3
\$\begingroup\$

I need a bidirectional IC with on/off state.

In on state, IC should have very small impedance on the line.

In off state, IC should provide very high impedance to isolate the bus.

It's for an 3-channel line isolation. To be more specific, it's for SPI bus isolation.

No need for level shifting. Just need to be able to isolate the bus with a logic high or low.

I am looking for a IC because I want to make my board more compact, not because it cannot be done use discrete components. I have been searching all this afternoon. Most of the ICs are for I2C level shifting from high to low or low to high.

Thanks

Updates: 1. Thanks for suggesting the analog IC switch. I am deciding in between analog IC switch and digital IC switch. It looks like the only difference is one is made of CMOS and one is only made of NMOS.

  1. I also found this Digital Bus Switch ICs Single FET. Is it bi-directional?

  2. I found 74HC243 does exactly what I want. The trick is the to search the right name. Now that I know what name to search, I found tons of chips from different companies that does the same. thing.

\$\endgroup\$
6
  • \$\begingroup\$ Any bus switch should be able to do this \$\endgroup\$
    – Passerby
    Nov 2, 2015 at 1:33
  • 2
    \$\begingroup\$ Why do you want a bidirectional switch for SPI? SPI uses only unidirectional signals. \$\endgroup\$
    – The Photon
    Nov 2, 2015 at 2:09
  • \$\begingroup\$ MISO and MOSI aren't in the same direction. But that's not the issue. I need to be able to isolate the bus, so that I can turn it off when I need to. \$\endgroup\$
    – hong pan
    Nov 2, 2015 at 2:38
  • \$\begingroup\$ For (2), did you read the datasheet? Everything you need to answer that is in there. \$\endgroup\$
    – Tom Carpenter
    Nov 2, 2015 at 4:09
  • \$\begingroup\$ The problem is that what I have been reading on the net is not the same as what I have learned from school. From school, MOSFET is always bi-directional. but I haven't seen a FET that doesn't have a body diode in them. Just found out that JFET is the type that doesn't have body diode effect. Looks like school was lying about the FETs. \$\endgroup\$
    – hong pan
    Nov 2, 2015 at 5:55

2 Answers 2

Reset to default
2
\$\begingroup\$

It sounds like a device similar to the TXB0108 would work for your needs.

It has an output enable pin which places all of the pins in high impedance state, otherwise it is a bi-directional signal translator.

txb0108

\$\endgroup\$
3
  • \$\begingroup\$ Just remember that these and similar self-direction-sensing level shifters tend to have \$\approx4\mathrm{k\Omega}\$ output impedance, so no pull-up or pull-down resistors should be used. Also again because of the impedance these will only do for short runs at higher speeds. \$\endgroup\$
    – Tom Carpenter
    Nov 2, 2015 at 4:06
  • \$\begingroup\$ Thanks for the suggestion. I found [74HC243][1] does exactly what I want. The trick is the to search the right name. Now that I know what name to search, I found tons of chips from different companies that does the same thing. [1]:http//nxp.com/documents/data_sheet/74HC243.pdf \$\endgroup\$
    – hong pan
    Nov 2, 2015 at 7:52
  • \$\begingroup\$ @TomCarpenter Thanks Tom, important thing to consider. \$\endgroup\$
    – justing
    Nov 2, 2015 at 15:08
1
\$\begingroup\$

A multiplexer IC should work -- e.g. CD4016 quad switch, or CD4066.

Note this isn't particularly low resistance; there are others that might be lower. The impact of this might affect the performance (maximum speed) of your SPI interface.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks for suggesting the analog IC switch. But I found this Digital Bus Switch ICs Single FET. Is it bi-directional? \$\endgroup\$
    – hong pan
    Nov 2, 2015 at 2:30
  • \$\begingroup\$ why do you need bi-directional? \$\endgroup\$
    – Jasen Слава Україні
    Nov 2, 2015 at 8:36
  • \$\begingroup\$ Yes, that digital Bus Switch is bidirectional. \$\endgroup\$
    – jp314
    Nov 8, 2015 at 5:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.