The FT4222 I2C master clock frequency was hard-coded to 400 kbps in Ft4222I2cBus, with no public way to change it (e.g. to 100 kbps). Per the LibFT4222 user guide, FT4222_I2CMaster_Init supports speeds from 60 to 3400 kbps.

Changes

• Ft4222I2cBus: Replaced the private 400 constant with public Default/Minimum/Maximum (400/60/3400) constants. Made Ft4222I2cBus public so those constants are part of the public API. Added constructor overloads for the default and explicit frequency cases, validating the range and surfacing the selected value via a read-only I2cMasterFrequencyKbps property.
• Ft4222Device: Added a public I2cBusFrequencyKbps property (default 400 kbps), forwarded to Ft4222I2cBus in CreateI2cBusCore.
• README: Documented configuring the frequency.

Usage

Ft4222Device device = new Ft4222Device(FtCommon.GetDevices()[0]);
device.I2cBusFrequencyKbps = 100;
using I2cBus ftI2c = device.CreateI2cBus();

Values outside 60–3400 kbps throw ArgumentOutOfRangeException. Existing callers are unaffected since the default preserves the prior 400 kbps behavior, and the constructor overload pair avoids relying on a default argument for compatibility.