Controller Area Network (CAN)
=============================

.. -------------------------------- Overview -----------------------------------

Overview
--------

The ESP32's peripherals contains a CAN Controller that supports Standard Frame 
Format (16-bit ID) and Extended Frame Format (29-bit ID) of the CAN2.0B specification.

.. warning::
    The ESP32 CAN controller is not compatible with CAN FD frames and will interpret 
    such frames as errors.

This programming guide is split into the following sections:

    1. :ref:`basic-can-concepts`

    2. :ref:`signals-lines-and-transceiver`

    3. :ref:`configuration`

    4. :ref:`driver-operation`

    5. :ref:`examples`


.. --------------------------- Basic CAN Concepts ------------------------------

.. _basic-can-concepts:

Basic CAN Concepts
------------------

.. note::
    The following section only covers the basic aspects of CAN. For full details,
    see the CAN2.0B specification

The CAN protocol is a multi-master, multi-cast communication protocol with error
detection/signalling and inbuilt message prioritization. The CAN protocol is 
commonly used as a communication bus in automotive applications.

**Multi-master:** Any node in a CAN bus is allowed initiate the transfer of data.

**Multi-cast:** When a node transmits a message, all nodes are able to receive
the message (broadcast). However some nodes can selective choose which messages
to accept via the use of acceptance filtering (multi-cast).

**Error Detection and Signalling:** Every CAN node will constantly monitor the 
CAN bus. When any node detects an error, it will signal the error by transmitting an error 
frame. Other nodes will receive the error frame and transmit their own error frames
in response. This will result in an error detection being propagated to all nodes on
the bus.

**Message Priorities:** If two nodes attempt to transmit simultaneously, the
node transmitting the message with the lower ID will win arbitration. All other
nodes will become receivers ensuring there is at most one transmitter at any time.

CAN Message Frames
^^^^^^^^^^^^^^^^^^

The CAN2.0B specification contains two frame formats known as **Extended Frame**
and **Standard Frame** which contain 29-bit IDs and 11-bit IDs respectively.
A CAN message consists of the following components

    - 29-bit or 11-bit ID
    - Data Length Code (DLC) between 0 to 8
    - Up to 8 bytes of data (should match DLC)

Error States and Counters
^^^^^^^^^^^^^^^^^^^^^^^^^

The CAN2.0B specification implements fault confinement by requiring every CAN node 
to maintain two internal error counters known as the **Transmit Error Counter (TEC)**
and the **Receive Error Counter (REC)**. The two error counters are used to determine
a CAN node's **error state**, and the counters are incremented and decremented 
following a set of rules (see CAN2.0B specification). These error states are known
as **Error Active**, **Error Passive**, and **Bus-Off**.

**Error Active:** A CAN node is Error Active when **both TEC and REC are less 
than 128** and indicates a CAN node is operating normally. Error Active nodes are
allowed to participate in CAN bus activities, and will actively signal any error
conditions it detects by transmitting an **Active Error Flag** over the CAN bus.

**Error Passive:** A CAN node is Error Passive when **either the TEC or REC becomes
greater than or equal to 128**. Error Passive nodes are still able to take part in
CAN bus activities, but will instead transmit a **Passive Error Flag** upon
detection of an error.

**Bus-Off:** A CAN node becomes Bus-Off when the **TEC becomes greater than or equal
to 256**. A Bus-Off node is unable take part in CAN bus activity and will remain so
until it undergoes bus recovery.


.. ---------------------- Signal Lines and Transceiver -------------------------

.. _signals-lines-and-transceiver:

Signals Lines and Transceiver
-----------------------------

The CAN controller does not contain a internal transceiver and therefore
**requires an external transceiver** to operate. The type of external transceiver will 
depend on the application's physical layer specification (e.g. using SN65HVD23X 
transceivers for ISO 11898-2 compatibility).

The CAN controller's interface consists of 4 signal lines known as **TX, RX, BUS-OFF,
and CLKOUT**. These four signal lines can be routed through the GPIO Matrix to GPIOs.

