Detailed Description

PWM driver implementation for Low Power F3 devices.

Overview

The general PWM API should be used in application code, i.e. PWM_open() should be used instead of PWMTimerLPF3_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 LPF3:

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 PWMTimerLPF3.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 LGPTimer driver. The following statements are valid:

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

Accuracy

The PWM output period and duty cycle are limited by the underlying timer. The LGPT peripherals have the following counter widths:

LGPT0: 16 bits
LGPT1: 16 bits
LGPT2: 16 bits
LGPT3: 24 bits

Note that some LPF3 device types do not support all four LGPT peripherals.

The frequency of the underlying timer counter can be divided by configuring the preScalerDivision element of the PWMTimerLPF3_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 LGPT peripheral used and the configured preScalerDivision value. When using the LGPT3 peripheral with the default preScalerDivision value of 1, the minimal frequency is 48MHz / (2^24-1) = 2.86Hz (349.525ms).

When using high output frequencies the duty cycle resolution is reduced correspondingly. For a 24MHz PWM only a 0%/50%/100% duty is available as the timer uses only counts 0 and 1. Similarly for a 12MHz period the duty cycle will be limited to a 12.5% resolution.

Note: The PWM signals are generated by the LGPT peripheral using the timer clock which is dependent on the high frequency clock (CLKSVT).

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/LGPTimerLPF3.h>

Include dependency graph for PWMTimerLPF3.h:

Go to the source code of this file.

Data Structures
struct	PWMTimerLPF3_HwAttrs
	PWMTimerLPF3 Hardware attributes. More...

struct	PWMTimerLPF3_Object
	PWMTimerLPF3 Object. More...

Macros
PWMTimerLPF3 specific control commands and arguments
#define	PWMTimerLPF3_CMD_DEBUG_STALL

Arguments for PWMTimerLPF3_CMD_DEBUG_STALL
#define	CMD_ARG_DEBUG_STALL_OFF

#define	CMD_ARG_DEBUG_STALL_ON

Typedefs
typedef struct PWMTimerLPF3_HwAttrs	PWMTimerLPF3_HwAttrs
	PWMTimerLPF3 Hardware attributes. More...

typedef struct PWMTimerLPF3_Object	PWMTimerLPF3_Object
	PWMTimerLPF3 Object. More...

Variables
const PWM_FxnTable	PWMTimerLPF3_fxnTable

Macro Definition Documentation

§ PWMTimerLPF3_CMD_DEBUG_STALL

#define PWMTimerLPF3_CMD_DEBUG_STALL

Timer debug stall mode (stop PWM output debugger halts CPU) When enabled, PWM output will be HIGH when CPU is halted

§ CMD_ARG_DEBUG_STALL_OFF

#define CMD_ARG_DEBUG_STALL_OFF

§ CMD_ARG_DEBUG_STALL_ON

#define CMD_ARG_DEBUG_STALL_ON

Typedef Documentation

§ PWMTimerLPF3_HwAttrs

typedef struct PWMTimerLPF3_HwAttrs PWMTimerLPF3_HwAttrs

PWMTimerLPF3 Hardware attributes.

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

§ PWMTimerLPF3_Object

typedef struct PWMTimerLPF3_Object PWMTimerLPF3_Object

PWMTimerLPF3 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. Applications should also not access member variables of this structure as backwards compatibility is not guaranteed.

Variable Documentation

§ PWMTimerLPF3_fxnTable

const PWM_FxnTable PWMTimerLPF3_fxnTable