nRF5 SDK for Mesh v4.0.0  79d0f3f
Resource usage

To be functional, the 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 @anchor resource_usage_radio_timeslot

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


Hardware peripherals @anchor resource_usage_hardware_peripherals

The following hardware peripherals are occupied by the mesh stack:

  • RTC1
  • QDEC
    • Although the quadrature decoder hardware is not used by the mesh, the interrupt request line dedicated to the QDEC module is utilized for post processing within the mesh stack. Because all the software interrupts available to the application on the nRF51 are frequently used in the nRF5 SDK, the mesh stack uses the QDEC IRQ handler for processing, as this peripheral is not commonly used. This makes combining the mesh stack with SDK applications easier.
      Note
      If the QDEC peripheral and its interrupt request line is needed by the application, the 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 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 mesh stack during the acquired Radio Timeslot sessions. The application must not modify the TIMER0 peripheral.
  • TIMER2
  • ECB
    • The mesh stack uses the SoftDevice interface for the ECB.
  • UART0
    • If built with serial support, the mesh stack uses the UART0 peripheral to serialize its API. The mesh stack takes full control over the peripheral. The application must not modify it.
  • PPI
    • The mesh uses PPI channels 8, 9, 10, and 11 for various timing-related tasks when controlling the radio.

RAM and flash usage @anchor resource_usage_ram_and_flash

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

When it comes to memory, the 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 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: <tt>MinSizeRel</tt> (<tt>-Os</tt>), Logging: On (default) @anchor resource_usage_ram_and_flash_nRF52832-1

Flash usage (kB) RAM usage (kB) Example
94.968 10.660 Beaconing
97.456 10.948 DFU without serial interface
107.424 14.060 DFU with serial interface
110.904 12.272 EnOcean switch translator client
108.872 11.872 Light switch dimming client
113.700 11.932 Light switch dimming server
122.000 12.172 Low Power node
107.880 12.140 Light switch client
106.244 10.936 Light switch provisioner
121.632 15.760 Light switch server
100.824 10.028 PB-remote client
98.976 10.496 PB-remote server
95.556 13.100 Serial

Build type: <tt>MinSizeRel</tt> (<tt>-Os</tt>), Logging: None @anchor resource_usage_ram_and_flash_nRF52832-2

Flash usage (kB) RAM usage (kB) Example
82.084 9.372 Beaconing
82.492 9.660 DFU without serial interface
91.772 12.772 DFU with serial interface
93.724 12.256 EnOcean switch translator client
92.732 11.856 Light switch dimming client
97.064 11.916 Light switch dimming server
107.208 12.156 Low Power node
92.380 12.124 Light switch client
85.480 10.920 Light switch provisioner
102.292 15.744 Light switch server
82.680 10.012 PB-remote client
83.140 9.208 PB-remote server
82.960 11.812 Serial

Flash hardware lifetime @anchor resource_usage_flash_lifetime

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

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

Name Description and Configuration parameter Default nRF51 Series Default nRF52 Series Unit
MSG_PER_SEC The number of messages created by the device every second (relayed messages not included). The message sequence number field is 24 bits. It cannot be depleted within one IV update period, which must be at least 192 hours. Because of this, a device cannot possibly send more than 2^24 / (192 * 60 * 60) = 24.3 messages per second on average without breaking the specification.

Configuration parameter: N/A
24.3 24.3 messages/s
BLOCK_SIZE The message sequence numbers are allocated in blocks. Every block represents a set number of messages.

Configuration parameter: NETWORK_SEQNUM_FLASH_BLOCK_SIZE
8192 8192 messages
ENTRY_SIZE The size of a single allocated block entry in flash storage.

Configuration parameter: N/A
8 8 bytes
AREA_SIZE Size of the storage area. Must be in flash-page-size increments. Defaults to a single page.

Configuration parameter: N/A
1024 4096 bytes
AREA_OVERHEAD Static overhead in the storage area, per page.

Configuration parameter: N/A
8 8 bytes
ERASE_CYCLES The number of times the device can erase a flash page before it starts faulting.

Configuration parameter: N/A
20000 10000 cycles

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 @anchor resource_usage_flash_lifetime_example_values

SoC Settings Case Result
nRF51 Default Worst case 26.97 years
nRF52 Default Worst case 54.58 years

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 @anchor resource_usage_flash_lifetime_configuration_values

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 @anchor resource_usage_flash_lifetime_configuration_values_sequence

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 @anchor resource_usage_flash_lifetime_configuration_values_flash_area

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.