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TCAN4550: TCAN4550 + MSP430 isolation solution

Yang Wu
Expert 3170 points
Part Number: TCAN4550
Other Parts Discussed in Thread: ISO7741

Hi Team,

I am considering an isolated CAN solution for my customer with MSP430 & TCAN4550. We know that there are 2 approaches for the isolation. One approach is using TCAN4550 only as CAN controller and then placing an isolated CAN TRX following up TCAN4550 (Figure 1). Another approach is to place the digital isolator between MSP430 & TCAN4550.

For the second approach, I am considering how we can simplify the digital isolator between MSP430 & TCAN4550. The straightforward way is using ISO776x between MSP430 & TCAN4550 (Figure 2). But can we simplify it to using ISO774x (Figure 3) solution to save cost?

Q1: Except for SCLK, SDI & SDO, what other logic signal are mandatory to make sure normal function between MSP430 & TCAN4550? Q2: In detail, There has only 1 SPI slave (TCAN4550) , can nCS of TCAN4550 be pulled down to ground? Can nWKRQ & nINT of TCAN4550 be left floating?

Q3: Can RST of TCAN4550 be left floating? 

Thanks a lot.

Best regards,

Yang

  • 0
    TI__Guru 52275 points

    Hi Yang,

    This is a good question and presents a good opportunity to overview some of the pin features of TCAN4550. 

    Yang Wu said:
    Q1: Except for SCLK, SDI & SDO, what other logic signal are mandatory to make sure normal function between MSP430 & TCAN4550?

    Required pins:
    SCLK, SDI, SDO: All mandatory for SPI communication as they are used to shift data in and out of the device in the synchronous protocol. 
    nCS: While it may seem that this pin is only used to select the pin, it is necessary to pulse this signal high in between SPI transactions. This allows the device to ensure the proper number of clock pulses were received during any single transaction. This pin must be connected to the MCU for proper SPI communication.

    Optional* pins:
    RST: The reset pin is used to restart the device in the event that it becomes unresponsive or to hold it in reset during some fault condition. If this feature is not needed, tie the pin to GND to ensure unintended resets do not occur. 
    nINT: This pin reports the state of the global interrupt status flag. This allows interrupt-based servicing of TCAN4550 functions and faults. Without access to this pin, the controller will need to be polled through SPI to determine when interrupts such as new messages are available. 
    nWKRQ: This pin reports when a remote or local wake request is detected by the device while in sleep mode. This can be used to indicate the wake event to the node if the INH output is not used to control a local voltage regulator for instance. 
    GPIO1, GPO2: These pins can be mapped to peripheral functions for extra or quick access to certain information. 

    *Note that without access to these pins, some functions or feature may be significantly limited or unavailable. 

    I would recommend maintaining the connections to the nINT signal as interrupt based servicing will likely preferable to a polling solution. Depending on your system needs, the other pins listed in this section may be excluded while maintaining critical functionality. 

    Yang Wu said:
    Q2: In detail, There has only 1 SPI slave (TCAN4550) , can nCS of TCAN4550 be pulled down to ground? Can nWKRQ & nINT of TCAN4550 be left floating?

    As described above, this input is needed to separate SPI frames from one another for CRC purposes. This pin is a necessary connection for SPI to work properly. 

    Yang Wu said:
    Q3: Can RST of TCAN4550 be left floating? 

    While this pin has an integrated pull-down, I would recommend pulling to ground externally to ensure noise does not cause an unintended reset of the device. 

    Let me know if you have any other questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 3170 points

    Hi Eric,

    Thanks for your detailed reply. It's very clear. 

    So, If customer can use SPI polling solution and only maintain critical CAN functionality, ISO7741 can be used for lower lost. Right?

    Thanks a lot.

    Best regards,

    Yang

  • 0 in reply to Yang Wu
    TI__Guru 52275 points

    Hi Yang,

    Yes, for basic functionality with a polling method, this solution is fine. 

    Regards,
    Eric Schott