It is for PID control of 4 DC motors. I have 2 hardware pwm outputs, and limited digital GPIOs. The pwm lines go to motor drivers. Currently the system has 2 DC motors, with a PID loop (and all data collecting) closed at 2kHz (for both motors). The idea was to split the pwm lines to portA and portB, and update a particular port every other loop: thus reducing the control rate to 1kHz (which is still acceptable).
I am using a gumstix basix to run this, and i2c seems to take about 0.25ms: which is way too long.
5V or 3.3V does not matter.