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ISO7762: Isolators required for SPI and GPIO

Naveen K1
Intellectual 870 points
Part Number: ISO7762
Other Parts Discussed in Thread: ISO7741, ISO7761, CC3220MODA,

Hi,

Subjected to the previous Post ADS1018, We are using Isolators between Energy Meter-ADC IC and Microcontroller (CC3220MODSF12MOBR) as shown in the block diagram below to monitor the AC Voltage and Current.

1. Kindly suggest the suitable Isolator IC for SPI pins, Pulse output pins and Interrupt pins.

Thanks and regards,

Naveen K

  • 0
    TI__Mastermind 29225 points

    Hello Naveen, 

    Thank you for your interest in our isolation's devices. The common recommendation in SPI isolation is ISO7741. ISO7761 is also an option if more I/O is needed for additional control signals.

    See Isolating SPI Signals (Rev. A), for more information and helpful links. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 870 points

    Hi Andrew,

    Thank you for the information and appreciated your quick response.

    We are having 6 Inputs, 8 Outputs and one SPI, so we need to isolate these pins. So, we need 3 to 4 Isolators in the Board. We thought to use same part numbers for all the isolators if possible.

    1. We will use ISO7761 or ISO7741, In both parts there will be two packages 16-SSOP and 16-SOIC, this decides Isolation Voltage whether 3000V or 5000V. Our application need 3000V or 5000V for Safety and Certification Prospective?
    2. We use CC3220MODA MCU on SPI operation. Isolator Default Output pin status should be High or Low? or How we can know this?
    3. 3. Can we use ISO7761 and ISO7741 for both SPI communication as well as General Purpose signals like Pulse Output signal, Interrupt Output Signal, UART, RS485, etc?
    4. In ISO7741, We will not control EN1, EN2. Can we keep EN1 and EN2 permanently High or permanently Open in the hardware itself ? All the Outputs are always  enabled, so there is no issue on the performance/Noise, right?
    5. requesting you to Please review the below Circuit and provide your feedback to improvise further.

          a. Left Side, labeled as  "1 EMIC" Isolator Signal pins are directly connected to EMIC-ADC IC pins and Labeled as "2-MCU" pins are directly                            connected  to Micro-controller pins.4th Isolator for SPI signals, GND1 and VCC1 is connected to MCU side and GND2,VCC2 is connected on EMIC            side compare to other 3-Isolators are in opposite. Hope this swapping will not impacting the operations, right?

         b. To protect the Isolators and MCU and EMIC-ADC each other from the Higher signal Current/Voltage, Need to place Current Limiting series resistors             between EMIC-Isolator (Left Side) and Isolator - MCU (Right Side)? or not important to have current Limiting resistors?

         c. SPI Slave Select(CS) pin need Pull up resistor. Pull Up resistor should be on Isolator Input pin or Output pin or any side will be fine?

    Thanks and regards,

    Naveen K

  • 0 in reply to Naveen K1
    TI__Mastermind 29225 points

     Hello Naveen, 

    Please see my comments below. 

    1. The level of isolation needed will be decided by your industries safety-standards. Typically, a reinforced isolator is either required or recommended when human safety is a concern. [FAQ] What is the difference between basic and reinforced digital isolators?
    2. Yes, you can have SPI channels and GPIO channels on the same isolator. Each channel can operate independently of each other. 
    3. The default state of the isolator is indicated by an "F" in the product number. See (+) [FAQ] What is the difference between ISOxxxxF and ISOxxxx devices (Ex. ISO7741 vs. ISO7741F)? 
    4.  ENx pin can either be tied high or left open for output channels that are always enabled. There is generally no issue on performance, however if you have a concern, then tying the pin to VCC would be a safe choice.  
    5. Please include decoupling capacitors on the VCCx pins placed as close as possible to the supply pin (less than 2mm away). 
      2. It is recommended for signals on side 1 and side 2 to remain completely isolated from each other with no cross over (or traces under neath the isolator). The device will continue to operate, but the application will not have full isolation performance.
        1. It would be better if the 4th isolator with EMIC signals on side 2 is rotated 180 degrees in the layout. 
        2. This way all EMIC signals will be on one side of the isolation barrier and the MCU signals will be on the other. 
      3. Series limiting resistors will help limit transients to some extent; however, are generally not needed unless you are expecting transients outside of the recommended operating conditions that could damage the device on the I/O pins. 
        1. Another case would be if you are looking to protect the device in the event of a single side isolator fail. Then it is recommended to place a resistor in series with VCC to prevent the failure from heating up the device and causing a secondary failure.
        2. See Understand and apply safety-limiting values for digital isolators for more details. 
      4. A pull up resistor on the input is preferred, however, in the end, the location of the pull up resistor is up to you. The device will still go to default state while powering up or down. Please keep that in mind. 

    Please mark this post as resolved if all questions have been answered.

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 870 points

    Hi Andrew,

    Thank you for the information.

    1. If the isolator input is connected to microcontroller(MCU) output pins and MCU default/configured pin status is High, both ISO7741 and ISO7741F default output state will be High. If MCU pins default/configured status are Low then both ISO7741 and ISO7741F default output will be Low. If MCU pins are Floating/High Impedance then ISO7741 default Output pin will be Low and ISO7741F pin will be High. Kindly advice, My understanding is correct for all the 3 cases when MCU default Output is High, Low, Floating/High Impedance? It will help us to choose, whether we need SO7741 or SO7741F.
    2. Yes, Communication is between the on board IC and not with the external devices, so series resistors between EMIC-Isolator-MCU is not important.
    • a. If we use 1KOhm series resistor between EMIC and ISO7762 or ISO7762 and MCU, 1KOhm will works for Isolators?
    • b. As per your advice we will use 20Ohm/0.65W resistor on VCC. Kindly confirm, we need to use 20Ohm resistor on both VCC1 and VCC2, right? 20Ohm is fine for both ISO7741 as well as for ISO7762 ?

    Thanks and regards,

    Naveen K

  • 0 in reply to Naveen K1
    TI__Mastermind 29225 points

    Hello Naveen, 

    1. The Default output state of an isolator is what state the output is at durring power up and power down. 
      1. This is shown in the below table from the datasheet and FAQ: [FAQ] What is the difference between ISOxxxxF and ISOxxxx devices (Ex. ISO7741 vs. ISO7741F)?
      3. The isolator's outputs will always match the inputs, assuming the MCU's output is a certain either high or low, and the isolator is powered on with its outputs enabled. 
      4. The isolator's output cannot be determined if the input is held to a high Z state by the MCU. 
    2. Smaller resistor values are recommended for series termination resistors on the I/O lines. Typically, 100ohm resistors are used. 
      1. 20 ohms is a good safety limiting resistor value.

    Best,
    Andrew 

  • 0 in reply to Andrew J
    Intellectual 870 points

    Hi Andrew,

    Thank you for your support.

    1.All unused input and Output should kept float or we need to pulled down or pulled up?

    2. Hope Output current will be 2mA @ 3.3V VCC and it will not drive the LED directly.

    3. Please review the Schematic and confirm if any modification required.

    Thanks and regards,

    Naveen K

  • +1 in reply to Naveen K1
    TI__Mastermind 29225 points

    Hello Naveen, 

    1. It is okay to keep unused Input and Output pins floating. 
    2. The ISO7762 has a recommended output current of 2mA at 3.3V. This is at the recommended max; however, it should be okay. 
    3. The schematic looks good, and I do not expect any issues. Please make sure that the decoupling capacitors are placed less than 2mm away from the supply pins when moving to the PCB layout. 

    Best,
    Andrew