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1. Bluetooth® Low Energy GATT Data Base SRAM computation
The Bluetooth® Low Energy GATT Data Base SRAM computation takes into account the default GATT and GAP services in addition to services and characteristics for specific Bluetooth® Low Energy user application.
This computation includes the calculation of the number of services, the number of attributes and the Size of the storage area for the attribute values.
In following chapters, there are descriptions of number of services (CFG_BLE_NUM_GATT_SERVICES), number of attributes (CFG_BLE_NUM_GATT_ATTRIBUTES) and Size of the storage area for the attribute values (CFG_BLE_ATT_VALUE_ARRAY_SIZE).
2. How to calculate CFG_BLE_NUM_GATT_SERVICES ?
CFG_BLE_NUM_GATT_SERVICES defines the number of services to be stored in the GATT database.
Note that the default GAP and GATT services are automatically added at initialization.
The parameter CFG_BLE_NUM_GATT_SERVICES must be the number of user services increased by 2 (number of default GAP and GATT services).
CFG_BLE_NUM_GATT_SERVICES = 2 + number of user's services
3. How to calculate CFG_BLE_NUM_GATT_ATTRIBUTES ?
CFG_BLE_NUM_GATT_ATTRIBUTES defines the number of attribute records related to all the characteristics for the specific user's Bluetooth® Low Energy application.
For each characteristic:
The number of attribute records goes from 2 to 5 depending on the characteristic properties:
- Minimum of 2 (1 for declaration and 1 for the value)
- Add 1 more record for each additional property: Notify or Indicate, Broadcast, Extended property
- Notify and Indicate properties correspond to CCCD (Client Characteristic Configuration Descriptor)
- Broadcast property corresponds to SCCD (Server Characteristic Configuration Descriptor)
- Extended property corresponds to CEPD (Characteristic Extended Property Descriptor)
- Notify and Indicate properties correspond to CCCD (Client Characteristic Configuration Descriptor)
The parameter CFG_BLE_NUM_GATT_ATTRIBUTES must be the number of user's application attributes increased by 9 (the number of attributes of GAP and GATT default services)
CFG_BLE_NUM_GATT_ATTRIBUTES = <number of SCCD attributes for default GAP and GATT services> + 9 + number of user's attributes
| Attributes of default GAP and GATT services (number = 9) |
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Example of service including characteristics with notification
| Attributes of service including characteristics with notification |
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4. How to calculate CFG_BLE_ATT_VALUE_ARRAY_SIZE ?
CFG_BLE_ATT_VALUE_ARRAY_SIZE defines the size of the storage area for the attribute values.
Each characteristic contributes to the Attribute Value Array Size as follow:
Characteristic value length plus:
– 5 bytes if characteristic UUID is 16 bits
– 19 bytes if characteristic UUID is 128 bits
– 2 bytes if characteristic has a server configuration descriptor
– 2 bytes * CFG_BLE_NUM_LINK if the characteristic has a client configuration descriptor
– 2 bytes if the characteristic has extended properties
And it is necessary to add characteristics related to the default GAP and GATT services:
- GATT - Service change characteristic (if not removed): 9 bytes (5+2+2), 5 for UUID, 2 for indicate, 2 for characteristic value length
- GAP - Device Name: 11 bytes (5+6), 5 for UUID, 6 for name length
- GAP - Appearance: 7 bytes (5+2), 5 for UUID, 2 for attribute length
- GAP - Peripheral Preferred Connection Parameters: 13 bytes (5+8), 5 for UUID, 8 for attribute length
| Attribute Value Array Size of default GAP and GATT services |
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For this case default value array size: 9 + 31 = 40 bytes
Example of service including characteristics with notification - Attribute Value Array Size
| Attribute Value Array Size of service including characteristics with notification |
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5. GATT database size in SRAM
5.1. STM32WB
Maximum size of SRAM allocation for the GATT database cannot be changed by the user's FW and varies for different stacks.
Calculation of the Max size allocated for GATT Data Base in SRAM is done in M0.
| Max size table | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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The limitation for number of services and number of attributes follows the rule:
CFG_BLE_NUM_GATT_SERVICES x 48 + CFG_BLE_NUM_GATT_ATTRIBUTES x 40 + CFG_BLE_ATT_VALUE_ARRAY_SIZE < Max size
To apply the limitation, it is necessary to calculate the number of services, the number of attributes and the attribute value array size.
The document GATT_DB_Size_SRAM_NVM.xlsx [1] is part of the STM32WBCube delivery in \Middlewares\ST\STM32_WPAN\ble\core\doc.
This file allows the GATT database SRAM computation as well as the Bluetooth® LE GATT database and security record in NVM depending on the user application.
5.2. STM32WBA
For STM32WBA, the application allocates the necessary GATT Data Base SRAM at Bluetooth® LE stack initialization. The value passed to the stack is equal to BLE_GATT_BUF_SIZE. This value is calculated thanks to CFG_BLE_NUM_GATT_ATTRIBUTES, CFG_BLE_NUM_GATT_SERVICES and CFG_BLE_ATT_VALUE_ARRAY_SIZE values initialized through CubeMX.
