LM3S3768 Luminary Micro, Inc, LM3S3768 Datasheet - Page 609

• Manufacturer Part Number: LM3S3768
• Description: Lm3s3768 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

19.2.5
19.2.6
June 02, 2008
Interrupt/ADC-Trigger Selector
The PWM generator also takes the same four (or six) counter events and uses them to generate
an interrupt or an ADC trigger. Any of these events or a set of these events can be selected as a
source for an interrupt; when any of the selected events occur, an interrupt is generated. Additionally,
the same event, a different event, the same set of events, or a different set of events can be selected
as a source for an ADC trigger; when any of these selected events occur, an ADC trigger pulse is
generated. The selection of events allows the interrupt or ADC trigger to occur at a specific position
within the PWM signal. Note that interrupts and ADC triggers are based on the raw events; delays
in the PWM signal edges caused by the dead-band generator are not taken into account.
Synchronization Methods
The PWM unit provides four PWM generators providing eight PWM outputs that may be used in a
wide variety of applications. Generally speaking, this falls into combinations of two categories of
operation:
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If multiple PWM generators are configured with the same counter load value, this can be used to
guarantee that they also have the same count value (this does imply that the PWM generators must
be configured before they are synchronized). With this, more than two PWM signals can be produced
with a known relationship between the edges of those signals since the counters always have the
same values. Other states in the unit provide mechanisms to maintain the common time base and
mutual synchronization.
The counter in a PWM unit generator can be reset to zero by writing the PWM Time Base Sync
(PWMSYNC) register and setting the Sync bit associated with the generator. Multiple PWM
generators can be synchronized together by setting all necessary Sync bits in one access. For
example, setting the Sync0 and Sync1 bits in the PWMSYNC register causes the counters in PWM
generators 0 and 1 to reset together.
Additionally, the state of a PWM unit is affected by writing to the registers of the PWM unit and the
PWM units' generators, which has an effect on the synchronization between multiple PWM generators.
Depending on the register accessed, the register state is updated in one of the following three ways:
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■
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Unsynchronized: The PWM generator and its two output signals are used by itself, independent
of other PWM generators.
Synchronized: The PWM generator and its two outputs signals are used in conjunction with other
PWM generators using a common, unified time base.
Immediately: The write value has immediate effect, and the hardware reacts immediately.
Locally Synchronized: The write value does not affect the logic until the counter reaches the
value zero. In this case, the effect of the write is deferred until the end of the PWM cycle (when
the counter reaches zero). By waiting for the counter to reach zero, a guaranteed behavior is
defined, and overly short or overly long output PWM pulses are prevented.
Globally Synchronized: The write value does not affect the logic until two sequential events have
occurred: (1) the global synchronization bit applicable to the generator is set, and (2) the counter
reaches zero. In this case, the effect of the write is deferred until the end of the PWM cycle (when
the counter reaches zero) following the end of all updates. This mode allows multiple items in
multiple PWM generators to be updated simultaneously without odd effects during the update;
everything runs from the old values until a point at which they all run from the new values. The
Update mode of the load and comparator match values can be individually configured in each
PWM generator block. It typically makes sense to use the synchronous update mechanism
Preliminary
LM3S3768 Microcontroller
609