Other Parts Discussed in Thread: TM4C1294NCPDT
I am having the worst time conceivable understanding and using the PWM module for my Tiva TM4C1294NCPDT. I am trying to do the two following base tasks:
- Set the Period (target: 20ms)
- Set the Pulse Width (target: any value 0-20ms)
I have consulted the Peripheral Library Reference Manual and the provided example code (TivaWare_C_Series-2.1.3.156/examples/peripherals/pwm) and am very painfully unable to correctly set Period or Pulse Width. See the attached code ZIP, and code below for full illustration.
Problems
- I cannot successfully change the period
- I cannot understand how the math here works
- e.g. how to calculate the values for the set functions below. I can provide numeric examples of my results and confusion in them if needed
Questions (the starters I think...!)
-
How to get a PWM signal with a 20ms pulse??
- How does the math work for setting the period?
This feels mystical and confusing, in strong contrast to my MSP430 experiences. What am I missing here?? :(
#define F_CPU_MHZ (20000000)
#define PWM_PER (6400000)
int main(void) {
/* Begin TivaWare Peripheral Driver Library Example (p.433)
Note: Uses 'PWM_BASE', replaced here with valid 'PWM0_BASE'
Changes - PWM_GEN_0 -> PWM_GEN_1
OUT_0 -> OUT_2, OUT_1 -> OUT_3
OUT_0_BIT -> OUT_2_BIT, OUT_1_BIT -> OUT_3_BIT
Goal - Set the period of the PWM signal
Goal - Set each pulse width, OUT_2 and OUT_3
A Target - Set PWM0 to 20ms period, with 1ms pulse on OUT_2 and 19ms pulse on OUT_3
Ultimate Goal - Have this dang thing make sense, and be able to use it like TivaWare states it can! */
//<Begin Added Code>
g_ui32SysClock = SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ | SYSCTL_OSC_MAIN | SYSCTL_USE_OSC), F_CPU_MHZ);
SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOF);
GPIOPinConfigure(GPIO_PF2_M0PWM2);
GPIOPinConfigure(GPIO_PF3_M0PWM3);
GPIOPinTypePWM(GPIO_PORTF_BASE, GPIO_PIN_2);
GPIOPinTypePWM(GPIO_PORTF_BASE, GPIO_PIN_3);
//<End Added Code>
//The following example shows how to use the PWM API to initialize the PWM0 with a 50 KHz frequency, and with a 25% duty cycle
//on PWM2 and a 75% duty cycle on PWM3.
// Enable the PWM0 peripheral
SysCtlPeripheralEnable(SYSCTL_PERIPH_PWM0);
// Wait for the PWM0 module to be ready.
while(!SysCtlPeripheralReady(SYSCTL_PERIPH_PWM0));
// Configure the PWM generator for count down mode with immediate updates to the parameters.
PWMGenConfigure(PWM0_BASE, PWM_GEN_1, PWM_GEN_MODE_DOWN | PWM_GEN_MODE_NO_SYNC);
// Set the period. For a 50 KHz frequency, the period = 1/50,000, or 20 microseconds. For a 20 MHz clock, this translates to 400
// clock ticks. Use this value to set the period
PWMGenPeriodSet(PWM0_BASE, PWM_GEN_1, 400);
// Set the pulse width of PWM2 for a 25% duty cycle
PWMPulseWidthSet(PWM0_BASE, PWM_OUT_2, 100);
// Set the pulse width of PWM3 for a 75% duty cycle
PWMPulseWidthSet(PWM0_BASE, PWM_OUT_3, 300);
// Start the timers in generator 0
PWMGenEnable(PWM0_BASE, PWM_GEN_1);
// Enable the outputs.
PWMOutputState(PWM0_BASE, (PWM_OUT_2_BIT | PWM_OUT_3_BIT), true);
for(;;);
}