.. blockdiag:: ../../../_static/diagrams/can/can_controller_signals.diag
    :caption: Signal lines of the CAN controller
    :align: center

**TX and RX:** The TX and RX signal lines are required to interface with an 
external CAN transceiver. Both signal lines represent/interpret a dominant bit 
as a low logic level (0V), and a recessive bit as a high logic level (3.3V).

**BUS-OFF:** The BUS-OFF signal line is **optional** and is set to a low logic level
(0V) whenever the CAN controller reaches a bus-off state. The BUS-OFF signal line
is set to a high logic level (3.3V) otherwise.

**CLKOUT:** The CLKOUT signal line is **optional** and outputs a prescaled version
of the CAN controller's source clock (APB Clock).

.. note::
    An external transceiver **must internally tie the TX input and the RX output**
    such that a change in logic level to the TX signal line can be observed on the 
    RX line. Failing to do so will cause the CAN controller to interpret differences 
    in logic levels between the two signal lines as a lost in arbitration or a 
    bit error.


.. ------------------------------ Configuration --------------------------------

.. _configuration:

Configuration
-------------

Operating Modes
^^^^^^^^^^^^^^^

The CAN driver supports the following modes of operations:

**Normal Mode:** The normal operating mode allows the CAN controller to take part
in bus activities such as transmitting and receiving messages/error frames. 
Acknowledgement from another CAN node is required when transmitting message frames.

**No Ack Mode:** The No Acknowledgement mode is similar to normal mode, however
acknowledgements are not required when transmitting message frames. This mode is
useful when self testing the CAN controller.

**Listen Only Mode:** This mode will prevent the CAN controller from taking part
in bus activities. Therefore transmissions of messages/acknowledgement/error frames
will be disabled. However the the CAN controller will still be able to receive
messages (without acknowledging). This mode is suited for applications such as
CAN bus monitoring.

Alerts
^^^^^^

The CAN driver contains an alert feature which is used to notify the application
level of certain CAN driver events. Alerts are selectively enabled when the
CAN driver is installed, but can be reconfigured during runtime by calling
:cpp:func:`can_reconfigure_alerts`. The application can then wait for any enabled
alerts to occur by calling :cpp:func:`can_read_alerts`. The CAN driver supports 
the following alerts:

+------------------------------------+------------------------------------------------------------------------+
| Alert                              | Description                                                            |
+====================================+=============================================+==========================+
| ``CAN_ALERT_TX_IDLE``              | No more messages queued for transmission                               |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_TX_SUCCESS``           | The previous transmission was successful                               |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_BELOW_ERR_WARN``       | Both error counters have dropped below error warning limit             |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_ERR_ACTIVE``           | CAN controller has become error active                                 |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_RECOVERY_IN_PROGRESS`` | CAN controller is undergoing bus recovery                              |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_BUS_RECOVERED``        | CAN controller has successfully completed bus recovery                 |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_ARB_LOST``             | The previous transmission lost arbitration                             |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_ABOVE_ERR_WARN``       | One of the error counters have exceeded the error warning limit        |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_BUS_ERROR``            | A (Bit, Stuff, CRC, Form, ACK) error has occurred on the bus           |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_TX_FAILED``            | The previous transmission has failed                                   |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_RX_QUEUE_FULL``        | The RX queue is full causing a received frame to be lost               |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_ERR_PASS``             | CAN controller has become error passive                                |
+------------------------------------+------------------------------------------------------------------------+
| ``CAN_ALERT_BUS_OFF``              | Bus-off condition occurred. CAN controller can no longer influence bus |
+------------------------------------+------------------------------------------------------------------------+

.. note::
    The **error warning limit** can be used to preemptively warn the application
    of bus errors before the error passive state is reached. By default the CAN
    driver sets the **error warning limit** to **96**. The ``CAN_ALERT_ABOVE_ERR_WARN``
    is raised when the TEC or REC becomes larger then or equal to the error warning
    limit. The ``CAN_ALERT_BELOW_ERR_WARN`` is raised when both TEC and REC return 
    back to values below **96**.

