Motorola M68HC08 manual PWM period = Periodpwmus = SETPERPWM*2 EQ, PWM Frequency Setting

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

User Guide

Tuning for Customer Motor

Range: <1,255>

The final PWM period is defined by setting SET_PER_PWM.

The PWM period [∝s] 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:

PWMfrequency =	106
	2SET------------------------------------------------_PER_PWM	(EQ 6-15.)

With default software setting.

Settings for some important PWM frequencies are listed in Table 6-3.

Table 6-3. PWM Frequency Setting

SET_PER_PWM	PWM Frequency	PWM Period
	(FREQUENCY_PWM)	(PERIOD_PWM_US)
16.0	31.250 Hz	32 ∝s
25.0	20.000 Hz	50 ∝s
32.0 (default)	15.625 Hz	64 ∝s
128.0	3.90625 Hz	256 ∝s

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 ∝s, the current regulator may not work properly.

DRM028 — Rev 0		Designer Reference Manual
MOTOROLA	User Guide	157
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