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ESP32-DEVKIT freewheeling

Introduction

Get to know this ESP32 v4 evaluation kit (as Server): Connecting with a HUZZAH32 - ESP32 Feather (as Client):

Set up a GATT-server and Client with these devices.

  • Client devices access remote resources over a BLE link using the GATT protocol. Usually, the central is the client (but not necessarily).
  • Server devices have a local database and access control methods and provide resources to the remote client. Usually, the peripheral is the server (but not necessarily).
  • You can use read, write, notify, or indicate operations to move data between the client and the server.
    • Read and write operations are requested by the client and the server responds (or acknowledges).
    • Notify and indicate operations are enabled by the client but initiated by the server, providing a way to push data to the client.
    • Notifications are unacknowledged, while indications are acknowledged. Notifications are therefore faster but less reliable

  • GATT client - a device which accesses data on the remote GATT server via read, write, notify, or indicate operations
  • GATT server - a device which stores data locally and provides data access methods to a remote GATT client
  • GATT stands for Generic Attributes and it defines a hierarchical data structure that is exposed to connected BLE devices. This means that GATT defines the way that two BLE devices send and receive standard messages.
    Example ->
  • Profile: standard collection of services for a specific use case
  • Service: collection of related information, like sensor readings, battery level, heart rate, etc. ; predefined UUID list
  • Characteristic: it is where the actual data is saved on the hierarchy (value);
    • Descriptor: metadata about the data;
    • Properties: describe how the characteristic value can be interacted with. For example: read, write, notify, broadcast, indicate, etc.
      Link Reference on randomTutorials

Attribute Protocol (ATT)

The following figure shows a logical representation of an Attribute:

  • Handle: This is a 16-bit value that the server assigns to each of its attributes — think of it as an address.
  • Attribute Permissions determine whether an attribute can be read or written

Prerequisite

Useful links

Getting started

One can start with the hello world example here
If all went well the last step will be

Project build complete. To flash, run this command:
C:\Users\davth\.espressif\python_env\idf4.4_py3.8_env\Scripts\python.exe ..\..\components\esptool_py\esptool\esptool.py -p (PORT) -b 460800 --before default_reset --after hard_reset --chip esp32  write_flash --flash_mode dio --flash_size detect --flash_freq 40m 0x1000 build\bootloader\bootloader.bin 0x8000 build\partition_table\partition-table.bin 0x10000 build\hello_world.bin
or run 'idf.py -p (PORT) flash'

serial connection

The Hello world output on your terminal should look like

Hello world!
This is esp32 chip with 2 CPU core(s), WiFi/BT/BLE, silicon revision 3, 2MB external flash
Minimum free heap size: 293260 bytes
Restarting in 10 seconds...
Restarting in 9 seconds...

Gatt Server - Example

C:\Users\<name>\esp\gat_server>xcopy /e /i %IDF_PATH%\examples\bluetooth\bluedroid\ble\gatt_server
C:\Users\<name>\esp\gat_server>idf.py build
..[1291]steps..
C:\Users\x\esp\gat_server>idf.py -p COM11 flash

How do we actually flash

  • the bootloader goes to address 0x01000
  • the partition table gets on address 0x08000
  • the application get on address 0x10000
C:\Users\x\.espressif\python_env\idf4.4_py3.8_env\Scripts\python.exe d:\esp-idf\components\esptool_py\esptool\esptool.py -p (PORT) -b 460800 --before default_reset --after hard_reset --chip esp32  write_flash --flash_mode dio --flash_size detect --flash_freq 40m 0x1000 build\bootloader\bootloader.bin 0x8000 build\partition_table\partition-table.bin 0x10000 build\gatt_server_demos.bin

Gatt Client - Example

C:\Users\<name>\esp\gat_client>xcopy /e /i %IDF_PATH%\examples\bluetooth\bluedroid\ble\gatt_client
C:\Users\<name>\esp\gat_client>idf.py build
..[1291]steps..
C:\Users\<name>\esp\gat_client>idf.py -p COM12 flash

The Client (Esp32 Feather) will connect on the server (ESP32 Devkit) and we get below output
Server output

I (7181196) GATTS_DEMO: ESP_GATTS_DISCONNECT_EVT, disconnect reason 0x8
I (7181596) GATTS_DEMO: ESP_GATTS_CONNECT_EVT, conn_id 0, remote 30:ae:a4:1a:27:c6:
I (7181596) GATTS_DEMO: CONNECT_EVT, conn_id 0, remote 30:ae:a4:1a:27:c6:
I (7181756) GATTS_DEMO: update connection params status = 0, min_int = 16, max_int = 32,conn_int = 32,latency = 0, timeout = 400
I (7182956) GATTS_DEMO: ESP_GATTS_MTU_EVT, MTU 500
I (7182956) GATTS_DEMO: ESP_GATTS_MTU_EVT, MTU 500
I (7183116) GATTS_DEMO: GATT_WRITE_EVT, conn_id 0, trans_id 2, handle 43
I (7183116) GATTS_DEMO: GATT_WRITE_EVT, value len 2, value :
I (7183126) GATTS_DEMO: 01 00
I (7183126) GATTS_DEMO: notify enable
I (7183136) GATTS_DEMO: ESP_GATTS_CONF_EVT, status 0 attr_handle 42
I (7183196) GATTS_DEMO: GATT_WRITE_EVT, conn_id 0, trans_id 3, handle 42
I (7183196) GATTS_DEMO: GATT_WRITE_EVT, value len 35, value :
I (7183206) GATTS_DEMO: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
I (7183206) GATTS_DEMO: 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
I (7183216) GATTS_DEMO: 20 21 22

