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SMBUS reltaed question MSP430

Aamir Ali
Expert 2525 points

1. I have seen in many devices specifying Machine Model voltage & ESD protection volatge.? What do they signify.

2.In many of SMBus devices I have seen resistors (R36 & R37) , what is their purpose

3. I want 2 design a SmBUS slave device for freq 10-100KHZ. How to caluculate timeout period after which I have to bailout slave device after start condition when no bus activity seen?

4. What factors affects selection of value of pull up resistor in SMBus or I2c device. I have 2 devices, one of 3.3V & other is 5V. How value of resistor vary in two?

  • 0
    Genius 4925 points

    1. Electrostatic discharge can happen in almost any environment, it's basically a short pulse of a high voltage, the exact pulse form depends on the model which is used (human body, machine and something else I don't remember just now). If a device is rated for ESD, it means that it can withstand a pulse with the given voltage without being damaged. That is from a functional point of view. Some parameters may derate, like the current consumption might increase due to more leakage in the protection diodes and other things might happen as well.

    2. They are probably (not 100% sure) there to pull the lines to ground if the device is not connected to the SMBus (so there is a defined level)

    3. and 4. Have a read of the application note here over at Maxim, it's a comparison between I2C and SMBus and covers the timeout and resistor question.

  • 0 in reply to Bernhard Weller
    Expert 2525 points

    Bernhard Weller,

    If you look at page 3 of Bq28550, you will notice that that are two values given in KV for machine & human body. Does that mean such an high voltage for some short duration if yes that what duration exactly because its  obvious a high voltage at any pin can destroy it due to excessive current consumption

  • 0 in reply to Aamir Ali
    Genius 4925 points

    Sure, no normal voltage IC will withstand a voltage of 200V or 2kV for a prolonged duration.

    The human body pulse has a duration of around 200 ns, the machine model pulse is a lot different and not very well defined in terms of duration. But of course it's not the voltage which kills the devices but the current flowing, the current in the machine model is around ten times higher than that of the human body model, and that's why you will see that most devices can handle a machine model voltage of around ten times less than human body model.

    Maybe a good resource for ESD is this page here at siliconfareast of course there are also official standards covering ESD and it's models like the JEDEC standard here or the MIL-STD-883 or something from IEC is also available. 

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