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SN65HVS881: ERROR IN DATA BEING READ

ashwin raj
Prodigy 40 points
Part Number: SN65HVS881

Hi,

    I am currently using 2 sn65hvs881 chip, in daisy chain, for getting 16 bit input for development purpose.  I am using raspberry pi for the controller part. currently, i have connected the chip to rpi using a mosfet (bss138) level shifter(from 3.3 to 5V and vice versa). I am giving a low clock pulse of 10Hz...Now the problem is that when i change inputs in the second chip( the chip whose 5 V is not connected) the output of 1st one is also getting affected. Also when i probe the  clock signal, while the other probe is connected to the SOP of 1st chip, The SOP output gets messed up, that is, some of the bits are not read properly..mainly zeros are not recognized.  If only one chip is there outputs are read properly. but when 2 chips are connected the 2nd output seems to messed up. cant understand why it is happening? can anyone shed some light one this? Thank you. Ps: the ciruit is attached, sorry for the shabbiness did it in a hurry,,

  • 0
    Prodigy 40 points
    If someone can at least tell me if the circuitry will work or not and, if not, a probable reason for it, then that will also be really helpful....thank you.
  • 0 in reply to ashwin raj
    TI__Intellectual 2165 points
    Hi Ashwin,
    To better understand the issue you're observing, could you please provide the following:

    -What are the operating conditions of the devices in your diagram.
    -Detailed schematics, the graph attached is not very clear.
    -Waveform associated with the anomaly observed, you mentioned probing a couple signals- capturing scope shots of those signals and labeling them would help.

    Thank you,
    Helen
  • 0 in reply to Helen Sun
    Prodigy 40 points
    HI Helen,
    thank you for responding....i was not able to reply as i was busy with other circuit designing....I wont be able to provide waveforms a this point but will explain the issue......

    I am using the gpio expander ic to read 16 inputs... this is a setup for the software development department to work on.. so I have not included any of the protectioin circuits.....
    The flow of diagram is like this:

    Raspberry pi gpios -----> 220ohm resistors------>level shifter board (4 input output....using bss138 mosfet as shown in figure)--------> 881 IC(SIP of which is connected to the second 881 ics SOP.......)-------->inputs( 12V directly given to input through a switch to the field side of the ic and RE pins are connected to an smd green led)

    High voltage(+5V) for level shifter is given from the 1st 881 ics 5V output(can this cause a problem?) and low voltage +(3.3V) is given from raspberry pi.

    PROBLEM: When i connect probe of DSO (2-channel) in data pin and clock pin of 881 the data coming out from SOP pin of the 881 ic, is getting corrupted, that is, some of the transitions to 0 or high to low are missing. why am i missing the data was my question? The frequency of clock at this point was 10Hz. Also i am not getting the IP7 bit of the 1st ic or the ic connected to the raspberry pi.
    If u read this with the diagram you will understand what i am getting at..... also is there a minimum frequency of operation for the 881 ic

    thank you,
    with regards,
    ashwin
  • 0 in reply to ashwin raj
    TI__Intellectual 2165 points
    Hi Ashwin,

    I would suggest to use waveform generator to follow the path of the data starting from Rpi GPIO output to find out where your data started to get corrupted, so the issue can be isolated to one or multiple ICs. Like you mentioned level shifter could be a potential issue depending on which part number you are using.

    Thank you,
    Helen
  • 0 in reply to Helen Sun
    Prodigy 40 points

    Hi Helen

                    the issue is partially resolved... initially data was pulled out from 881ic at 10Hz clock frequency.....when the frequency was increased to 100khz  data was coming properly and probing effect has gone....also i shorted the resistance in series with rpi gpio and mosfet level shifter....... what is the excact cause i dont know...the 881ic  was connected to SPI_CLK of rpi ...may be it has a lower frequency limit....don't know just guessing...  had to move on to  other designs thats why couldnt provide u with waveforms....  now i wont be able to go back and do a thorough check soon.. so am going to leave it at that.....thank you for ur valuable time and suggestions  Helen.

    With regards,

    Ashwin Raj