Resource usage

To be functional, the Bluetooth mesh stack requires a minimum set of the hardware resources provided by the Nordic SoCs. The stack is designed to be built together with the user application and it resides in the application code space. Moreover, it relies on the SoftDevice being present and thus requires the same hardware resources as the SoftDevice.

For information on SoftDevice hardware resource requirements, see the relevant SoftDevice specification.

Table of contents

• SoftDevice Radio Timeslot API
• Hardware peripherals
• RAM and flash usage
  • Build type: `MinSizeRel` (`-Os`), Logging: On (default)
  • Build type: `MinSizeRel` (`-Os`), Logging: None
• Flash hardware lifetime
  • Calculating flash lifetime
    • Flash example values
  • Flash configuration parameters
    • Sequence number block size
    • Flash area size

SoftDevice Radio Timeslot API

The Bluetooth mesh stack operates concurrently with the SoftDevice through the SoftDevice Radio Timeslot API. Because the stack takes complete control over the Radio Timeslot API, this API is unavailable to the application.

Hardware peripherals

The following hardware peripherals are occupied by the Bluetooth mesh stack:

• RTC1
  • App timer uses RTC1 as hardware base.
• QDEC
  • Although the quadrature decoder hardware is not used by the mesh network, the interrupt request line dedicated to the QDEC module is utilized for post processing within the Bluetooth mesh stack. Because all the software interrupts available to the application on the nRF51 are frequently used in the nRF5 SDK, the Bluetooth mesh stack uses the QDEC IRQ handler for processing, as this peripheral is not commonly used. This makes combining the Bluetooth mesh stack with SDK applications easier.

    Note

    If the QDEC peripheral and its interrupt request line is needed by the application, the Bluetooth mesh stack can be configured to use the SWI0 IRQ by defining BEARER_EVENT_USE_SWI0 during the build.

• RADIO
  • Shared with the SoftDevice, the RADIO peripheral is occupied by the Bluetooth mesh stack during the acquired Radio Timeslot sessions. The application must not modify the RADIO peripheral.
• TIMER0
  • Shared with the SoftDevice, the TIMER0 peripheral is occupied by the Bluetooth mesh stack during the acquired Radio Timeslot sessions. The application must not modify the TIMER0 peripheral.
• TIMER2
  • By default, the Bluetooth mesh stack uses TIMER2 to manage its radio timing for all low-level operations. Which timer to use can be controlled by changing BEARER_ACTION_TIMER_INDEX in mesh/bearer/api/nrf_mesh_config_bearer.h.
• ECB
  • The Bluetooth mesh stack uses the SoftDevice interface for the ECB.
• UART0
  • If built with serial support, the Bluetooth mesh stack uses the UART0 peripheral to serialize its API. The Bluetooth mesh stack takes full control over the peripheral. The application must not modify it.
• PPI
  • The Bluetooth mesh uses PPI channels 8, 9, 10, and 11 for various timing-related tasks when controlling the radio.

RAM and flash usage

Depending on the application needs, the mesh core can be configured to achieve either higher performance and functionality or a reduced footprint.

When it comes to memory, the Bluetooth mesh stack:

• shares its call stack with the application and the SoftDevice;
• requires a minimum call stack size of 2 kB;
• requires the presence of a heap (of minimum MESH_MEM_SIZE_MIN bytes), unless it is configured with a custom memory allocator to replace the need for stdlib.h's malloc().

See the Mesh memory manager interface for more details on how to replace the memory manager backend.

The following tables show the flash and RAM requirements for the Bluetooth mesh examples. The values are valid for all fully compatible configurations based on the nRF52 Series Development Kits.

The examples are built with the minimum recommended version of GNU Arm Embedded Toolchain.

Build type: MinSizeRel (-Os), Logging: On (default)

Build type: MinSizeRel (-Os), Logging: None

Flash hardware lifetime

The flash hardware can withstand a limited number of write and erase cycles. As the Bluetooth mesh stack uses the flash to store state across power failures, the device flash will eventually start failing, resulting in unexpected behavior in the Bluetooth mesh stack.

To improve flash lifetime, flash manager does wear leveling by writing a new data to the flash page by allocating a new entry and then invalidating the old one. Eventually, flash page fills up and must be erased and re-written (see flash manager documentation).

The Bluetooth mesh stack uses flash to store the following states:

• Encryption keys
• Mesh addresses
• Access model composition
• Access model configuration
• Network message sequence number
• Network IV index state
• DFU metadata

Based on the assumption that the reconfiguration of keys, addresses, and access configuration is rare, the most likely source of flash write exhaustion are the network states. The network message sequence number is written to flash continuously, in user-configurable blocks.

Calculating flash lifetime

The following table lists parameters that must be defined to calculate the flash lifetime of a device.

The formula for the network state flash exhaustion is as follows:

FLASH LIFETIME [seconds] = ((AREA_SIZE - AREA_OVERHEAD) * ERASE_CYCLES) / (ENTRY_SIZE * MSG_PER_SEC / BLOCK_SIZE)

Flash example values

As any changes made to the default flash configuration may significantly reduce the product lifetime, recalculate the network state flash exhaustion time if any of the parameters change.

Flash configuration parameters

While the default settings will be sufficient for most applications, there are tradeoffs in the flash configuration that you might want to take advantage of.

Sequence number block size

The sequence number block size affects the number of power resets that the device can do within a 192-hour IV update period.

For security reasons, the device can never send a message with the same sequence number twice within an IV update period. This means that the device must allocate a new block of sequence numbers before it sends its first packet after a power reset, to avoid a scenario where it reuses the same sequence number on next powerup. As a consequence, every power reset requires a sequence number block allocation, which can exhaust the sequence number space faster than accounted for in the lifetime calculations.

With the default block size of 8192, the device may reset 2048 times in a 192-hour interval. If you expect a higher rate of resets, consider a smaller block size. Keep in mind that this will directly affect the flash lifetime, because more frequent writes are required during the normal operation.

The block size can also be increased if the number of power resets is expected to be lower than 2048, resulting in longer device lifetime.

Flash area size

The flash area size affects the number of erases required for the configuration and network state areas.

This does not alter the device lifetime significantly, because the flash manager defragmentation process requires a separate backup page that will be erased for every backed-up page. Adding pages to the flash area will therefore result in fewer, but more expensive defragmentations, with effectively no change to the number of erases required.