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SN65LVDM176D to PIC18F27J13 ?

Ahmad Firdaus
Prodigy 60 points
Other Parts Discussed in Thread: SN65MLVD204A

Hello everyone, I am new to this forum. I am working on this project where I have to design an underwater corrosion sensor. It will take the measurement and send the data to the main electronic module. Usually I am using 485, but this one is aim at low power consumption where LVDM is the perfect answer for long distance and low power comm. I have never use LVDM before, so I am bit struggling. I am planning to use SN65LVDM176D as transceiver and PIC18F27J13 as microchip. Assuming that I am using two transceivers (one for sensor and one for main unit), do i need to concern about the PIC usart? Can I send the signal straight from receiver to the PIC? Will the PIC able to receive it and store the data?

Cheers

  • 0
    TI__Intellectual 1360 points

    Ahmad

     

    What is your specific concerns or questions regarding MLVDS usage?

     

    What is your datarate?

     

    Do you in general want to know how MLVDS works?

  • 0 in reply to Ajinder Singh
    Prodigy 60 points

     

    Well, first thing I would like to know how MLVDS works. I need to design two seperate units; one is sensor for taking measurement and one for power supply and data storage. Those two will be about 80-100 meter apart. The idea is to take one measurement per day and send the data to main unit. Speed wont be an issue, and the main unit will be using rechargeable battery(s). What I need to know is how to design the MLVDS part. I am planning to use SN65LVDM176D as transceiver.

    Here are few questions:

    Does it capable of doing the desire operation?

    What else do i need to consider?

    What other components do I need?

     

    Many thanks

  • 0 in reply to Ahmad Firdaus
    TI__Intellectual 1360 points

    Ahmad

    If you are familiar with LVDS, MLVDS is very similar to MLVDS except that it can be used for multipoint configuration as well as the driver has more strength so it can drive long distances.

     

    I am providing links to 2 app notes that can explain you how MLVDS works

    How MLVDS works: http://focus.ti.com/lit/an/slla108/slla108.pdf

    MLVDS speed vs distance: http://focus.ti.com/lit/an/slla127/slla127.pdf

    Your application requires that the 2 MLVDS transceivers are separated by ~80m. However your speed is low.  The rule of thumb is that you need to ensure that the loss in the media whether it is cable of backplane does not account for more than 6dB.

    for example attentuation of CAT5 beldin cable is:

    Attenuation/100m = 1.808*sqrt(f) +.017*f + .05/sqrt(f), here f is in MHz. so if your speed is only 1 MHz, and attenuation is per 100m. then you might be ok with your application

     

    Does it capable of doing the desire operation?

    [as] possibly, but you need to do calculation based on your speed and the media you have as i have shown above

    What else do i need to consider?

    [as] speed, media loss

    What other components do I need?

    [as] MLVDS requires a 100ohm termination on the source and sink side as explained in the app note http://focus.ti.com/lit/an/slla127/slla127.pdf

    figure 1.

     

     

  • 0 in reply to Ajinder Singh
    Prodigy 60 points

    Hello,

    I never use LVDS before so thats why I am bit struggling.

    I dont know the value of speed needed for my project but presumably low because I only send one signal per day. Does this mean that I need to choose MLVDS with low signalling rate i.e SN65MLVD204A (100Mbps, Type 2) in order to achieve longer distance comm.?

    I know I need two transceivers, A and B pins are for carrying signals. I also need 2 100 ohms resistors in parallel at each transceiver, correct me if I am wrong. What are DE and RE pins for? D and R can be send straight to the microcontroller.

    Many thanks.

     

  • 0 in reply to Ahmad Firdaus
    TI__Intellectual 1360 points

    Ahmad

     

    LVDS is low voltage differential signaling.

     

    I think more than the frequency of how many times you will be sending signal, the important thing is when you send the signal, what will be the data rate?

     

    DE stands for MLVDS data enable.

    RE stands for LVTTL data enable.

     

    Let me know if you have more questions

     

    /Ajinder

  • 0 in reply to Ajinder Singh
    Prodigy 60 points

    I have attached my circuit diagram. This is at the reciver end (main storage unit).

    r0049.pdf
  • 0 in reply to Ahmad Firdaus
    Prodigy 60 points

     

    In the circuit, I also design some logic gates that allow the pic to choose whether to use LVDS to send/receive signal from sensor (left hand side) or rs485 to another wireless module (right hand side).

     

    How does RE and DE works?

    Do I need another components for long distance transmission? (Repeater?)

     

    Thanks

     

  • 0 in reply to Ahmad Firdaus
    TI__Intellectual 1360 points

     

    Ahmad

    RE and DE are just enable pins, so if you can provide a DC high level or low level, that is all that is required.

     

    As indicated you will be needing 100 ohm termination on both source and sink side or transmit and receive side.

     

    As far as the need for a repeater, it will depend on your actual data rate and loss.

     

    If you can provide me the loss of the media and the data rate, i can make recommendation.

     

    /Ajinder

  • 0 in reply to Ajinder Singh
    Prodigy 60 points

    I am not sure whats my data rate, but the baud rate I use normally is 9.6kb.