Motorola M68HC08 - Manual (Page 157)

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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