ADSP-21368 SHARC Processor Hardware Reference 8-13
Pulse Width Modulation
• Full on. The PWM for any pair of PWM signals is said to operate
in full on when the desired high side output of the three-phase tim-
ing unit is in the on state (low) between successive
PWMSYNC rising
edges. This state may be entered by virtue of the commanded duty
cycle values in conjunction with the setting in the PWMDTx registers.
• Full off. The PWM for any pair of PWM signals is said to operate
in full off when the desired high side output of the three-phase tim-
ing unit is in the off state (high) between successive PWMSYNC pulses.
This state may be entered by virtue of the commanded duty cycle
values in conjunction with the setting in the PWMDTx registers.
• Normal modulation. The PWM for any pair of PWM signals is
said to operate in normal modulation when the desired output duty
cycle is other than 0% or 100% between successive PWMSYNC pulses.
There are certain situations, when transitioning either into or out of either
full on or full off, where it is necessary to insert additional emergency dead
time delays to prevent potential shoot through conditions in the inverter.
The use of crossover, described in “Crossover” on page 8-16, can also cause
outputs to violate the shoot through conditions criteria. These transitions
are detected automatically and, if appropriate, the emergency dead time is
inserted to prevent the shoot through conditions.
Inserting additional emergency dead time into one of the PWM signals of
a given pair during these transitions is only needed if both PWM signals
would otherwise be required to toggle within a dead time of each other.
The additional emergency dead time delay is inserted into the PWM sig-
nal that is toggling into the on state. In effect, the turn on (if turning on
during this dead time region) of this signal is delayed by an amount
2 × PWMDT × t
PCLK
from the rising edge of the opposite output. After
this delay, the PWM signal is allowed to turn on, provided the desired
output is still scheduled to be in the on state after the emergency dead
time delay.
To Next Page
Loading...
Need help?
General
