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Do you think it is legal to clock the I2C bus at 400 kHz when there are devices on the bus that are spec'ed only for 100 kHz? I may be wrong,

but

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such an idea sounds awfully risky to me.

Per Philips AN10216-01 I2C Manual (March 24, 2003) you can do this legally if you use I2C repeater such as PCA9515. This device isolates the 100kHZ only segment when you are talking to 400kHz segment at fast mode.

Some additional info:

The I2C-Bus Specification Version 2.1 (Jan 2000) section 13.5 says:

"13.5 Mixed speed modes on one serial bus system.

If a system has a combination of Hs-, Fast-, and/or Standard-mode devices, it's possible, by using an interconnection bridge, to have different bit rates between different devices (see Fig 24 and Fig 25). .... Such a bridge can be implemented in any IC as an autonomous circuit. ..."