LM3S5R36_13 Datasheet, PDF (883/1063 Pages) Texas Instruments – LM3S5R36 Microcontroller

NRND: Not recommended for new designs.
Stellaris^® LM3S5R36 Microcontroller
19.4
This function can be used, for example, to perform commutation of a brushless DC motor with a
single register write (and without modifying the individual PWM generators, which are modified by
the feedback control loop). In addition, the updating of the bits in the PWMENABLE register can
be configured to be immediate or locally or globally synchronized to the next synchronous update
using the PWM Enable Update (PWMENUPD) register.
During fault conditions, the PWM output signals, PWMn, usually must be driven to safe values so
that external equipment may be safely controlled. The PWMFAULT register specifies whether during
a fault condition, the generated signal continues to be passed driven or to an encoding specified in
the PWMFAULTVAL register.
A final inversion can be applied to any of the PWMn signals, making them active Low instead of the
default active High using the PWM Output Inversion (PWMINVERT). The inversion is applied even
if a value has been enabled in the PWMFAULT register and specified in the PWMFAULTVAL
register. In other words, if a bit is set in the PWMFAULT, PWMFAULTVAL, and PWMINVERT
registers, the output on the PWMn signal is 0, not 1 as specified in the PWMFAULTVAL register.
Initialization and Configuration
The following example shows how to initialize PWM Generator 0 with a 25-kHz frequency, a 25%
duty cycle on the PWM0 pin, and a 75% duty cycle on the PWM1 pin. This example assumes the
system clock is 20 MHz.
1. Enable the PWM clock by writing a value of 0x0010.0000 to the RCGC0 register in the System
Control module (see page 257).
2. Enable the clock to the appropriate GPIO module via the RCGC2 register in the System Control
module (see page 274).
3. In the GPIO module, enable the appropriate pins for their alternate function using the
GPIOAFSEL register. To determine which GPIOs to configure, see Table 22-4 on page 986.
4. Configure the PMCn fields in the GPIOPCTL register to assign the PWM signals to the appropriate
pins (see page 445 and Table 22-5 on page 992).
5. Configure the Run-Mode Clock Configuration (RCC) register in the System Control module
to use the PWM divide (USEPWMDIV) and set the divider (PWMDIV) to divide by 2 (000).
6. Configure the PWM generator for countdown mode with immediate updates to the parameters.
■ Write the PWM0CTL register with a value of 0x0000.0000.
■ Write the PWM0GENA register with a value of 0x0000.008C.
■ Write the PWM0GENB register with a value of 0x0000.080C.
7. Set the period. For a 25-KHz frequency, the period = 1/25,000, or 40 microseconds. The PWM
clock source is 10 MHz; the system clock divided by 2. Thus there are 400 clock ticks per period.
Use this value to set the PWM0LOAD register. In Count-Down mode, set the LOAD field in the
PWM0LOAD register to the requested period minus one.
■ Write the PWM0LOAD register with a value of 0x0000.018F.
8. Set the pulse width of the PWM0 pin for a 25% duty cycle.
■ Write the PWM0CMPA register with a value of 0x0000.012B.
October 06, 2012
883
Texas Instruments-Production Data