What is the point of defining VOL and IOL separately in I2C and SMBUS?

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  • \$\begingroup\$ I'm not exactly sure, but if you have hardware I2C and hardware SMBus, they're physically different circuits that may and will have different sinking capabilities and voltage performance. They can be closely matched, but they're still different circuits on hardware level. But I don't know for sure. Besides, maybe you should specify what IC you're talking about \$\endgroup\$
    – Ilya
    Aug 31 '20 at 8:42
  • \$\begingroup\$ look at smbus on wikipedia. It has two current specs of 350 uA and 4 mA where I2C just has 3 mA, so all the more puzzling! \$\endgroup\$
    – Neil_UK
    Aug 31 '20 at 10:01

Am assuming these specs refer to bus connections logic levels. Keep in mind that these lines are pulled up to some unknown positive voltage with a pull-up resistor, and are pulled down to ground by a NMOS transistor, or by a NPN bipolar transistor, or possibly even by a very fast switch.
The NMOS transistor might be viewed as a pull-down resistor when turned on. Its resistance varies as its drain voltage on the bus drops, but we're mostly interested in its characteristics when it yanks the bus line low.

  • The \$V_{OL}\$ spec says that the NMOS drain-to-source resistance should be no more than 100 ohms when it is turned on, and is pulling the bus low.
  • The \$I_{OL}\$ spec says that the NMOS transistor should maintain its less-than-100 ohm drain-to-source resistance while 3mA flows through it.

Viewing the bus pull-down device as a turned-on resistor this way is a gross simplification - a transistor of any type is non-linear.


That is because while I2C and SMBUS are closely related and the devices are quite interoperable, they still have different electrical specifications, so they can't be listed as one common entry in the specs. It just highlights to the reader that do note that these parameters are different in different standards and it might be important when connecting various chips together.

And it seems there is an error in the table. I2C should have 3mA sink current and SMBUS should have 4mA sink current.

  • \$\begingroup\$ Actually even in I2C data sheet they specify seperate VOL and IOL specifications?Why is it so? \$\endgroup\$ Aug 31 '20 at 10:32

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