LTC4306 - Downstream Bus Connection Fault

Question

Hi, 
 We have a design implementing the LTC4306 on a backplane to provide I2C to 4 downstream modules. In some scenarios, 1 or more of these modules may be present. 
 We would like to use the 'Downstream Bus Connection Fault" to detect if a module is missing, which requires less than 1V on the downstream SDA/SCL pins. Is the suggested implementation for this to have the downstream I2C pullup resistors on the module, so that the downstream bus pins of the LTC4306 are floating when no module is connected? 
 Or do we need to include a weak pulldown on the backplane to ensure the downstream bus pins are below the 1V threshold? 
 Thanks in advance for your support. 
 Cooper

ericb · Accepted Answer

Hi Cooper, 
 You will want the SCL/SDA for the missing module to be below 1V when the module is not present in order to trigger the Downstream Bus Connection Fault when the LTC4306 attempts to connect to that bus segment. When the module is present and its bus segment is idle, SCL/SDA should be high to avoid the fault when the LTC4306 attempts connection. 
 Placing a weak pull-down on the backplane will keep the backplane portion of SCL/SDA low when the module is not present. SCL/SDA will still need a pull-up, but placing it on the module will assure that these pins only go high when the module is physically present and idle. 
 Note that the pull-up and pull-down resistor values will need to be coordinated to make sure that high voltage is sufficiently high. I'd expect that most weak pull-down and normal pull-up resistor values will work, but it would still be wise to confirm this to be true. 
 
 Eric)

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LTC4306 - Downstream Bus Connection Fault