Motorola M68HC08 Reference Manual page 98

Software Design
Designer Reference Manual
98
Freescale Semiconductor, Inc.
Service of Commutation
As already explained, the motor phase commutation is performed in
the OC interrupt service routine. The phase commutated flag
(PC_F=1) indicates this action to the scheduler, which allows the
performed commutation to be serviced. Detailed explanation of this
state is in Processes Commutation and Zero Crossing Preset and
Set.
Service of Received Back-EMF Zero Crossing
The back-EMF zero crossing is detected by PWM middle function
block. Then the appropriate flag (captured received the zero crossing
event - ICR_F) is set by PWM centre interrupt service routine. This
indicates to the scheduler that the zero crossing event must be
served.
The following actions are taken:
1. Commutation parameters are recalculated more precisely based
on the received feedback
2. Commutation time is preset to the output compare register of timer
2
For a better understanding of how the commutation process works,
see 3.3.1 Sensorless Commutation
— Commutation Time
BEMF Zero Crossing Sensing Interrupt Service Routine
This ISR is used to evaluate the back-EMF zero crossing. Back-EMF
is evaluated here in order to synchronize zero crossing capture with
the middle of central-aligned PWM. This technique rejects the noise
caused by PWM from the back-EMF signal. When this ISR is initiated,
then three samples of the zero crossing input (BEMF_IN) are taken
and the state is evaluated. Based on the expected edge (V_TASC2,
ELS2A_ELS2B) and the evaluated state of the BEMF_IN pin, the
zero crossing event is verified. If it is accepted, then the captured time
is stored in variable (T_ZCros) and the PWM ISR is finished. The
appropriate flag (captured received the zero crossing event — ICR_F)
is set.
Current Measurement Interrupt Service Routine
Software Design
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Control, (see
Calculation).
3.3.1.5 Starting
DRM028 — Rev 0
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