ST7MC1K2-Auto datasheet(224/371 Pages) STMICROELECTRONICS

224 / 371 page

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ST7MC1K2-Auto, ST7MC1K6-Auto, ST7MC2S4-Auto, ST7MC2S6-Auto

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Figure 98.

Simulated commutation and Z event

Figure 99 gives the step ratio register value (left axis) and the number of BEMF sampling during one

electrical step with the corresponding accuracy on the measure (right axis) as a function of the

mechanical frequency.

For a given prescaler value (step ratio register) the mechanical frequency can vary between two fixed

values shown on the graph as the segment ends. In autoswitched mode, this register is automatically

incremented/decremented when the step frequency goes out of this segment.

At fMTC = 4 MHz, the range covered by the step ratio mechanism goes from 2.39 to 235000

(pole pair x rpm) with a minimum accuracy of 1.2% on the step period.

To read the number of samples for Zn within one step (right Y axis), select the mechanical frequency on

the X axis and the sampling frequency curve used for BEMF detection (PWM frequency or measurement

window frequency). For example, for N.Frpm = 15000 and a sampling frequency of 15 kHz, there are

approximately 10 samples in one step and there is a 10% error rate on the measurement.

SC bit is reset. The result

MCOMP

of the hardware multiplication

is put in MCOMP-->CH and

compared with MTIM once

SC bit is set during Z IT. The

hardware multiplication is taken into

account but the value in MCOMP can

be overwritten

SC bit is already set when Z IT occurs.

The hardware multiplication is not

taken into account. A value has to be

written in the MCOMP register.

Zero-crossing event

Demagnetization event

Hardware commutation

Simulated commutation

written.

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