Pulse-Width Modulator for Motor Control (PWMMC)
Output Control
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA Pulse-Width Modulator for Motor Control (PWMMC) 155
Figure 9-18. Current Convention
To allow for correction based on different current sensing methods or
correction controlled by software, the ISENS1 and ISENS0 bits i n PWM
control register 1 are provided to choose the correction method. These
bits provide correction according to Table 9-5.
If correction is to be done in software or is not necessary, setting
ISENS1:ISENS0 = 00 or = 01 causes the correction to be based on bits
IPOL1, IPOL2, and IPOL3 in PWM control register 2. If corr ection is no t
required, the user can initialize the IPOLx bits and then only load one
PWM value register per PWM pair.
To allow the user to use a current sense scheme based upon sensed
phase voltage during dead-time, setting ISENS1:ISENS0 = 10 causes
the polarity of the Ix pin to be latched when both the top and bottom
PWMs are off (for example, during the dead- time) . At t he 0 p ercen t and
100 percent duty cycle boundaries, there is no dead-time so no new
current value is sensed.
Table 9-5. Correction Methods
Current Correction Bits
ISENS1 and ISENS0 Correction Method
00
01 Bits IPOL1, IPOL2, and IPOL3 used for correctio n
10 Current sensing on pins IS1, IS2, and IS3 occurs
during the dead-time.
11 Current sensing on pins IS1, IS2, and IS3 occurs at
the half cycle in center-aligned mode and at the
end of the cycle in edge-aligned mode.
I+I-