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SN74LV4051A-Q1: Multiplex 4 Single-Wire PWM Output

sns22
Prodigy 100 points
Part Number: SN74LV4051A-Q1
Other Parts Discussed in Thread: TS3A5017, CD4051B

Hi all, 

I want to connect 4 Single-Wire PWM Outputs from MAX6672 channels to LPC24xx microcontroller (uC). Previously I was using 4 different timer capture inputs from the uC but 

these ports need to be freed now. 

So I am planning to use these 4 sensors via MUX SN74LV4051A-Q1 by switching between the inputs as the uC only reads one sensor at a time. And connect the output to 

only one timer capture port instead of 4 hence freeing 3 lines at the uC.

I want to ask if this will work as the measurement is done by reading the time stamp of the rising and falling edges of the signal and if there is any risk of cross talk between 

the sensor channels. 

Regards.

  

  • 0
    TI__Mastermind 45380 points

    sns,

    You are correct there is always the potential for cross talk between channels inside a switch IC.  Unfortunately we do not have the cross talk specified in the switch devices like the SN74LV4051A-Q1 which are predominantly used for switching digital signals where signal integrity is less significant of an issue.  If you feel that crosstalk will cause an issue in your system I would recommend looking at a 4:1 switch like the TS3A5017 that is specified for applications where signal integrity is important.

     

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 100 points

    Hi Adam,

    Thanks for your quick reply. 

    I would think cross talk is a major issue in case of fast channel switching in the IC. In my design, there is a significant delay (3x more than as mentioned in the datasheet) in the between turning on and off the channels. Can cross-talk still be an issue? 

    The signals are rather slow. But I saw some overlapping between ADC channels with another multiplexer. And the PWM capture counter values got destroyed as channel 0 was not completely switched off when channel 1 was being turned on.

    I will test prototype it quickly to avoid any surprises. 
    Cheers

    Sns.  

  • 0 in reply to sns22
    TI__Mastermind 45380 points

    Sns,

    The cross talk specified in the datasheet like the TS3A5017 is taken with device control signal at a static voltage.  The spec is measuring how much on the signal on the connected path couples to the unconnected path.  As you correctly stated this measurement varies with frequency as shown below for the TS3A5017.  There is less of a cross talk concern with lower frequency signals. 

    If you are concerned about the interference between channels while the control signal is toggling the device between states you need to look for a device that guarantees a feature called "break-before-make".  This feature ensures the IC isolates the signal path for before connecting the other. 

    Good luck on your prototype let us know if you have additional questions.

    Adam

  • 0 in reply to Adam Torma
    Prodigy 100 points

    Hi Adam,

    Thank you for your reply. I was also looking into this device.

    Either CD4051B or TS3A5017 should work good for me. 

    Cheers

    Sns