Freescale Semiconductor MC68HC908MR32 - Manual (Page 131)

Output Control
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
Freescale Semiconductor 131
For a typical motor drive inverter as shown in Figure 12-13 , for a given top/bottom transistor pair, only one
of the transistors will be effective in controlling the output voltage at any given time depending on the
direction of the motor current for that pair. To achieve distortion correction, one of two different correction
factors must be added to the desired PWM value, depending on whether the top or bottom transistor is
controlling the output voltage. Therefore, the software is responsible for calculating both compensated
PWM values and placing them in an odd/even PWM register pair. By supplying the PWM module with
information regarding which transistor (top or bottom) is controlling the output voltage at any given time
(for instance, the current polarity for that motor phase), the PWM module selects either the odd or even
numbered PWM value register to be used by the PWM generator.
Current sensing or programmable software bits are then used to determine which PWM value to use. If
the current sensed at the motor for that PWM pair is positive (voltage on current pin ISx is low) or bit IPOLx
in PWM control register 2 is low, the top PWM value is used for the PWM pair. Likewise, if the current
sensed at the motor for that PWM pair is negative (voltage on current pin ISx is high) or bit IPOLx in PWM
control register 2 is high, the bottom PWM value is used. See
Table 12-4 .
NOTE
This text assumes the user will provide current sense circuitry which causes
the voltage at the corresponding input pin to be low for positive current and
high for negative current. See
Figure 12-19 for current convention. In
addition, it assumes the top PWMs are PWMs 1, 3, and 5 while the bottom
PWMs are PWMs 2, 4, and 6.
Figure 12-19. Current Convention
Table 12-4. Current Sense Pins
Current
Sense Pin
or Bit
Voltage
on Current
Sense Pin
or IPOLx Bit
PWM Value
Register Used
PWMs
Affected
IS1 or IPOL1 Logic 0 PWM value register 1 PWMs 1 and 2
IS1 or IPOL1 Logic 1 PWM value register 2 PWMs 1 and 2
I+
I-