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
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.
The following hardware peripherals are occupied by the mesh stack:
BEARER_EVENT_USE_SWI0 during the build.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:
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 on nRF52832. The examples are build with the GNU Arm Embedded Toolchain (arm-none-eabi-gcc) v7.3.1.
MinSizeRel (-Os), Logging: On (default)| Flash usage (kB) | RAM usage (kB) | Example |
|---|---|---|
| 92.524 | 10.112 | Beaconing |
| 94.988 | 10.384 | DFU with serial interface |
| 104.876 | 13.504 | DFU without serial interface |
| 108.128 | 11.684 | EnOcean switch translator client |
| 105.636 | 11.320 | Light switch dimming client |
| 110.352 | 11.388 | Light switch dimming server |
| 118.628 | 11.656 | Low Power node |
| 104.676 | 11.584 | Light switch client |
| 103.936 | 10.376 | Light switch provisioner |
| 115.180 | 13.448 | Light switch server |
| 97.980 | 9.512 | PB-remote client |
| 96.260 | 9.992 | PB-remote server |
| 90.220 | 12.432 | Serial |
MinSizeRel (-Os), Logging: None| Flash usage (kB) | RAM usage (kB) | Example |
|---|---|---|
| 80.408 | 8.824 | Beaconing |
| 80.792 | 9.096 | DFU with serial interface |
| 90.008 | 12.216 | DFU without serial interface |
| 91.940 | 11.668 | EnOcean switch translator client |
| 90.744 | 11.304 | Light switch dimming client |
| 94.964 | 11.372 | Light switch dimming server |
| 105.132 | 11.640 | Low Power node |
| 90.408 | 11.568 | Light switch client |
| 84.004 | 10.360 | Light switch provisioner |
| 97.376 | 13.432 | Light switch server |
| 80.764 | 9.496 | PB-remote client |
| 81.352 | 8.704 | PB-remote server |
| 77.944 | 11.144 | Serial |
The memory usage values for build types based on nRF52840 are similar to those valid for nRF52832.
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:
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.
The following table lists parameters that must be defined to calculate the flash lifetime of a device.
| Name | Description and Configuration parameter | Default nRF51 | Default nRF52 | 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)
| 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.
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.
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.
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.