BLE_GATT_BUF_SIZE = CFG_BLE_NUM_GATT_SERVICES x 48 + CFG_BLE_NUM_GATT_ATTRIBUTES x 40 + CFG_BLE_ATT_VALUE_ARRAY_SIZE
| Application configuration - BLE stack |
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6. Bluetooth® Low Energy GATT database and security record in NVM
Some flags are available to store either the full GATT database or a reduce GATT database in NVM which allow to store more bonded devices.
6.1. Bluetooth® Low Energy GATT Database record in NVM - FULL_GATTDB_NVM flag
See CFG_BLE_OPTIONS define in app_conf.h file, SHCI_C2_BLE_INIT_OPTIONS_FULL_GATTDB_NVM flag.
| Application parameters |
|---|
When SHCI_C2_BLE_INIT_OPTIONS_FULL_GATTDB_NVM flag is set in CFG_BLE_OPTIONS, the GATT Database record in NVM is composed:
- Per service:
- 2 bytes for handle
- 3 bytes if 16-bit UUID or 17 bytes if 128-bit UUID
- 2 bytes for handle
- Per characteristic attribute:
- 2 bytes for handle
- 3 bytes if 16-bit UUID or 17 bytes if 128-bit UUID
- 2 bytes for CCCD value (only if the attribute is a CCCD)
- 2 bytes for handle
- if GATT Caching enabled
- Add (3 x number of links) bytes for Client Supported Feature characteristic
- if Enhanced ATT enabled
- Add (3 x number of links) bytes for Client Supported Feature characteristic
- Add (3 x number of links) bytes for Server Supported Feature characteristic
The total corresponds to size_of_gatt record. The security record in NVM is fixed and equal to 80 bytes, corresponding to size_of_sec record.
6.2. Bluetooth® Low Energy GATT Database record in NVM - REDUC_GATTDB_NVM and NO_SVC_CHANGE_DESC flags
See CFG_BLE_OPTIONS define in app_conf.h file, SHCI_C2_BLE_INIT_OPTIONS_REDUC_GATTDB_NVM and SHCI_C2_BLE_INIT_OPTIONS_NO_SVC_CHANGE_DESC flags.
| Application parameters |
|---|
When SHCI_C2_BLE_INIT_OPTIONS_REDUC_GATTDB_NVM and SHCI_C2_BLE_INIT_OPTIONS_NO_SVC_CHANGE_DESC flags are set in CFG_BLE_OPTIONS, the GATT database record in NVM is composed:
- Per characteristic attribute:
- 3 bytes for CCCD value (only if the attribute is a CCCD)
- if GATT Caching enabled
- Add (3 x number of links) bytes for Client Supported Feature characteristic
- if Enhanced ATT enabled
- Add (3 x number of links) bytes for Client Supported Feature characteristic
- Add (3 x number of links) bytes for Server Supported Feature characteristic
This is size_of_gatt record.
6.3. Bluetooth® Low Energy GATT DB record in NVM - REDUC_GATTDB_NVM and WITH_SVC_CHANGE_DESC flags
See CFG_BLE_OPTIONS define in app_conf.h file, SHCI_C2_BLE_INIT_OPTIONS_REDUC_GATTDB_NVM and SHCI_C2_BLE_INIT_OPTIONS_WITH_SVC_CHANGE_DESC flags.
| Application parameters |
|---|
When SHCI_C2_BLE_INIT_OPTIONS_REDUC_GATTDB_NVM and SHCI_C2_BLE_INIT_OPTIONS_WITH_SVC_CHANGE_DESC flags are set in CFG_BLE_OPTIONS, the GATT database record in NVM is composed:
- Per characteristic attribute:
- 3 bytes for CCCD value (only if the attribute is a CCCD)
- 3 bytes for CCCD value (only if the attribute is a CCCD)
- if GATT Caching enabled
- Add (3 x number of links) bytes for Client Supported Feature characteristic
- if Enhanced ATT enabled
- Add (3 x number of links) bytes for Client Supported Feature characteristic
- Add (3 x number of links) bytes for Server Supported Feature characteristic
+ 16 bytes (The Data Base hash stores a 128 bits value calculated from the Data Base structure.
Any change in the Data Base structure results in a different hash value.)
This is size_of_gatt record.
6.4. Bluetooth® Low Energy security record
The security record in NVM is fixed and equal to 80 bytes, corresponding to size_of_sec record.
The security record includes the BD address of bonded device, and different keys: IRK, LTK and CSRK.
7. STM32WB - Bluetooth® Low Energy number of bonded devices in NVM
Regarding the size of NVM, the computation of number n of bonded devices that can be stored in NVM is the following:
n = (total_size_of_nvm-1) / [ (size_of_sec_record+1) + (size_of_gatt record+1) ]
- total_size_of_nvm = 2028 bytes (fixed in M0)
- size_of_sec_record = 80 bytes (this is a constant from the BLE stack)
- size_of_gatt_record is dependent of the flags chosen in the application in app_conf.h file.