Client output

I (4403) GATTC_DEMO: ESP_GATTC_CONNECT_EVT conn_id 0, if 1
I (4403) GATTC_DEMO: REMOTE BDA:
I (4403) GATTC_DEMO: a8 03 2a eb e7 de
I (4403) GATTC_DEMO: open success
I (4603) GATTC_DEMO: update connection params status = 0, min_int = 16, max_int = 32,conn_int = 32,latency = 0, timeout = 400
I (5723) GATTC_DEMO: discover service complete conn_id 0
I (5723) GATTC_DEMO: SEARCH RES: conn_id = 0 is primary service 1
I (5733) GATTC_DEMO: start handle 40 end handle 43 current handle value 40
I (5733) GATTC_DEMO: service found
I (5743) GATTC_DEMO: UUID16: ff
I (5743) GATTC_DEMO: Get service information from remote device
I (5753) GATTC_DEMO: ESP_GATTC_SEARCH_CMPL_EVT
I (5753) GATTC_DEMO: ESP_GATTC_REG_FOR_NOTIFY_EVT
I (5803) GATTC_DEMO: ESP_GATTC_CFG_MTU_EVT, Status 0, MTU 500, conn_id 0
I (5963) GATTC_DEMO: ESP_GATTC_NOTIFY_EVT, receive notify value:
I (5963) GATTC_DEMO: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e
I (5973) GATTC_DEMO: write descr success
I (6043) GATTC_DEMO: write char success

Task

The code actually starts with GAP Generic Access Profile , it is what makes your device visible to the outside world, and determines how two device interact. This is low-level compared to GATT.

Code changes were applied to the gatts_demo.c and test were performed from Android Smartphone. The client ESP32 FEATHER was replaced by a smartphone for easy debugging.

1) Read/write characteristics of an 8 byte array

We can write and read-back values upto 8 byte. Only the READ and WRITE properties were set. The UUID is 0x00FF the charateristic id is 0x00FF. The default array is {0x11,0x22,..,0x88}
nRF Connect Screenshot
LOG:

I (15900) GATTS_DEMO: ESP_GATTS_CONNECT_EVT, conn_id 0, remote 54:2f:14:c8:6a:f2:
I (16380) GATTS_DEMO: update connection params status = 0, min_int = 16, max_int = 32,conn_int = 24,latency = 0, timeout = 400
I (16680) GATTS_DEMO: update connection params status = 0, min_int = 0, max_int = 0,conn_int = 6,latency = 0, timeout = 500
I (16940) GATTS_DEMO: update connection params status = 0, min_int = 0, max_int = 0,conn_int = 24,latency = 0, timeout = 400
I (20220) GATTS_DEMO: GATT_READ_EVT, conn_id 0, trans_id 1, handle 42

I (34170) GATTS_DEMO: GATT_WRITE_EVT, conn_id 0, trans_id 2, handle 42
I (34180) GATTS_DEMO: GATT_WRITE_EVT, value len 4, value :
I (34180) GATTS_DEMO: 01 02 03 04
I (38040) GATTS_DEMO: GATT_READ_EVT, conn_id 0, trans_id 3, handle 42

2) Boot button

The BOOT button is connected to GPIO0 (which is also a bootstrapping pin to set the boot mode), so pressing it will pull GPIO0 low. You can use this as a general purpose button after your firmware is running.
This GPIO will have a notification trigger with a counter of the amount of boot buttons pressed.

A task called testNotify() is started when the notification is enabled. Now the Notification is sent every 1s, could be changed to on-change. The default value is 0 and increases by 1 every enable button push.

The UUID is 0x00EE the charateristic id is 0xEE01. We can NOTIFY and READ this single byte variable.

nRF Connect Screenshot
Console Log:

I (97301) GATTS_DEMO: GATT_WRITE_EVT, value len 2, value :
I (97301) GATTS_DEMO: 01 00
I (97301) GATTS_DEMO: Notify data send 3
I (97311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (98311) GATTS_DEMO: Notify data send 3
I (98311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (99311) GATTS_DEMO: Notify data send 3
I (99311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (100311) GATTS_DEMO: Notify data send 3
I (100311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (101311) GATTS_DEMO: Notify data send 3
I (101311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (102311) GATTS_DEMO: Notify data send 3
I (102311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (103121) GATTS_DEMO: GPIO[0] intr, val: 1, EnablePushedCnt: 4

I (103311) GATTS_DEMO: Notify data send 4
I (103311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (104311) GATTS_DEMO: Notify data send 4
I (104311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (104551) GATTS_DEMO: GPIO[0] intr, val: 1, EnablePushedCnt: 5

I (105311) GATTS_DEMO: Notify data send 5
I (105311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (106311) GATTS_DEMO: Notify data send 5
I (106311) GATTS_DEMO: ESP_GATTS_CONF_EVT status 0 attr_handle 46
I (107041) GATTS_DEMO: GATT_WRITE_EVT, conn_id 0, trans_id 10, handle 47

I (107051) GATTS_DEMO: GATT_WRITE_EVT, value len 2, value :
I (107051) GATTS_DEMO: 00 00
I (107051) GATTS_DEMO: task suspended
I (107061) GATTS_DEMO: notify disable

As this code is only a proof of concept no effort has been put in coding guidelines nor structural readability.

David.