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SN74CB3Q3257: Question for Propagation delay

William Hu
Prodigy 60 points
Part Number: SN74CB3Q3257
Other Parts Discussed in Thread: SN74LVC1G66, , SN74AVC4T774

Tool/software:

We used two parts on SPI bus, SN74CB3Q3257 has CLK/CS/IO1/IO2 and SN74LVC1G66 has IO3/IO4.

Propagation delay is MAX 0.8 ns on SN74LVC1G66,Propagation delay is MAX 0.2 ns on SN74CB3Q3257.

Is there any parts of single switch that  Propagation delay is MAX 0.2 ns or any else suggestions? How to has the same Propagation delay on SPI bus?

  • 0
    Guru 297640 points

    The footnote for tpd says:

    The propagation delay is the calculated RC time constant of the typical ON-state resistance of the switch and the specified load capacitance, when driven by an ideal voltage source (zero output impedance).

    This test condition is not realistic; the actual propagation delay will be higher. The actual rise/fall time depends on the drive strength of your SPI devices and on the actual load capacitance.

  • 0
    TI__Genius 12425 points

    Hello William,

    Looking at our portfolio the closest device we have is the SN74CBTLV3257. It has a max prop delay of 0.25ns. As Celemens mentioned the prop delay on the SPI bus depends on the drive strength of your spi devices .

    Thanks,

    Nir 

  • 0 in reply to Nir Gilgur
    Prodigy 60 points

    Could you review the schematics if the circuit is fesible? Can we used two parts on SPI bus, SN74CB3Q3257 has CLK/CS/IO1/IO2 and SN74LVC1G66 has IO3/IO4?

  • +1 in reply to William Hu
    Guru 297640 points

    The schematic looks OK, but it is not possible to say anything about the propagation delays because there is not any information about the SPI devices or their drive strength. (Also, the delays will be dominated by the tpd of the SN74AVC4T774, but I don't know if it is actually used in the case you're asking about.)