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PMBus vs I2C bus

Hi,

I am a beginner with PMBus. I am trying to understand the difference between I2C and PMBus. I understand PMBus standard is based off I2C spec. 

Do systems have different PMBus and I2C bus, or can the same bus be used for serial communication between power management IC's and other IC's?

Are the capacitance limits for PMBus the same as I2C bus (400 pF for Fast mode?).

Thanks,

Rajan

  • Rajan,

    [ACK] -- excuse the joke :)

    Yes, the capacitance specifications are the same.

    The theoretical difference is that PMBus is a superset of SMBus which is a superset of I2C. If you take a look at the 7-layers of the OSI Model, I2C is just the Physical Layer. SMBus adds the Data Link and Network Layers, and PMBus adds the Transport layer and a set of >200 commands that are specific to communicating with power management devices.

    Some practical differences include the fact that with the exception of the Alert Response Address (ARA) call, PMBus will always include a command byte after sending an address, whereas I2C can simply send a slave address and a read bit if it so chooses, and expect the slave to send back some data. PMBus on the other hand will send a slave address, read/write bit, and then send a command byte after receiving an ACK from the slave.

    In many instances, PMBus and I2C devices can talk to eachother, but there are some incompatibilities such as the one I highlighted above. I suppose what you suggest could be done in a pinch, but don't expect to put a device supporting PMBus on a regular I2C bus and have it fully function. For one thing, youll be missing the PMBus CTRL and PMBus Alert lines. If a system supports both I2C and PMBus, it will keep the two buses seperate.

    I would suggest reading through the I2C, PMBus, and SMBus specifications while learning.