.. note::
    When enabling alerts, the ``CAN_ALERT_AND_LOG`` flag can be used to cause the
    CAN driver to log any raised alerts to UART. The ``CAN_ALERT_ALL`` and
    ``CAN_ALERT_NONE`` macros can also be used to enable/disable all alerts during
    configuration/reconfiguration.

Bit Timing
^^^^^^^^^^

The operating bit rate of the CAN controller is configured using the
:cpp:type:`can_timing_config_t` structure. The period of each bit is made up of 
multiple **time quanta**, and the period of a **time quanta** is determined by a 
prescaled version of the CAN controller's source clock. A single bit contains the
following segments in the following order:

    1. The **Synchronization Segment** consists of a single time quanta
    2. **Timing Segment 1** consists of 1 to 16 time quanta before sample point
    3. **Timing Segment 2** consists of 1 to 8 time quanta after sample point

The **Baudrate Prescaler** is used to determine the period of each time quanta by
dividing the CAN controller's source clock (80 MHz APB clock). The ``brp`` can be
**any even number from 2 to 128**.

.. packetdiag:: ../../../_static/diagrams/can/can_bit_timing.diag
    :caption: Bit timing configuration for 500kbit/s given BRP = 8
    :align: center

The sample point of a bit is located on the intersection of Timing Segment 1 and
2. Enabling **Triple Sampling** will cause 3 time quanta to be sampled per bit 
instead of 1 (extra samples are located at the tail end of Timing Segment 1).

The **Synchronization Jump Width** is used to determined the maximum number of 
time quanta a single bit time can be lengthened/shortened for synchronization
purposes. ``sjw`` can **range from 1 to 4**.

.. note::
    Multiple combinations of ``brp``, ``tseg_1``, ``tseg_2``, and ``sjw`` can 
    achieve the same bit rate. Users should tune these values to the physical 
    characteristics of their CAN bus by taking into account factors such as 
    **propagation delay, node information processing time, and phase errors**.

Bit timing **macro initializers** are also available for commonly used CAN bus bit rates.
The following macro initiliazers are provided by the CAN driver.

    - ``CAN_TIMING_CONFIG_25KBITS()``
    - ``CAN_TIMING_CONFIG_50KBITS()``
    - ``CAN_TIMING_CONFIG_100KBITS()``
    - ``CAN_TIMING_CONFIG_125KBITS()``
    - ``CAN_TIMING_CONFIG_250KBITS()``
    - ``CAN_TIMING_CONFIG_500KBITS()``
    - ``CAN_TIMING_CONFIG_800KBITS()``
    - ``CAN_TIMING_CONFIG_1MBITS()``

Acceptance Filter
^^^^^^^^^^^^^^^^^

The CAN controller contains a hardware acceptance filter which can be used to
filter CAN messages of a particular ID. A node that filters out a message 
**will not receive the message, but will still acknowledge it**. Acceptances 
filters can make a node more efficient by filtering out messages sent over the 
CAN bus that are irrelevant to the CAN node in question. The CAN controller's 
acceptance filter is configured using two 32-bit values within :cpp:type:`can_filter_config_t`
known as the **acceptance code** and the **acceptance mask**.  

The **acceptance code** specifies the bit sequence which a message's ID, RTR, and
data bytes must match in order for the message to be received by the CAN 
controller. The **acceptance mask** is a bit sequence specifying which bits of 
the acceptance code can be ignored. This allows for a messages of different IDs 
to be accepted by a single acceptance code.

The acceptance filter can be used under **Single or Dual Filter Mode**. 
Single Filter Mode will use the acceptance code and mask to define a single 
filter. This allows for the first two data bytes of a standard frame to be filtered, 
or the entirety of an extended frame's 29-bit ID. The following diagram illustrates
how the 32-bit acceptance code and mask will be interpreted under Single Filter Mode 
(Note: The yellow and blue fields represent standard and extended CAN frames respectively).

