Detailed Description

PWM driver implementation for CC26XX/CC13XX.

Overview

The general PWM API should be used in application code, i.e. PWM_open() should be used instead of PWMTimerCC26XX_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 CC26XX:

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

Error handling

If providing unsupported arguments 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 TI-RTOS 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 PWMTimerCC26XX.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 Standby. 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 high-frequency 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 standby.

Accuracy

The PWM output period and duty cycle are limited by the underlying timer. In PWM mode the timer is effectively 24 bits which results in a minimum frequency of 48MHz / (2^24-1) = 2.86Hz (349.525ms) The driver will round off the configured duty and period to a value limited by the timer resolution and the application is responsible for selecting duty and period that works with the underlying timer if high accuracy is needed. The effect of this is most visible when using high output frequencies as the available 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. The PWM signals are generated using the high-frequency clock as source.

If very high-accuracy outputs are needed, the application should request using the external HF crystal:

#include <ti/sysbios/family/arm/cc26xx/Power.h>
#include <ti/sysbios/family/arm/cc26xx/PowerCC2650.h>
Power_setDependency(XOSC_HF);

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.
When changing duty cycle there will be a period where the high level is either too short or too high since the timer match value is updated.
PWM usage

Basic PWM output

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

void taskFxn(UArg a0, UArg a1) {
  PWM_Handle hPWM;
  PWM_Params pp;
  PWM_Params_init(&pp);
  pp.idleLevel      = PWM_IDLE_LOW;
  pp.period.unit    = PWM_PERIOD_HZ;
  pp.period.value   = 8e6;
  pp.duty.unit      = PWM_DUTY_FRACTION;
  pp.duty.value     = PWM_DUTY_FRACTION_MAX / 2;
  hPWM = PWM_open(Board_PWM0);
  if(hPWM == NULL) {
    Log_error0("Failed to open PWM");
    Task_exit();
  }
  PWM_start(hPWM);
  while(1) {
    Task_sleep(BIOS_WAIT_FOREVER);
  }

#include <stdint.h>
#include <stdbool.h>
#include <ti/drivers/PIN.h>
#include <ti/drivers/PWM.h>
#include <ti/drivers/timer/GPTimerCC26XX.h>

Include dependency graph for PWMTimerCC26XX.h:

Go to the source code of this file.

Data Structures
struct	PWMTimerCC26XX_HwAttrs
	PWMTimer26XX Hardware attributes. More...

struct	PWMTimerCC26XX_Object
	PWMTimer26XX Object. More...

Macros
#define	PWMTimerCC26XX_CMD_DEBUG_STALL PWM_CMD_RESERVED + 0

#define	CMD_ARG_DEBUG_STALL_OFF (uint32_t)GPTimerCC26XX_DEBUG_STALL_OFF

#define	CMD_ARG_DEBUG_STALL_ON (uint32_t)GPTimerCC26XX_DEBUG_STALL_ON

Typedefs
typedef const struct PWMTimerCC26XX_HwAttrs	PWMTimerCC26XX_HwAttrs
	PWMTimer26XX Hardware attributes. More...

typedef struct PWMTimerCC26XX_Object	PWMTimerCC26XX_Object
	PWMTimer26XX Object. More...

Macro Definition Documentation

#define PWMTimerCC26XX_CMD_DEBUG_STALL PWM_CMD_RESERVED + 0

PWMTimerCC26XX specific commands

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

#define CMD_ARG_DEBUG_STALL_OFF (uint32_t)GPTimerCC26XX_DEBUG_STALL_OFF

#define CMD_ARG_DEBUG_STALL_ON (uint32_t)GPTimerCC26XX_DEBUG_STALL_ON

Typedef Documentation

typedef const struct PWMTimerCC26XX_HwAttrs PWMTimerCC26XX_HwAttrs

PWMTimer26XX Hardware attributes.

These fields are used by the driver to set up underlying PIN and GPTimer driver statically. A sample structure is shown below:

 // PWM configuration, one per PWM output
 PWMTimerCC26XX_HwAttrs pwmtimerCC26xxHWAttrs[CC2650_PWMCOUNT] = {
  { .pwmPin = Board_PWMPIN0, .gpTimerUnit = CC2650_GPTIMER0A } ,
  { .pwmPin = Board_PWMPIN1, .gpTimerUnit = CC2650_GPTIMER0B } ,
  { .pwmPin = Board_PWMPIN2, .gpTimerUnit = CC2650_GPTIMER1A } ,
  { .pwmPin = Board_PWMPIN3, .gpTimerUnit = CC2650_GPTIMER1B } ,
  { .pwmPin = Board_PWMPIN4, .gpTimerUnit = CC2650_GPTIMER2A } ,
  { .pwmPin = Board_PWMPIN5, .gpTimerUnit = CC2650_GPTIMER2B } ,
  { .pwmPin = Board_PWMPIN6, .gpTimerUnit = CC2650_GPTIMER3A } ,
  { .pwmPin = Board_PWMPIN7, .gpTimerUnit = CC2650_GPTIMER3B } ,
 };

typedef struct PWMTimerCC26XX_Object PWMTimerCC26XX_Object

PWMTimer26XX 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. A sample structure is shown below:

1 // PWM object, one per PWM output

2 PWMTimerCC26XX_Object pwmtimerCC26xxObjects[CC2650_PWMCOUNT];