Filter apparatus and method for brushless DC motors

In a multi-phase brushless DC motor, a zero crossing N-bit filter comprising:

• a comparator;
• a phase multiplexer;
• said phase multiplexer connecting each motor phase to each of a positive and a negative input of said comparator, with a switch in each connection;
• a microprocessor disposed to operate said switches,
• said microprocessor being configured to measure a BEMF for a first phase by opening said switches connecting all other phases to said positive input of said comparator and by opening said switch connecting said first phase being measured to said negative input of said comparator;
• said comparator outputting a signal that is received by a shift register;
• and
• said microprocessor being further configured to respond to a zero crossing when a majority of bits in said shift register change between high and low;
• said shift register in operative communication with an output of said comparator;
• an extra bit, said extra bit being a hysteresis bit;
• wherein said extra bit has a present state that is the present state of a zero cross.