.. packetdiag:: ../../../_static/diagrams/can/can_acceptance_filter_single.diag
    :caption: Bit layout of single filter mode (Right side MSBit)
    :align: center

**Dual Filter Mode** will use the acceptance code and mask to define two separate 
filters allowing for increased flexibility of ID's to accept, but does not allow 
for all 29-bits of an extended ID to be filtered. The following diagram illustrates
how the 32-bit acceptance code and mask will be interpreted under **Dual Filter Mode**
(Note: The yellow and blue fields represent standard and extended CAN frames respectively).

.. packetdiag:: ../../../_static/diagrams/can/can_acceptance_filter_dual.diag
    :caption: Bit layout of dual filter mode (Right side MSBit)
    :align: center

Disabling TX Queue
^^^^^^^^^^^^^^^^^^

The TX queue can be disabled during configuration by setting the ``tx_queue_len``
member of :cpp:type:`can_general_config_t` to ``0``. This will allow applications
that do not require message transmission to save a small amount of memory when 
using the CAN driver.


.. -------------------------------- CAN Driver ---------------------------------

.. _driver-operation:

Driver Operation
----------------

The CAN driver is designed with distinct states and strict rules regarding the
functions or conditions that trigger a state transition. The following diagram
illustrates the various states and their transitions.

.. blockdiag:: ../../../_static/diagrams/can/can_state_transition.diag
    :caption: State transition diagram of the CAN driver (see table below)
    :align: center

+-------+------------------------+------------------------------------+
| Label |       Transition       |          Action/Condition          |
+=======+========================+====================================+
|   A   | Uninstalled -> Stopped |   :cpp:func:`can_driver_install`   |
+-------+------------------------+------------------------------------+
|   B   | Stopped -> Uninstalled |  :cpp:func:`can_driver_uninstall`  |
+-------+------------------------+------------------------------------+
|   C   |   Stopped -> Running   |       :cpp:func:`can_start`        |
+-------+------------------------+------------------------------------+
|   D   |   Running -> Stopped   |        :cpp:func:`can_stop`        |
+-------+------------------------+------------------------------------+
|   E   |   Running -> Bus-Off   |   Transmit Error Counter >= 256    |
+-------+------------------------+------------------------------------+
|   F   | Bus-Off -> Uninstalled |  :cpp:func:`can_driver_uninstall`  |
+-------+------------------------+------------------------------------+
|   G   | Bus-Off -> Recovering  | :cpp:func:`can_initiate_recovery`  |
+-------+------------------------+------------------------------------+
|   H   | Recovering -> Stopped  | 128 occurrences of bus-free signal |
+-------+------------------------+------------------------------------+

Driver States
^^^^^^^^^^^^^

**Uninstalled**: In the uninstalled state, no memory is allocated for the driver
and the CAN controller is powered OFF.

**Stopped**: In this state, the CAN controller is powered ON and the CAN driver
has been installed. However the CAN controller will be unable to take part in
any CAN bus activities such as transmitting, receiving, or acknowledging messages.

**Running**: In the running state, the CAN controller is able to take part in 
bus activities. Therefore messages can be transmitted/received/acknowledged. 
Furthermore the CAN controller will be able to transmit error frames upon detection
of errors on the CAN bus.

**Bus-Off**: The bus-off state is automatically entered when the CAN controller's
Transmit Error Counter becomes greater than or equal to 256 (see CAN2.0B specification
regarding error counter rules). The bus-off state indicates the occurrence of severe
errors on the CAN bus or in the CAN controller. Whilst in the bus-off state, the
CAN controller will be unable to take part in any CAN bus activities. To exit
the bus-off state, the CAN controller must undergo the bus recovery process.

**Recovering**: The recovering state is entered when the CAN driver undergoes
bus recovery. The CAN driver/controller will remain in the recovering state until
the 128 occurrences of the bus-free signal (see CAN2.0B specification) is observed
on the CAN bus.

