104 lines
6.1 KiB
ReStructuredText
104 lines
6.1 KiB
ReStructuredText
ESP-NOW
|
|
=======
|
|
|
|
Overview
|
|
--------
|
|
|
|
ESP-NOW is a kind of connectionless WiFi communication protocol which is defined by Espressif. In ESP-NOW, application data is
|
|
encapsulated in vendor-specific action frame and then transmitted from one WiFi device to another without connection.
|
|
CTR with CBC-MAC Protocol(CCMP) is used to protect the action frame for security. ESP-NOW is widely used in smart light, remote
|
|
controlling, sensor, etc.
|
|
|
|
Frame Format
|
|
------------
|
|
|
|
ESP-NOW uses vendor-specific action frame to transmit ESP-NOW data. The format of vendor-specific action frame is as follows:
|
|
|
|
.. highlight:: none
|
|
|
|
::
|
|
|
|
----------------------------------------------------------------------------------------
|
|
| MAC Header | Category Code | Organization Identifier | Vendor Specific Content | FCS |
|
|
----------------------------------------------------------------------------------------
|
|
1 byte 3 bytes 7~255 bytes
|
|
|
|
- Category Code: The Category field is set to the value(127) indicating the vendor-specific category.
|
|
- Organization Identifier: The Organization Identifier contains a unique identifier(0x18fe34) which is the first three bytes
|
|
of MAC address applied by Espressif.
|
|
- Vendor Specific Content: The Vendor Specific Content contains vendor-specific field as follows:
|
|
|
|
.. highlight:: none
|
|
|
|
::
|
|
|
|
-------------------------------------------------------------------------------
|
|
| Element ID | Length | Organization Identifier | Type | Version | Body |
|
|
-------------------------------------------------------------------------------
|
|
1 byte 1 byte 3 bytes 1 byte 1 byte 0~250 bytes
|
|
|
|
- Element ID: The Element ID field is set to the value(221) indicating the vendor-specific element.
|
|
- Length: The length is the total length of Organization Identifier, Type, Version and Body.
|
|
- Organization Identifier: The Organization Identifier contains a unique identifier(0x18fe34) which is the first three bytes
|
|
of MAC address applied by Espressif.
|
|
- Type: The Type field is set to the value(4) indicating ESP-NOW.
|
|
- Version: The Version field is set to the version of ESP-NOW.
|
|
- Body: The Body contains the ESP-NOW data.
|
|
|
|
As ESP-NOW is connectionless, the MAC header is a little different from that of standard frames. The FromDS and ToDS bits of
|
|
FrameControl field are both 0. The first address field is set to the destination address. The second address field is set to
|
|
the source address. The third address field is set to broadcast address(0xff:0xff:0xff:0xff:0xff:0xff).
|
|
|
|
Security
|
|
--------
|
|
|
|
ESP-NOW use CCMP method which can be referenced in IEEE Std. 802.11-2012 to protect the vendor-specific action frame. The WiFi
|
|
device maintains a Primary Master Key(PMK) and several Local Master Keys(LMK). The lengths of them are 16 bytes. PMK is used
|
|
to encrypt LMK with AES-128 algorithm. Call ``esp_now_set_pmk()`` to set PMK. If PMK is not set, a default PMK will be used.
|
|
If LMK of the paired device is set, it will be used to encrypt the vendor-specific action frame with CCMP method. The maximum
|
|
number of different LMKs is six. Do not support encrypting multicast vendor-specific action frame.
|
|
|
|
Initialization and De-initialization
|
|
------------------------------------
|
|
|
|
Call ``esp_now_init()`` to initialize ESP-NOW and ``esp_now_deinit()`` to de-initialize ESP-NOW. ESP-NOW data must be transmitted
|
|
after WiFi is started, so it is recommended to start WiFi before initializing ESP-NOW and stop WiFi after de-initializing ESP-NOW.
|
|
When ``esp_now_deinit()`` is called, all of the information of paired devices will be deleted.
|
|
|
|
Add Paired Device
|
|
-----------------
|
|
|
|
Before sending data to other device, call ``esp_now_add_peer()`` to add it to the paired device list first. The maximum number of
|
|
paired devices is twenty. If security is enabled, the LMK must be set. ESP-NOW data can be sent from station or softap interface.
|
|
Make sure that the interface is enabled before sending ESP-NOW data. A device with broadcast MAC address must be added before
|
|
sending broadcast data. The range of the channel of paired device is from 0 to 14. If the channel is set to 0, data will be sent
|
|
on the current channel. Otherwise, the channel must be set as the channel that the local device is on.
|
|
|
|
Send ESP-NOW Data
|
|
-----------------
|
|
|
|
Call ``esp_now_send()`` to send ESP-NOW data and ``esp_now_register_send_cb`` to register sending callback function. It will return
|
|
`ESP_NOW_SEND_SUCCESS` in sending callback function if the data is received successfully on MAC layer. Otherwise, it will return
|
|
`ESP_NOW_SEND_FAIL`. There are several reasons failing to send ESP-NOW data, for example, the destination device doesn't exist, the
|
|
channels of the devices are not the same, the action frame is lost when transmiting on the air, etc. It is not guaranteed that
|
|
application layer can receive the data. If necessary, send back ack data when receiving ESP-NOW data. If receiving ack data timeout
|
|
happens, retransmit the ESP-NOW data. A sequence number can also be assigned to ESP-NOW data to drop the duplicated data.
|
|
|
|
If there is a lot of ESP-NOW data to send, call ``esp_now_send()`` to send less than or equal to 250 bytes of data once a time.
|
|
Note that too short interval between sending two ESP-NOW datas may lead to disorder of sending callback function. So, it is
|
|
recommended that sending the next ESP-NOW data after the sending callback function of previous sending has returned. The sending
|
|
callback function runs from a high-priority WiFi task. So, do not do lengthy operations in the callback function. Instead, post
|
|
necessary data to a queue and handle it from a lower priority task.
|
|
|
|
Receiving ESP-NOW Data
|
|
----------------------
|
|
|
|
Call ``esp_now_register_recv_cb`` to register receiving callback function. When receiving ESP-NOW data, receiving callback function
|
|
is called. The receiving callback function also runs from WiFi task. So, do not do lengthy operations in the callback function.
|
|
Instead, post necessary data to a queue and handle it from a lower priority task.
|
|
|
|
API Reference
|
|
-------------
|
|
|
|
.. include:: /_build/inc/esp_now.inc
|