ST7MC1K2 datasheet(227/371 Pages) STMICROELECTRONICS

227 / 371 page

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On-chip peripherals

227/371

Speed measurement mode

Motor speed can be measured using two methods depending on sensor type: period measurement or

pulse counting. Typical sensor handling is described here.

Incremental encoders allows accurate speed measurement by providing a large number of pulses per

revolution (ppr) with ppr rates up to several thousands; the higher the ppr rate, the higher the resolution.

The proposed method consists of counting the number of clock cycles issued by the incremental encoder

interface (encoder clock) during a fixed time window (refer to Figure 101).

The tachogenerator has a much lower ppr rate than the encoder (typically factor 10). In this context, it is

more meaningful to measure the period between tacho captures (that is, relevant transitions of the

incoming signals). Accuracy is imposed by the reference clock, that is, the CPU clock (refer to

Figure 100).

Figure 100. Tachogenerator period acquisition using MTIM timer

Figure 101. Encoder clock frequency measure using MTIM timer

Comparator

output

Tacho

capture

Compare

value

MTIM counter

value

Interrupts

to interrupt generator

(capture event)

(speed error event)

Decreasing speed

Encoder clock

Capture (triggered by

software or real time clack

MTIM counter value

Interrupts

to interrupt generator

(capture event)

Decreasing speed

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