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ADUM3151 Tying several MI Lines together and more questions

Category: Hardware
Product Number: ADUM3151

Hello ,We have few questions regarding ADUM3151. 

We are planning to use the digital Isolator for SPI.

A) It's written in the data sheet page 18 that "the MI pin is always active and does not tristate and therefore , this precludes tying several MI Lines together" 

I think this means I can't tie the MI pin of Multiple ADUM3151 ICs without a Multiplexer is that correct? Are there any IC Alternatives that do not require external Multiplexer for Tying the lines together ?

B) What about MO , SO , SI and the clock , can I tie the different ADUM3151 ICs together on the same isolated side so that I control the Communication through the Slave select alone ? or I need also Multiplexing same as MI ?

C) Can I use the low speed data lines (whose direction from master to slave) as extra slave select signals in case it won't affect my overall system speed or will that cause any issues with the IC ?

D) Is the isolation considered Galvanic Isolation ? I am planning to use MAX253 with a pulsed transformer to Isolate the supply so I want to make sure that the IC will provide the necessary Isolation.

Thank you very much in advance.

BR

Amr Wael

R&D Electronics Engineer

BEDO Co.

  • _______ UPDATE ______

    I just came by ADUM3154 which is specifically designed for multiple slaves systems and We are planning to use it since it's more convenient for our application but the questions A & D still apply for that IC as well.

    Looking forward for your reply.

    BR 

    Amr

  • Hello Amr,

    A) The MI output pin on both the ADuM3151 (and ADuM3154) is always driven either higher or low.  This means that this pin cannot be directly paralleled with other SPI targets/peripherals.  Either a mux or 2nd SPI driver on the controller must be used.  The isolated side SO pin is an input and so would allow multiple SPI device's data output pins to be connected to it provided that the particular SPI devices will high Z their output when not selected.  See the following block diagram.  

    To simplify the drawing, I'm only showing the SPI data lines from the target to the controller.

    B) The other pins MO, SI and clock can be connected to multiple devices without issue.  SO can also be paralleled but with the requirement noted above that the target devices high Z their output when not asserted.

    C) yes, you can use these lines as additional select lines; however, because they have a variable propagation delay due to their low speed sampling, you will need to use the worst case propagation delay in your SPI transaction timing.  In particular, you will need to add 2.6us to what ever set up time your isolated side SPI devices require when being selected to assure that the target device is ready when the clock starts.   Note that while this won't impact the data rate/ clock frequency within the actual transaction; it may have an impact on the effective SPI bus speed, especially at higher SPI clock frequencies.

    D) Yes, the ADuM315x parts offer true galvanic isolation.

    Eric

  • Sounds fine.  Don't forget to include default state biasing resistors (e.g. pull-up or pull-down) since the select output lines on the ADuM3154 are high-Z unless selected by the input mux.

  • Hi Eric , 

    Thank you for your fast reply.

    A) Actually I am not planning to tie multiple targets directly together , I am planning to use multiple ADuM3151 as shown in figure below , is this disallowed as well ?

    Thank you for replying on the other questions.

    Looking forward to your reply

    BR ,

    Amr

  • Thank you very much , I really forgot them ! 

  • Hi Amr,

       Unfortunately, the MI pins of 2x ADuM3151 (or ADuM3154 or any combination) cannot be tied together.  You'll need to use a mux or tri-state buffers which are controlled by the chip select from the controller.  You could also use a 2nd SPI controller on the micro, though if you have multiple ADuM315x, the mux (or tri-state buffer) will probably be a better solution.

    Eric