Message Flags
^^^^^^^^^^^^^

The CAN driver distinguishes different types of CAN messages by using the message 
flags in the ``flags`` field of :cpp:type:`can_message_t`. These flags help
distinguish whether a message is in standard or extended format, an RTR, and the
type of transmission to use when transmitting such a message. The CAN driver
supports the following flags:

+-------------------------------+---------------------------------------------------------------+
| Flag                          |  Description                                                  |
+===============================+===============================================================+
| ``CAN_MSG_FLAG_EXTD``         | Message is in Extended Frame Format (29bit ID)                |
+-------------------------------+---------------------------------------------------------------+
| ``CAN_MSG_FLAG_RTR``          | Message is a Remote Transmit Request                          |
+-------------------------------+---------------------------------------------------------------+
| ``CAN_MSG_FLAG_SS``           | Transmit message using Single Shot Transmission (Message will |
|                               | note be retransmitted upon error or loss of arbitration)      |
+-------------------------------+---------------------------------------------------------------+
| ``CAN_MSG_FLAG_SELF``         | Transmit message using Self Reception Request (Transmitted    |
|                               | message will also received by the same node)                  |
+-------------------------------+---------------------------------------------------------------+
| ``CAN_MSG_FLAG_DLC_NON_COMP`` | Message's Data length code is larger than 8. This             |
|                               | will break compliance with CAN2.0B                            |
+-------------------------------+---------------------------------------------------------------+

.. note::
    The ``CAN_MSG_FLAG_NONE`` flag can be used for Standard Frame Format messages


.. -------------------------------- Examples -----------------------------------

.. _examples:

Examples
--------

Configuration & Installation
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The following code snippet demonstrates how to configure, install, and start the
CAN driver via the use of the various configuration structures, macro initializers,
the :cpp:func:`can_driver_install` function, and the :cpp:func:`can_start` function.

.. code-block:: c

    #include "driver/gpio.h"
    #include "driver/can.h"

    void app_main()
    {
        //Initialize configuration structures using macro initializers
        can_general_config_t g_config = CAN_GENERAL_CONFIG_DEFAULT(GPIO_NUM_21, GPIO_NUM_22, CAN_MODE_NORMAL);
        can_timing_config_t t_config = CAN_TIMING_CONFIG_500KBITS();
        can_filter_config_t f_config = CAN_FILTER_CONFIG_ACCEPT_ALL();

        //Install CAN driver
        if (can_driver_install(&g_config, &t_config, &f_config) == ESP_OK) {
            printf("Driver installed\n");
        } else {
            printf("Failed to install driver\n");
            return;
        }

        //Start CAN driver
        if (can_start() == ESP_OK) {
            printf("Driver started\n");
        } else {
            printf("Failed to start driver\n");
            return;
        }

        ...

    }

The usage of macro initializers are not mandatory and each of the configuration
structures can be manually.

Message Transmission
^^^^^^^^^^^^^^^^^^^^

The following code snippet demonstrates how to transmit a message via the usage
of the :cpp:type:`can_message_t` type and :cpp:func:`can_transmit` function.

.. code-block:: c

    #include "driver/can.h"

    ...

    //Configure message to transmit
    can_message_t message;
    message.identifier = 0xAAAA;
    message.flags = CAN_MSG_FLAG_EXTD;
    message.data_length_code = 4;
    for (int i = 0; i < 4; i++) {
        message.data[i] = 0;
    }

    //Queue message for transmission
    if (can_transmit(&message, pdMS_TO_TICKS(1000)) == ESP_OK) {
        printf("Message queued for transmission\n");
    } else {
        printf("Failed to queue message for transmission\n");
    }

Message Reception
^^^^^^^^^^^^^^^^^

The following code snippet demonstrates how to receive a message via the usage
of the :cpp:type:`can_message_t` type and :cpp:func:`can_receive` function.

.. code-block:: c

    #include "driver/can.h"

    ...

