistions • Xilinx resistors controlled by SRAM • Care on internal tri-state lines • get example from XQR4000XL • Dedicated Inputs • Actel unused inputs were handled by s/w • Not true for some SX, SX-S clocks  Check each case carefully

5. Input Stages - TerminationCase Study: SX-S Clock Pin

6. Input Transition Times

7. Input Transition TimesReferences • [1] ACT ™ 1 Field Programmable Gate Arrays, March 1991. • [2] ACT 1 and ACT 2 Military FPGAs, April, 1992. • [3] ACT ™ 1 Series FPGAs, April 1996. • [4] Radiation‑Hardened FPGAs, v3.0, January 2000. • [5] ACT ™ 2 Series FPGAs, April 1996. • [6] Accelerator Series FPGAs – ACT ™ 3 Family, September, 1997. • [7] 54SX Family FPGAs RadTolerant and HiRel, Preliminary V1.5, March 2000. • [8] HiRel SX-A Family FPGAs, Advanced v.1, April 2000. • [9] RT54SX-S RadTolerant FPGAs for Space Applications, Advanced 0.2, November, 2000. • [10] QPRO XQR4000XL Radiation Hardened FPGAs, DS071 (v1.1) June 25, 2000. • [11] QPRO™ Virtex™ 2.5V Radiation Hardened FPGAs, DS028 (v1.0) April 25, 2000 Advance Product Specification. • [12] Not in data sheet. • [13] Configurable Logic Data Book, Atmel, August 1995. • [14] AT6000LV, Atmel, October 1999.

8. Clock Transition Time SpecificationA Difficult Case

9. Transition Time RequirementsImplications - Pullup Resistors • Often used for tri-state or bi-directional busses • Rise time (10% - 90%) =  = 2.2 RC • Example C = 50 pF R = 10 k (keep power levels reasonable)  = 500 ns  violates many devices' specifications (see table)

10. Transition Time RequirementsImplications - Filters and Protection Circuits • Often used on signals • Elimination of noise • ESD protection • Etc. • RC filters or clamps (high C) can often substantially degrade transition times • Consider discrete hysteresis buffers, particularly for clock signals

11. One Bus Hold Circuit Bus Hold Circuit in an FPGA Supplies leakage current only.

12. Transition Time RequirementsImplications - Interfacing with older logic families • Case Study (1) • CD4000B CMOS NOR gate • VDD = 5V • tT (typ) = 100 ns • Case Study (2) • CD4050B (used as a level shifter, for example) • VDD = 5V • tT (max, 25 C) = 160 ns

13. Transition Time RequirementsImplications - Interfacing with older logic families (cont'd) • Case Study (3) - 54HC00 CMOS NOR gate • 5962-8403701VDA, NAND GATE, QUAD 2-INPUT Test Symbol Test conditions 1/ Limits Unit -55°C TC+125°C Min Max unless otherwise specified Transition time, tTHL, TC = +25C VCC=2.0 75 output rise and tTLH CL = 50 pF VCC=4.5 15 ns fall 3/ See figure 4 VCC=6.0 13 TC = -55C, -55C VCC=2.0 110 CL = 50 pF VCC=4.5 22 ns See figure 4 VCC=6.0 19 3/ Transition time (tTLH, tTHL), if not tested, shall be guaranteed to the specified limits in table 1.

14. Transition Time RequirementsParameter Measurement From: Figure 4, 5962-8403701VDA, NAND GATE, QUAD 2-INPUT

15. Transition Time RequirementsCase Study: RH1020 • Production Parts • Input stage was modified for clock upset • VCC = +5VDC • T=25°C • CLKBUF monitored on output • Because of design of the buffer, difficult to see effects on the input pin • Used a low-impedance signal generator, triangle waveform • Commercial specification is tR, tF of 500 ns • RH1020 did not meet this specification • SMD 5962-90965 does not specify this parameter

16. Transition Time RequirementsCase Study: RH1020 CLKBUF

17. Transition Time RequirementsRH1020 CLKBUF @ VIN threshold

18. Transition Time RequirementsCase Study: RH1020 CLKBUF Notes • Conditions: Room temp; VCC = 5.0 V. • Oscillations detected consistently at tR = 360 ns • Sporadic output pulses at tR = 300 ns • Transition time requirement not symmetric • Oscillations detected consistently at tF = 1.5 s • Sporadic output pulses observed at tF = 1.0 s

19. Interfacing - Voltage Margin • TTL  CMOS • Problem with discrete circuits (still seen) • Normally not a problem with 5V FPGAs • Issue with new FPGAs • 0.35 m may only pull up to 3.3 VDC • 0.25 m may only pull up to 2.5 VDC • Can be issue with parts having a VIH = 70% VDD • Ringing can cause false triggering • VIL = 0.8V and fast devices are sensitive to ringing on a backplane.

20. Inputs: RT54SX16 tT RT54SX16 output (bottom trace) with a slow rising input (top trace) which clocks a divide by two counter resulting in a "glitch." The clock input was provided by an HP8110A pulse generator.

21. Inputs: RT54SX16 tT RT54SX16 output (bottom trace) with a slow rising input (top trace) which clocks a divide by two counter resulting in a "glitch." The clock input was provided by an HP8110A pulse generator.

22. JTAG and Loss of Control • Run TCK with TMS=‘1’ • Guaranteed to return to TEST_LOGIC_RESET state within 5 clocks. • Share system clock with TCK • JTAG Hit • Inputs turn to outputs • Clock pin turns to output, clamps system clock  No TCK, system hangs.