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  1. #1
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    2:1 Multiplexer with open drain Outputs IC

    In our design,

    we want to connect a GPIO (3.3V) to 2 devices (selected by another GPIO, one device is 3.3V and other device works with 1.8V.

    I am seeing 2:1 mux with Open drain outputs so, that i can connect each output to any voltage(1.8V/3.3V) tolerant device.

    any other alternate way to fulfil this in single IC is fine.

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  2. #2
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    Re: 2:1 Multiplexer with open drain Outputs IC

    Sorry, this should be 1:2 demux not 1:2 Mux
    Click image for larger version. 

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ID:	156505



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    Re: 2:1 Multiplexer with open drain Outputs IC

    How about something like this: https://www.onsemi.com/pub/Collateral/NC7SZ157-D.PDF

    Looking at the datasheet, the switching thresholds are 0.7 for H and 0.3 for L, so as long as your input levels can swing beyond those points it should work.



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    Re: 2:1 Multiplexer with open drain Outputs IC

    @bking: that's a straight forward 1:2 demux.
    Out2 in my diagram is 1.8V (not 3.3V complaint)- these devices with 3.3V VCC will generate Min Voh as 2.4V which will damage 1.8V device....T
    That's the reason i asked for open drain outputs....



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    Re: 2:1 Multiplexer with open drain Outputs IC

    Hmm... I didn't have the post with the picture until now. I see you also flipped it to a 1:2.

    How about a 74LS156. That has open collector drivers.



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    Re: 2:1 Multiplexer with open drain Outputs IC

    Hi,

    An analog MUX usually is bidirectional. I think you donīt need this.
    OpenCollector outputs need a pullup. This may be waste of power or slow rising edges.

    Thus I think lowVoltage powerd single gates with 3.3V input tolerance could be a good solution without drawback in signals.

    But there are informations missing:


    HTML Code:
    IN  | SEL || OUT1  | OUT2
    -------------------------
     0  |  0  ||   0   |  ?  
     0  |  1  ||   ?   |  0  
     1  |  0  ||   1   |  ?  
     1  |  1  ||   ?   |  1

    Please replace the "?" with your desired values.

    Klaus
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    Re: 2:1 Multiplexer with open drain Outputs IC

    @bking: Those are old IC's with big packages. I can't fit in my board.

    - - - Updated - - -

    @KlausST: Can you specify which IC can generate this.
    I see configurable gates generate 2:1 mux because gates will have multiple inputs and single output.I haven't see any configurable gates which can generate 1:2 DEMUX



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    Re: 2:1 Multiplexer with open drain Outputs IC

    Hi,

    @KlausST: Can you specify which IC can generate this.
    I see configurable gates generate 2:1 mux because gates will have multiple inputs and single output.I haven't see any configurable gates which can generate 1:2 DEMUX
    First you need to give complete informations.
    --> replace the "?" with the logic levels you desire.

    Klaus
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    Re: 2:1 Multiplexer with open drain Outputs IC

    @KlauST: Thanks,

    all '?''s will be replaced by tri-state/disabled as like SN74LVC1G18( 1-of-2 Noninverting Demultiplexer With 3-State Deselected Output).

    Basically based on Selection pin IN state should be transferred to OUT1/ OUT2. OUT1 and OUT2 should be Open drain outputs.



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    Re: 2:1 Multiplexer with open drain Outputs IC

    Hi,

    all '?''s will be replaced by tri-state/disabled as like SN74LVC1G18( 1-of-2 Noninverting Demultiplexer With 3-State Deselected Output).
    Why you want tristate? It is an undefined logic level. The receiver may see HIGH or LOW, or oscillation...

    Klaus
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    Re: 2:1 Multiplexer with open drain Outputs IC

    "Why you want tristate? It is an undefined logic level. The receiver may see HIGH or LOW, or oscillation..."
    --> when it is tri-stated other output will be connected to receiver.

    Actual case is,
    I have a JTAG chain, with 2 devices connected to a connector (where an ICE or equalent will access the chain to program each device one by one).
    Device 1 - 3.3V device
    Device 2 - 1.8V device(last in chain)

    I need to have option to bypass device2 when required.
    TCK,TMS,TST & TDO going to Device2 are going to voltage translating switch(SN74AVC4T245 - name U1) - this translates 3.3V logic to 1.8V logic.
    which can disabled by driving OE#

    Device1 TDO - can go to Device 2 TDI(in image above it is OUT2) through voltage translating buffer (Device1,Device2 both are in chain) or directly to Connector TDI(when Device2 bypassed- in image it is OUT1)
    to implement this,i need a demux(U2)

    Case1 : Device2 is bypassed in chain - demux will send IN to Out1
    Out2 will be tri-stated,and U1 also disabled by driving OE# of U1


    Case2: Device1,Device2 are in chain
    demux will select Out2 (from IN),
    Out1 will be tristate and U1 will be enabled(OE# LOW), because U1 is enabled TDO from device2 will go to TDO of the connector.



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    Re: 2:1 Multiplexer with open drain Outputs IC

    Hi,

    Daisy chaining JTAG devices should need no extra logic. It can simply be done with JTAG protocol.
    You should be able program to program one device only or both as you like.

    You are not the first one to have this problem, thus I recommend to look for existing solutions.
    The device manufacturers should provide such informations.

    Many JTAG inputs are 3.3V or even 5V tolerant. Please check datasheet.

    In case you still want a special solution, then please post a draft/schematic about your idea.
    The better your informations the better our chance to help.

    Klaus
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  13. #13
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    Re: 2:1 Multiplexer with open drain Outputs IC

    "Daisy chaining JTAG devices should need no extra logic. It can simply be done with JTAG protocol.
    You should be able program to program one device only or both as you like.
    "
    --> Multi voltage jtag chain requires voltage translators.
    If there is a need to bypass/Include certain device on need basis - yes we need to add glue logic to do it.

    Below is the ruff implementation.

    Click image for larger version. 

Name:	jtag_chain.jpg 
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ID:	156596



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    Re: 2:1 Multiplexer with open drain Outputs IC

    The original request can be solved with a single 74HC4066 and 4 resistors:

    Click image for larger version. 

Name:	level_conversion_demux.png 
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ID:	156598

    The unmarked resistors should not be critikal. Use 10K or whatever.
    The 1k resistor will affect the maximum speed for the 1.8V TDO output. Reduce the value to increase the speed.



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    Re: 2:1 Multiplexer with open drain Outputs IC

    I think it would be better to use a MUX on the signal going to the TDO pin on the connector.
    If you need to disconnect the ASIC TDI pin when the ASIC is bypassed, use another mux before the 3.3V to 1.8V level converter.
    One 74HC157 contains 4 dual-input muxes controlled by the same select input, and should solve the problem.



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