BEMF Zero crossing detection internal peripheral

AN4370

8/23

DocID025336 Rev 1

BC pin will not update the counter value. This implies that the first value of the counter must

be written before selecting the BC mode. Besides, in BC mode, the BEMF counter will

automatically increase or decrease its value at the reception of a positive pulse according to

the BEMFDIR bit value. Definitely the BC mode is useful for motors with a very high step

switching frequency that would be for sure limited by the SPI communication itself.

Commutation. The offset sign is automatically updated in BC mode by the positive pulses,

while it has to be overridden in SPI mode at each BEMFCNT update event. Be aware that

demagnetization pulses could appear at the BEMFOUT pin, to filter them the BEMFCNT will

have to be updated after a certain delay from a commutation event taking into account the

demagnetization time. To understand better the demag pulses filtering at the BEMFOUT pin

by delaying the BEMFCNT update events respect to the commutation events, we will

consider Figure 4 concerning a 100% duty cycle PWM. The concept is the same with a duty

cycle different than 100%, in fact the only difference would be the sampling frequency. In

comparator, will be done at 10MHz. Let’s focus on the red vertical line of Figure 4. The line

shows where, after the commutation, we updated the BEMFCNT, thus changing the signal

to be sampled, in order to detect the next crossing event, from SH1 to SH3. As it can be

seen from the picture, if we had continued sampling SH1 until the end of the demag pulse,

we would have completely filtered the SH3 demag pulse at the BEMFOUT pin. This could

have been made possible, merely by delaying the BEMFCNT update event respect to the

related commutation event until the end of the demag pulse itself. In the example shown by

Figure 4 the BEMFCNT was updated right during the demag pulse, that is why the pulse is

partially but not totally filtered at the BEMFOUT pin.

Figure 4. BEMFCNT update event

Customers not interested in detecting the rotor position through the BEMF Detection IP,

could use it to read the motor speed. In this latter case two consecutive zero crossing

events, corresponding to signal transitions (one rising edge and one falling edge, excluding

of course demagnetization pulses) at the BEMFOUT pin, will provide the step time.