ST7MC1K2 datasheet(228/371 Pages) STMICROELECTRONICS

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Hall sensors (or equivalent sensors providing position information) are widely used for motor

control. There are two cases to be considered:

●

BLDC motor or six-step synchronous motor drive; ‘sensor mode’ is recommended in

this case, as most tasks are performed by hardware in the delay manager.

●

BLAC, asynchronous or motors supplied with 3-phase sinewave-modulated PWM

signals in general. In this case ‘speed sensor mode’ allows high accuracy speed

measurement (the sensor mode of the delay manager being unsuitable for sinewave

generation). Position information is handled by software to lock the statoric field to the

rotoric one for driving synchronous motors.

Hall sensors are usually arranged in a 120° configuration. In that case they provide 3 ppr

with both rising and falling edge triggering; the tachogenerator measurement method can

therefore be applied. The main difference lies in the fact that one must use the position

information they provide. This can be done using the three MCIx pins and the analog

multiplexer to know which of the three sensors toggled; an interrupt is generated just after

the expected transition (refer to Figure 102).

As described in Figure 103, the MTIM timer is re-configured depending on the selected

sensor. This means that most of delay manager registers are used for a different purpose,

with modified functionalities.

For greater precision, the MTIM up-counter is extended to 16 bits using MTIM and an

additional MTIML register. On a capture event, the current counter value is captured and the

counter [MTIM:MTIML] is cleared. The counting direction is not affected by the EDIR bit

when using an encoder sensor.

A 16-bit capture register is used to store the captured value of the extended MTIM counter:

the speed result is either a period in clock cycles or a number of encoder pulses. This 16-bit

value is not corrupted between the high and low byte accesses, a read access to the MSB of

this register (MZREG) locks the LSB (that is, MZPRV content is locked) until it is read and

any other capture event in between these two accesses is discarded.

A compare unit allows a maximum value to be entered for the tacho periods. If the 16-bit

counter [MTIM:MTIML] exceeds this value, a speed error interrupt is generated. This may be

used to warn the user that the tachogenerator signal is lost (wires disconnected, motor

stalled,...). As 8-bit accuracy is sufficient for this purpose, only the MSByte of the counter

(that is, MTIM) is compared to 8-bit compare register, mapped in the MDREG register

location. The LSByte is nevertheless compared with a fixed FFh value. Available values for

comparison are therefore FFFFh, FEFFh, FDFFh, ..., 01FFh, 00FFh.

Note:

This functionality is not useful when using an encoder. With an encoder, user must monitor

the captured values by software during the periodic capture interrupts: for instance, when

driving an AC motor, if the values are too low compared to the stator frequency, a software

interrupt may be triggered.

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