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ISO1540-Q1: Connection to side 1 and side 2

Part Number: ISO1540-Q1
Other Parts Discussed in Thread: ISO1540


My customer is designing I2C using ISO540 - Q1,
There is a part with less margin against device specification.

Would you please let me know if they can avoid this in the following way?
Device: ISO 1540-Q1
SPEC: I2C port input capacity limit Side1=max 40 pF
SPEC: I2C port input capacity limit Side2=max 400 pF     

Customer Design value: Input capacity of port theoretical value
Side 1: 37.4 pF (device 36 pF, pattern 1.4 pF)
Side 2: 8.4 pF (device 8 pF, pattern 0.4 pF)

Since the device capacity limit of Side 2 is 400 pF, we think that by replacing Side 1 and Side 2, capacity margin can be secured.

Is it OK to connect I2C HOST (Master) to Side 2?
Customers believe that ISO 1540 - Q 1 has no problem because CLK and DATA are bidirectional.

Thank you for your help.

Best Regards,Orobianco

  • Hi Orobianco,

    Thank you for sharing details about your application requirement relating to question. Please note that ISO1540-Q1 Side1 (SDA1/SCL1) & Side2 (SDA2/SCL2) are designed keeping in mind the component they would be interfaced to.

    VIL1 & VOL1 specs of ISO1540-Q1 are tightly designed to achieve bidirectional function and still avoid interlocking due to back-to-back driving. For this reason it is advisable to interface host to Side1 and peripherals to Side2. For this to function smoothly, host should have a VIL of >0.8V and VOL of <0.5V which is mostly the case.

    If the peripherals you are planning to interface to Side1 meet logic threshold specs of VIL > 0.8V and VOL < 0.5V, then yes the peripheral can be interfaced to Side1.

    Answering your other question on margins, it should be fine to use 37.4pF of load on 40pF Side of ISO1540. Thanks.

    Koteshwar Rao
  • Hello Koteshwar Rao,

    Thank you for your prompt reply.
    In conclusion, if the devices connected to side 1, side 2 satisfy the electrical specifications of SIDE 1, SIDE 2 described in the data sheet "6.9 Electrical Characteristics",Is it OK to connect the slave device to side 1 and connect the master device to side 2?

    Thank you for your help.
    Best Regards,Orobianco
  • Hi Orobianco,

    Yes, your understand is correct. If you have the slave and host comply to device specifications in datasheet for given sides, it should be fine to interface them accordingly. Please do make sure to comply with electrical specifications of ISO1540's VOL1 & VIL1 as these parameters are designed tightly to achieve bidirectional operation. Thank you.

    Koteshwar Rao