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LTC4332 remote INT duration

Thread Summary

The user is experiencing issues with the LTC4332 SPI extender where a 20us remote INT pulse is sometimes not transferred to the local side. The final answer explains that the INT input is sampled based on the link heartbeat refresh, which varies with the speed configuration. To ensure the INT is observed, the pulse must be held longer than the heartbeat period, which ranges from 845 us to 11 us depending on the speed index setting.
AI Generated Content
Category: Hardware
Product Number: LTC4332

Hello everyone,

I have an issue with LTC4332 SPI extender regarding remote INT duration. I use extender with the remote device which generates 20us pulse when the interrupt occurs, but sometimes it is not passed to local side (Fig1). I've attached several scope screenshots where this manifestation can be observed. I also use LTC4332 in other application where INT is active until the data is read, and that works perfectly.

Fig1. Interrupt generated on remote side, but isn't transferred to local

Fig2. Delay between remote and local

Fig3. Delay between remote and local

Fig4. Remote interrupt not transferred to local

  • Hi Miodrag,

      Absent any SPI traffic, the INT input on the LTC4332 is periodically sampled based on the link heart beat refresh which is based on the speed configuration settings.  When there is SPI traffic, the INT is sampled at the start of the transaction and then again after the transaction completes when a heartbeat occurs again.  The LTC4332 assumes that once asserted the INT would remain asserted until the SPI controller responds to the target device asserting the INT.

      In order to assure that the INT is observed, it will need to be held down long enough for at least one heartbeat refresh to occur.  I've measured the heartbeat period for the different speed index values using some eval boards on the bench:

    Speed Index LTC4332
    0 845 us
    1 679 us
    2 340 us
    3 255 us 
    4 171 us
    5 84 us
    6 43 us
    7 22 us
    8 11 us

    Depending on the selected speed, you'll want to make sure that your INT is held down longer than the above times.

    Eric

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