I²C bus at 400 kHz — what it sets for the bus budget
The 400 kHz clock is standard fast-mode I²C. That limits the total bus capacitance (including trace, cable, and device input capacitance) to about 400 pF for reliable edge rates. If you are running this expander on a long backplane or through a ribbon cable, you will need a bus buffer or repeater. The JTAG interface shares the same pins as the I²C lines, selected via the configuration EEPROM — so you get boundary-scan test access without extra package pins.