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Pwm frequency setting – Motorola M68HC08 User Manual

Page 157

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User Guide

Tuning for Customer Motor

DRM028 — Rev 0

Designer Reference Manual

MOTOROLA

User Guide

157

Range: <1,255>

The final PWM period is defined by setting SET_PER_PWM.

The PWM period [

µs] is:

PWM period = PERIOD_PWM_US = SET_PER_PWM*2 (EQ 6-14.)

With default software setting (oscillator clock, etc.).

The final PWM frequency [Hz] is:

(EQ 6-15.)

With default software setting.

Settings for some important PWM frequencies are listed in

Table 6-3

.

CAUTION:

Current measurement sampling period is synchronized with PWM.
Therefore, changing PWM frequency automatically changes the current
sampling period. This, apart from other things, effects the current
regulator. Therefore, after changing PWM frequency, changing (or
checking) current sampling period is strongly recommended.

6.5.7.2 Current Sampling Period

Current sampling period should usually be changed in two cases:

1. When PWM frequency is changed

2. Motors with externally low electrical constant

If the motor electrical constant is lower than default current sampling
period of 128

µs, the current regulator may not work properly.

Table 6-3. PWM Frequency Setting

SET_PER_PWM

PWM Frequency

(FREQUENCY_PWM)

PWM Period

(PERIOD_PWM_US)

16.0

31.250 Hz

32

µs

25.0

20.000 Hz

50

µs

32.0 (default)

15.625 Hz

64

µs

128.0

3.90625 Hz

256

µs

PWMfrequency

10

6

2SET_PER_PWM

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