LM3S9B92_11 Datasheet, PDF (1125/1386 Pages) Texas Instruments – Stellaris LM3S9B92 Microcontroller

Stellaris® LM3S9B92 Microcontroller
21.4
21.5
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 269).
2. Enable the clock to the appropriate GPIO module via the RCGC2 register in the System Control
module (see page 286).
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 24-4 on page 1238.
4. Configure the PMCn fields in the GPIOPCTL register to assign the PWM signals to the appropriate
pins (see page 439 and Table 24-5 on page 1247).
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.
9. Set the pulse width of the PWM1 pin for a 75% duty cycle.
■ Write the PWM0CMPB register with a value of 0x0000.0063.
10. Start the timers in PWM generator 0.
■ Write the PWM0CTL register with a value of 0x0000.0001.
11. Enable PWM outputs.
■ Write the PWMENABLE register with a value of 0x0000.0003.
Register Map
Table 21-3 on page 1126 lists the PWM registers. The offset listed is a hexadecimal increment to the
register's address, relative to the PWM module's base address:
March 19, 2011
Texas Instruments-Advance Information
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