LM3S6965 Microcontroller

18 Pulse Width Modulator (PWM)

Pulse width modulation (PWM) is a powerful technique for digitally encoding analog signal levels. High-resolution counters are used to generate a square wave, and the duty cycle of the square waveismodulatedtoencodeananalogsignal. Typicalapplicationsincludeswitchingpowersupplies and motor control.

TheStellaris® PWMmoduleconsistsofthreePWMgeneratorblocksandacontrolblock. Thecontrol blockdeterminesthepolarityofthePWMsignals,andwhichsignalsarepassedthroughtothepins.

Each PWM generator block produces two PWM signals that can either be independent signals (other than being based on the same timer and therefore having the same frequency) or a single pair of complementary signals with dead-band delays inserted. The output of the PWM generation blocks are managed by the output control block before being passed to the device pins.

The Stellaris® PWM module provides a great deal of flexibility. It can generate simple PWM signals, such as those required by a simple charge pump. It can also generate paired PWM signals with dead-band delays, such as those required by a half-H bridge driver. Three generator blocks can also generate the full six channels of gate controls required by a 3-phase inverter bridge.

Each Stellaris® PWM module has the following features:

■Three PWM generator blocks, each with one 16-bit counter, two PWM comparators, a PWM signal generator, a dead-band generator, and an interrupt/ADC-trigger selector

■One 16-bit counter

–Runs in Down or Up/Down mode

–Output frequency controlled by a 16-bit load value

–Load value updates can be synchronized

–Produces output signals at zero and load value

■Two PWM comparators

–Comparator value updates can be synchronized

–Produces output signals on match

■PWM generator

–Output PWM signal is constructed based on actions taken as a result of the counter and PWM comparator output signals

–Produces two independent PWM signals

■Dead-band generator

–ProducestwoPWMsignalswithprogrammabledead-banddelayssuitablefordrivingahalf-H bridge

–Can be bypassed, leaving input PWM signals unmodified

■Flexible output control block with PWM output enable of each PWM signal

Preliminary

Pulse Width Modulator (PWM)

–PWM output enable of each PWM signal

–Optional output inversion of each PWM signal (polarity control)

–Optional fault handling for each PWM signal

–Synchronization of timers in the PWM generator blocks

–Synchronization of timer/comparator updates across the PWM generator blocks

–Interrupt status summary of the PWM generator blocks

■Can initiate an ADC sample sequence

18.1Block Diagram

Figure18-1onpage486providestheStellaris® PWMmoduleunitdiagramandFigure18-2onpage 487 provides a more detailed diagram of a Stellaris® PWM generator. The LM3S6965 controller contains three generator blocks (PWM0, PWM1, and PWM2) and generates six independent PWM signals or three paired PWM signals with dead-band delays inserted.

Preliminary