LM3S5K36 Datasheet, PDF (877/1050 Pages) Texas Instruments – Stellaris® LM3S5K36 Microcontroller

Stellaris^® LM3S5K36 Microcontroller
19.3.7
The counter in a PWM generator can be reset to zero by writing the PWM Time Base Sync
(PWMSYNC) register and setting the SYNCn bit associated with the generator. Multiple PWM
generators can be synchronized together by setting all necessary SYNCn 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.
Additional synchronization can occur between multiple PWM generators by updating register contents
in one of the following three ways:
■ 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 at the end of the PWM cycle. In this case, the effect of the write is deferred, providing
a guaranteed defined behavior and preventing overly short or overly long output PWM pulses.
■ Globally Synchronized. The write value does not affect the logic until two sequential events
have occurred: (1) the Update mode for the generator function is programmed for global
synchronization in the PWMnCTL register, and (2) the counter reaches zero at the end of the
PWM cycle. In this case, the effect of the write is deferred until the end of the PWM cycle 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 across PWM generator blocks when the timers
in those blocks are synchronized, although this is not required in order for this mechanism to
function properly.
The following registers provide either local or global synchronization based on the state of various
Update mode bits and fields in the PWMnCTL register (LOADUPD; CMPAUPD; CMPBUPD):
■ Generator Registers: PWMnLOAD, PWMnCMPA, and PWMnCMPB
The following registers default to immediate update, but are provided with the optional functionality
of synchronously updating rather than having all updates take immediate effect:
■ Module-Level Register: PWMENABLE (based on the state of the ENUPDn bits in the PWMENUPD
register).
■ Generator Register: PWMnGENA, PWMnGENB, PWMnDBCTL, PWMnDBRISE, and
PWMnDBFALL (based on the state of various Update mode bits and fields in the PWMnCTL
register (GENAUPD; GENBUPD; DBCTLUPD; DBRISEUPD; DBFALLUPD)).
All other registers are considered statically provisioned for the execution of an application or are
used dynamically for purposes unrelated to maintaining synchronization and therefore do not need
synchronous update functionality.
Fault Conditions
A fault condition is one in which the controller must be signaled to stop normal PWM function and
then set the PWMn signals to a safe state. Two basic situations cause fault conditions:
■ The microcontroller is stalled and cannot perform the necessary computation in the time required
for motion control
■ An external error or event is detected
January 21, 2012
877
Texas Instruments-Production Data