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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.

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  • \$\begingroup\$ Any bus switch should be able to do this \$\endgroup\$ – Passerby Nov 2 '15 at 1:33
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    \$\begingroup\$ Why do you want a bidirectional switch for SPI? SPI uses only unidirectional signals. \$\endgroup\$ – The Photon Nov 2 '15 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 '15 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 '15 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 '15 at 5:55
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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

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  • \$\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 '15 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 '15 at 7:52
  • \$\begingroup\$ @TomCarpenter Thanks Tom, important thing to consider. \$\endgroup\$ – justing Nov 2 '15 at 15:08
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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.

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  • \$\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 '15 at 2:30
  • \$\begingroup\$ why do you need bi-directional? \$\endgroup\$ – Jasen Nov 2 '15 at 8:36
  • \$\begingroup\$ Yes, that digital Bus Switch is bidirectional. \$\endgroup\$ – jp314 Nov 8 '15 at 5:24

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