    //Wait for message to be received
    can_message_t message;
    if (can_receive(&message, pdMS_TO_TICKS(10000)) == ESP_OK) {
        printf("Message received\n");
    } else {
        printf("Failed to receive message\n");
        return;
    }

    //Process received message
    if (message.flags & CAN_MSG_FLAG_EXTD) {
        printf("Message is in Extended Format\n");
    } else {
        printf("Message is in Standard Format\n");
    }
    printf("ID is %d\n", message.identifier);
    if (!(message.flags & CAN_MSG_FLAG_RTR)) {
        for (int i = 0; i < message.data_length_code; i++) {
            printf("Data byte %d = %d\n", i, message.data[i]);
        }
    }

Reconfiguring and Reading Alerts
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The following code snippet demonstrates how to reconfigure and read CAN driver 
alerts via the use of the :cpp:func:`can_reconfigure_alerts` and 
:cpp:func:`can_read_alerts` functions.

.. code-block:: c

    #include "driver/can.h"

    ...

    //Reconfigure alerts to detect Error Passive and Bus-Off error states
    uint32_t alerts_to_enable = CAN_ALERT_ERR_PASS | CAN_ALERT_BUS_OFF;
    if (can_reconfigure_alerts(alerts_to_enable, NULL) == ESP_OK) {
        printf("Alerts reconfigured\n");
    } else {
        printf("Failed to reconfigure alerts");
    }

    //Block indefinitely until an alert occurs
    uint32_t alerts_triggered;
    can_read_alerts(&alerts_triggered, portMAX_DELAY);

Stop and Uninstall
^^^^^^^^^^^^^^^^^^

The following code demonstrates how to stop and uninstall the CAN driver via the
use of the :cpp:func:`can_stop` and :cpp:func:`can_driver_uninstall` functions.

.. code-block:: c

    #include "driver/can.h"

    ...

    //Stop the CAN driver
    if (can_stop() == ESP_OK) {
        printf("Driver stopped\n");
    } else {
        printf("Failed to stop driver\n");
        return;
    }

    //Uninstall the CAN driver
    if (can_driver_uninstall() == ESP_OK) {
        printf("Driver uninstalled\n");
    } else {
        printf("Failed to uninstall driver\n");
        return;
    }

Multiple ID Filter Configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The acceptance mask in :cpp:type:`can_filter_config_t` can be configured such that 
two or more IDs will be accepted for a single filter. For a particular filter to 
accept multiple IDs, the conflicting bit positions amongst the IDs must be set 
in the acceptance mask. The acceptance code can be set to any one of the IDs.

The following example shows how the calculate the acceptance mask given multiple 
IDs::

    ID1 =  11'b101 1010 0000
    ID2 =  11'b101 1010 0001
    ID3 =  11'b101 1010 0100
    ID4 =  11'b101 1010 1000
    //Acceptance Mask
    MASK = 11'b000 0000 1101

Application Examples
^^^^^^^^^^^^^^^^^^^^

**Network Example:** The CAN Network example demonstrates communication between 
two ESP32s using the CAN driver API. One CAN node acts as a network master initiate
and ceasing the transfer of a data from another CAN node acting as a network slave. 
The example can be found via :example:`peripherals/can/can_network`.

**Alert and Recovery Example:** This example demonstrates how to use the CAN driver's
alert and bus recovery API. The example purposely introduces errors on the CAN
bus to put the CAN controller into the Bus-Off state. An alert is used to detect
the Bus-Off state and trigger the bus recovery process. The example can be found
via :example:`peripherals/can/can_alert_and_recovery`.

**Self Test Example:** This example uses the No Acknowledge Mode and Self Reception
Request to cause the CAN controller to send and simultaneously receive a series
of messages. This example can be used to verify if the connections between the CAN 
controller and the external transceiver are working correctly. The example can be
found via :example:`peripherals/can/can_self_test`.


.. ---------------------------- API Reference ----------------------------------

API Reference
-------------

.. include:: /_build/inc/can.inc