Detailed Description

PWM driver implementation for Wi-Fi F3 devices.

Overview

The general PWM API should be used in application code, i.e. PWM_open() should be used instead of PWMTimerWFF3_open(). The board file will define the device specific config, and casting in the general API will ensure that the correct device specific functions are called.

General Behavior

Before using PWM on WFF3:

The Timer HW is configured and system dependencies (for example IOs, power, etc.) are set by calling PWM_open().

Error handling

If unsupported arguments are provided to an API returning an error code, the PWM configuration will not be updated and PWM will stay in the mode it was already configured to.

Power Management

The SimpleLink power management framework will try to put the device into the most power efficient mode whenever possible. Please see the technical reference manual for further details on each power mode.

The PWMTimerWFF3.h driver is not explicitly setting a power constraint when the PWM is running to prevent standby as this is assumed to be done in the underlying GPTimer driver. The following statements are valid:

After PWM_open(): The device is still allowed to enter Sleep. When the device is active the underlying GPTimer peripheral will be enabled and clocked.
After PWM_start(): The device can only go to Idle power mode since the system clock is needed for PWM operation:
After PWM_stop(): Conditions are equal as for after PWM_open
After PWM_close(): The underlying GPTimer is turned off and the device is allowed to go to sleep.

Accuracy

The PWM output period and duty cycle are limited by the underlying timer. For GPT counter widths see GPT peripheral properties.

The frequency of the underlying timer counter can be divided by configuring the preScalerDivision element of the PWMTimerWFF3_HwAttrs struct. This configuration can be used to extend the PWM signal period but will decrease the accuracy. The minimum obtainable PWM signal frequency is dependent on the width of the counter for the GPT peripheral used and the configured preScalerDivision value. When using either the GPT0 or the GPT1 peripheral with the default preScalerDivision value of 1, the minimal frequency is 40MHz / (2^32-1) = 0.0093Hz (107.374sec).

When using high output frequencies the duty cycle resolution is reduced correspondingly. For a 20MHz PWM only a 0%/50%/100% duty is available as the timer uses only counts 0 and 1. Similarly for a 10MHz period the duty cycle will be limited to a 12.5% resolution.

Note: The PWM signals are generated by the GPT peripheral using the timer clock which is dependent on the system clock.

Limitations

The PWM output can currently not be synchronized with other PWM outputs.
The PWM driver does not support updating duty and period using DMA.
Attempts to change both the period and duty cycle at the same time for an active PWM signal, can cause pulses to be too long or short if the change is applied close to the end of the current counter cycle. This does not apply to changing only the period or only the duty cycle, as any of these separate changes will take effect on the next counter cycle.
PWM usage

Basic PWM output

The below example will output a 8MHz PWM signal with 50% duty cycle.

PWM_Handle pwmHandle;
PWM_Params params;
PWM_Params_init(&params);
params.idleLevel      = PWM_IDLE_LOW;
params.periodUnits    = PWM_PERIOD_HZ;
params.periodValue    = 8e6;
params.dutyUnits      = PWM_DUTY_FRACTION;
params.dutyValue      = PWM_DUTY_FRACTION_MAX / 2;
pwmHandle = PWM_open(CONFIG_PWM0, &params);
if(pwmHandle == NULL) {
  Log_error0("Failed to open PWM");
  Task_exit();
}
PWM_start(pwmHandle);

#include <stdint.h>
#include <stdbool.h>
#include <ti/drivers/PWM.h>
#include <ti/drivers/timer/GPTimerWFF3.h>

Include dependency graph for PWMTimerWFF3.h:

Go to the source code of this file.

Data Structures
struct	PWMTimerWFF3_HwAttrs
	PWMTimerWFF3 Hardware attributes. More...

struct	PWMTimerWFF3_Object
	PWMTimerWFF3 Object. More...

Typedefs
typedef struct PWMTimerWFF3_HwAttrs	PWMTimerWFF3_HwAttrs
	PWMTimerWFF3 Hardware attributes. More...

typedef struct PWMTimerWFF3_Object	PWMTimerWFF3_Object
	PWMTimerWFF3 Object. More...

Variables
const PWM_FxnTable	PWMTimerWFF3_fxnTable

Typedef Documentation

§ PWMTimerWFF3_HwAttrs

typedef struct PWMTimerWFF3_HwAttrs PWMTimerWFF3_HwAttrs

PWMTimerWFF3 Hardware attributes.

These fields are used by the driver to set up the underlying GPTimer driver statically.

§ PWMTimerWFF3_Object

typedef struct PWMTimerWFF3_Object PWMTimerWFF3_Object

PWMTimerWFF3 Object.

These fields are used by the driver to store and modify PWM configuration during run-time. The application must not edit any member variables of this structure. Appplications should also not access member variables of this structure as backwards compatibility is not guaranteed.

Variable Documentation

§ PWMTimerWFF3_fxnTable

const PWM_FxnTable PWMTimerWFF3_fxnTable