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PWM MANAGEMENT FOR 3-PHASE BLDC MOTOR DRIVES USING THE ST7MC

– Select the group you want to read the BEMF from by programming the REO bit in the MPOL

register. The group the BEMF is read from depends on the sensorless control method used.

If the ST method is used, the high side group has to be selected because the PWM has to

be applied on the high side as seen in the previous chapter, therefore, the BEMF can be read

only from the high side group, REO bit is reset. If the classic method is used, the BEMF can

be read either from the high side group or from the low side group, it depends on where the

PWM signal will be applied as described below. So in this configuration, the REO bit can be

set or reset. Select also the polarity of the output channel through the MPOL register.

POLARITY REGISTER (MPOL)

Read/Write (some bits write-once)

Reset Value: 0011 1111 (3Fh)

Bit 7 = ZVD: Z vs D edge polarity.

0: Zero-crossing and End of Demagnetization have opposite edges

1: Zero-crossing and End of Demagnetization have same edge

Bit 6 = REO: Read on High or Low channel bit

0: Read the BEMF signal on High channels

1: Read on Low channels

Note: This bit always has to be configured whatever the sampling method.

Bits 5:0 = OP[5:0]: Output channel polarity.

These bits are used together with the OO[5:0] bits in the MPHST register to control the output

channels.

0: Output channel is Active Low

1: Output channel is Active High.

– Select the PWM direction after Demagnetization (D) event just before the Zero-crossing (Z)

event. The demagnetization event and the corresponding PWM configuration will be ex-

plained in Section 5. So the PWM direction can be selected using the OS1 bit in MCRB reg-

ister. The same is true for the group to read the BEMF from, the PWM can be applied

differently depending on the sensorless control method used. If the ST method is used, then

the PWM signal has to be applied on the high side, which means that the OS1 bit is reset in

MCRB register. If the classic sensorless control method is used, then the PWM signal can

be applied either on the high side or on the low side. In both cases, the microcontroller is able

7

6

5

4

3

2

1

0

ZVD

REO

OP5

OP4

OP3

OP2

OP1

OP0