13.2 Low Power BLE Application Design

1. The user must enable the system Sleep Mode in BLE stack H3 component configuration. After enabling this, the system requests to enable dependent components like RTC (timer source during sleep).

   

2. Upon enabling Sleep mode, the system automatically sets the FreeRTOS related settings.
   1. In the “RTOS Configuration” drop-down menu, perfrom the following steps:
      1. Set the “Tick Mode” to Tickless_Idle. This allows the system to suppress periodic tick interrupts when idle, reducing power consumption.
      2. Set the “Expected idle time before sleep” to 5 (ms).

      

   2. Enabling Tick Hook.
      1. Find the option labeled “Tick Hook.”
      2. Check the Use Tick Hook. This allows the user to add custom code that executes during each tick interrupt.

      

   3. RTC peripheral library will be added and configured.
      Note: RTC counter must not be reset (RTC_Timer32CounterSet()) arbitrarily when the system is running.
3. The user must manually set the RTC clock source, the following are the four options:
   1. FRC (±1% offset)
   2. LPRC (with larger offset, < ±5%)
   3. POSC – Candidate of the clock source (better clock accuracy)
   4. SOSC – Candidate of the clock source (better clock accuracy)
   Note: Select POSC/SOSC as the RTC clock source, as other clock sources can impact BLE connection stability.

   

4. Manually setting RTC clock source – POSC.
   1. Choose “POSC” in “Clock Configuration”.
   2. The user must configure as illustrated in the following figure.

   

5. Manually Setting RTC clock source – SOSC, by turning on SOSC and then choose SOSC as illustrated in the following figure.

   

   Note: The users can only select one clock source POSC or SOSC, steps mention how to choose either.
6. It is recommended to use 48MHz as SYS_CLOCK for better power savings. This can be configured by setting SPLLPOSTDIV1 to 2 as shown below.

   

7. Ensure that JTAG Enable is disabled by clearing the JTAGEN bit in CFGCON0 (Configuration Control Register 0) as shown below. This code snippet can be added to SYS_Initialize().

   CFG_REGS->CFG_CFGCON0CLR = CFG_CFGCON0_JTAGEN_Msk;

8. All unused pins in the application needs to be set to Input mode and the pull-down must be enabled for these pins.

   The user can configure this through pin configuration in Harmony3 Configurator. For more details, see the following figure.

   

9. For more details on code generation, refer to the MPLAB Code Configurator (MCC) Code Generation from Related Links.

FreeRTOS provides Tickless IDLE mode for power saving, allowing the user can to stop periodic tick interrupts during idle periods (periods when no application tasks can execute). To account for the lost time during the IDLE mode, the user can use the RTC timer to make a correcting adjustment to the RTOS tick count value when it restarts (after waking up from sleep). For more information on Low Power Tickless mode, refer to the Low Power Support in Reference Documentation from Related Links. The system automatically executes the Tickless Idle mode when the Idle task is the only task able to run because all the application tasks are either in Blocked or Suspended state. To prevent the system from entering Sleep/Standby Low mode and waking up immediately, it automatically sets the minimum sleep time (IDLE time) to 5 ms.

1. In order for the system to enter Sleep mode, the system needs to request Bluetooth® wireless subsystem to sleep. This is accomplished by calling the BT_SYS_EnterSleepMode() API for BLE.

   

   
2. The API to call to ensure subsystem is sleeping (inactive) or ready for system to enter Sleep mode is the BT_SYS_AllowSystemSleep API.

   

3. If the expected sleep time is greater than 5 ms, the system is allowed to enter Sleep mode by checking for two conditions:
   1. Bluetooth subsystem is inactive.
   2. eTaskConfirmSleepModeStatus() returns eNoTasksWaitingTimeout. For more information, refer to the eTaskConfirmSleepModeStatus in Reference Documentation from Related Links.
   Note: The user can also add their own condition to be checked before system goes to sleep, for example, do not enter system sleep if data transmission over UART is active.

   Pseudo code in RTC based Tickless Idle Mode:
   
   ```
   if ((BT_SYS_AllowSystemSleep() || ZB_ReadyToSleep()) && ( eTaskConfirmSleepModeStatus() != eAbortSleep ) && (user_condition))
   {
           //Enter System Sleep Mode
           DEVICE_EnterSleepMode ();        //RTC Based Tickless Idle Mode       
   }
   ```

4. When both the conditions as mentioned in point 3 are met, enter RTC based Tickless Idle mode (stops the system tick, uses the RTC timer to set the sleep time, disables interrupts).
   1. The system enters Sleep mode after setting the RTC based Tickless Idle mode by calling Device_EnterSleepMode()API and then the system executes the Wait for Interrupt (WFI) instruction.

      

When the system is in Sleep or Standby Low Power mode, it can be awakened by one of the following events:

• RTC timeout
• BLE event
• GPIO interrupt

To exit Sleep mode, perform the following steps:

1. Calling DEVICE_ExitSleepMode() API to begin the wake-up process.
2. After exiting the Sleep mode, interrupts must be re-enabled to allow execution of the interrupt service routines.
3. Interrupts are initially disabled because the system tick needs to be compensated (as required by Tickless IDLE mode in FreeRTOS).

   

The device needs to back up all GPIO register settings before entering Deep Sleep mode and recover these settings when the device wakes up from deep sleep, before clearing the Deep Sleep (DSCON) register. The following API, clears this register.

DEVICE_ClearDeepSleepReg()

1. In MPLAB® X IDE, open MCC and create a new Harmony project.
2. Add required components to the Project Graph.
   Note: The application being developed determines whether some components are optional.

   

3. Select the BLE Stack in the Project Graph tab.
   1. Configure the following settings for “BLE stack” component. The slower the advertising interval, the less current consumption. User can select their desired interval.

      

4. Select “RTC” component in the Project Graph tab.
   1. Configure the following settings for “RTC” component.

      

5. Go to MPLAB® Code Configurator>Harmony>Clock Configuration.
   1. Go to Clock Diagram in MCC.
   2. Set LPCLK source to POSC. From the drop-down list, select POSC.

   

   

   

6. Go to MPLAB® Code Configurator>Harmony>Clock Configuration.
   1. Go to Clock Diagram in MCC.
   2. Set LPCLK source to SOSC. From the drop-down list, select SOSC.

      

      

7. Clock Switching mechanism , if LPCLK source is set as POSC clock source using clock configuration, the FW switch to LPRC as LPCLK source as POSC clock source is unavailable in Deep Sleep Low Power Mode.