See the description in previous chapter.
7.1. Example 1 - with FULL_GATTDB_NVM flag set
Default GATT configuration without adding services:
5 bytes for the GATT service (2 bytes for handle + 3 bytes for service 16-bit UUID)
+ 7 bytes for the Service Changed characteristic (2 bytes for handle + 3 bytes for service 16-bit UUID + 2 bytes for CCCD)
+ 5 bytes for the GAP service (2 bytes for handle + 3 bytes for service 16-bit UUID)
+ 5 bytes for the Device Name characteristic (2 bytes for handle + 3 bytes for service 16-bit UUID)
+ 5 bytes for the Appearance characteristic (2 bytes for handle + 3 bytes for service 16-bit UUID)
+ 5 bytes for the Peripheral Preferred Connection Parameters characteristic (2 bytes for handle + 3 bytes for service 16-bit UUID)
= 32 bytes
Size of default GATT record in NVM is equal to 32 bytes.
CFG_BLE_NUM_GATT_SERVICES x 48 + CFG_BLE_NUM_GATT_ATTRIBUTES x 40 + CFG_BLE_ATT_VALUE_ARRAY_SIZE
= 2 x 48 + 9 x 40 + 40
= 496 bytes
Size of default GATT record in SRAM is equal to 496 bytes.
7.2. Example 2 - FULL_GATTDB_NVM flag
Default GATT configuration + 1 empty service with UUID_16
GATT record = 37 bytes
32 bytes for default + 5 bytes for the service
7.3. Example 3 - FULL_GATTDB_NVM flag
Default GATT configuration + 1 service with UUID_16 + 3 characteristics with UUID_16, 1 characteristic with notification property)
- GATT record in NVM = 54 bytes
32 bytes for default
+ 5 bytes for the service
+ 5 bytes for the 1st characteristic attribute
+ 5 bytes for the 2nd characteristic attribute
+ 7 bytes for the 3rd characteristic attribute (CCCD)
7.4. Example 4 - REDUC_GATTDB_NVM and WITH_SVC_CHANGE_DESC flags
Default GATT configuration + 1 service with UUID_16 + 3 characteristics with UUID_16
- GATT record = 6 bytes
3 bytes for default
+ 3 bytes for the 3rd characteristic attribute (CCCD)
7.5. Example 5 - REDUC_GATTDB_NVM and NO_SVC_CHANGE_DESC flags
Default GATT configuration + 1 service with UUID_16 + 3 characteristics with UUID_16
- GATT record = 3 bytes
0 bytes for default + 3 bytes for the 3rd characteristic attribute (CCCD)
7.6. Example 6 - GATT record in NVM with different flag configurations
- GATT record if SHCI_C2_BLE_INIT_OPTIONS_FULL_GATTDB_NVM selected.
- GATT record if SHCI_C2_BLE_INIT_OPTIONS_REDUC_GATTDB_NVM and SHCI_C2_BLE_INIT_OPTIONS_WITH_SVC_CHANGE_DESC flags selected.
- GATT record if SHCI_C2_BLE_INIT_OPTIONS_REDUC_GATTDB_NVM and SHCI_C2_BLE_INIT_OPTIONS_NO_SVC_CHANGE_DESC flags selected.
| Example 4 - Gatt record size difference depending on flags choice |
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- Formula to calculate Total Buffer Gatt Size
(number of services) x 48 + (number of attributes) x 40 + CFG_BLE_ATT_VALUE_ARRAY_SIZE
= 6 x 48 + 64 x 40 + 614
= 3462 bytes
In previous example, size_of_gatt record in SRAM is equal to 3462 bytes
And size of gatt record in NVM is equal to 469 bytes.
- Formula to calculate the number of bonded devices that can be stored in NVM
n = (total_size_of_nvm-1) / [ (size_of_sec_record+1) + (size_of_gatt record+1) ]
n = ((507x4)-1) / [(80+1) + (469+1)] = 3
8. STM32WBA - Bluetooth® Low Energy number of bonded devices in NVM
Like in STM32WB, regarding the size of NVM, the computation of number n of bonded devices that can be stored in NVM is the following:
n = (total_size_of_nvm-1) / [ (size_of_sec_record+1) + (size_of_gatt record+1) ]
The total_size_of_nvm in STM32WBA can be adapted to the user's application needs and configured through CubeMX interface.
| Application configuration - NVM |
|---|
By default, in our application examples, SNVMA_NVM_NUMBER = 1, that defines 1 NVM with a 8192 bytes size.
- total_size_of_nvm = 8192 bytes
- size_of_sec_record = 80 bytes (this is a constant from the BLE stack)
- size_of_gatt_record is dependent of the flags chosen in the application in app_conf.h file.
See the description in previous point.
Refer to STM32CubeWBA Memory management wiki page for information on memory management.
9. General information concerning NVM
We don’t store values of attributes, only the CCCD value. It is applicable for GATT server only.
On server side it is used to: - check if services have changed - keep CCCD in memory