The Two-Wire Automotive Interface (TWAI) is a real-time serial communication protocol suited for automotive and industrial applications. It is compatible with ISO11898-1 Classical frames, thus can support Standard Frame Format (11-bit ID) and Extended Frame Format (29-bit ID). The {IDF_TARGET_NAME} contains {IDF_TARGET_CONFIG_SOC_TWAI_CONTROLLER_NUM} TWAI controller(s) that can be configured to communicate on a TWAI bus via an external transceiver.
The TWAI is a multi-master, multi-cast, asynchronous, serial communication protocol. TWAI also supports error detection and signalling, and inbuilt message prioritization.
**Multi-master:** Any node on the bus can initiate the transfer of a message.
**Multi-cast:** When a node transmits a message, all nodes on the bus will receive the message (i.e., broadcast) thus ensuring data consistency across all nodes. However, some nodes can selectively choose which messages to accept via the use of acceptance filtering (multi-cast).
**Asynchronous:** The bus does not contain a clock signal. All nodes on the bus operate at the same bit rate and synchronize using the edges of the bits transmitted on the bus.
**Error Detection and Signaling:** Every node constantly monitors the bus. When any node detects an error, it signals the detection by transmitting an error frame. Other nodes will receive the error frame and transmit their own error frames in response. This results in an error detection being propagated to all nodes on the bus.
**Message Priorities:** Messages contain an ID field. If two or more nodes attempt to transmit simultaneously, the node transmitting the message with the lower ID value will win arbitration of the bus. All other nodes will become receivers ensuring that there is at most one transmitter at any time.
TWAI Messages are split into Data Frames and Remote Frames. Data Frames are used to deliver a data payload to other nodes, whereas a Remote Frame is used to request a Data Frame from other nodes, and other nodes can optionally respond with a Data Frame. Data and Remote Frames have two frame formats known as **Extended Frame** and **Standard Frame** which contain a 29-bit ID and an 11-bit ID respectively. A TWAI message consists of the following fields:
- 29-bit or 11-bit ID: Determines the priority of the message, and lower value has higher priority.
- Data Length Code (DLC) between 0 to 8: Indicates in bytes the size of the data payload for a Data Frame, or the amount of data to request for a Remote Frame.
- Up to 8 bytes of data for a Data Frame, which should match DLC.
The TWAI protocol implements a feature known as "fault confinement" where a persistently erroneous node will eventually eliminate itself from the bus. This is implemented by requiring every 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 incremented and decremented according to a set of rules where the counters increase on an error, and decrease on a successful message transmission/reception. The values of the counters are used to determine a node's **error state**, namely **Error Active**, **Error Passive**, and **Bus-Off**.
**Error Active:** A node is Error Active when **both TEC and REC are less than 128** and indicates that the node is operating normally. Error Active nodes are allowed to participate in bus communications, and will actively signal the detection of any errors by automatically transmitting an **Active Error Flag** over the bus.
**Error Passive:** A 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 bus communications, but will instead transmit a **Passive Error Flag** upon detection of an error.
**Bus-Off:** A node becomes Bus-Off when the **TEC becomes greater than or equal to 256**. A Bus-Off node is unable to influence the bus in any manner, essentially disconnected from the bus, thus eliminating itself from the bus. A node will remain in the Bus-Off state until it undergoes bus-off recovery.
The TWAI controller does not contain an integrated transceiver. Therefore, to connect the TWAI controller to a TWAI bus, **an external transceiver is required**. The type of external transceiver used should depend on the application's physical layer specification. E.g., using SN65HVD23x transceivers for ISO 11898-2 compatibility.
The TWAI controller's interface consists of four signal lines known as **TX, RX, BUS-OFF, and CLKOUT**. These four signal lines can be routed through the GPIO Matrix to the {IDF_TARGET_NAME}'s GPIO pads.
**TX and RX:** The TX and RX signal lines are required to interface with an external transceiver. Both signal lines represent/interpret a dominant bit as a low logic level (0 V) and a recessive bit as a high logic level (3.3 V).
**BUS-OFF:** The BUS-OFF signal line is **optional** and is set to a low logic level (0 V) whenever the TWAI controller reaches a bus-off state. The BUS-OFF signal line is set to a high logic level (3.3 V) otherwise.
An external transceiver **must internally loop back the TX to RX** 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 TWAI controller to interpret differences in logic levels between the two signal lines as a loss in arbitration or a bit error.
The TWAI driver provides two sets of API. One is handle-free and is widely used in IDF versions earlier than v5.2, but it can only support one TWAI hardware controller. The other set is with handles, and the function name is usually suffixed with "v2", which can support any number of TWAI controllers. These two sets of API can be used at the same time, but it is recommended to use the "v2" version in your new projects.
**Normal Mode:** The normal operating mode allows the TWAI controller to take part in bus activities such as transmitting and receiving messages/error frames. Acknowledgment from another node is required when transmitting a message.
**No Ack Mode:** The No Acknowledgement mode is similar to normal mode, however, acknowledgments are not required for a message transmission to be considered successful. This mode is useful when self-testing the TWAI controller (loopback of transmissions).
**Listen Only Mode:** This mode prevents the TWAI controller from influencing the bus. Therefore, the transmission of messages/acknowledgment/error frames will be disabled. However, the TWAI controller is still able to receive messages but will not acknowledge the message. This mode is suited for bus monitor applications.
The TWAI driver contains an alert feature that is used to notify the application layer of certain TWAI controller or TWAI bus events. Alerts are selectively enabled when the TWAI driver is installed, but can be reconfigured during runtime by calling :cpp:func:`twai_reconfigure_alerts`. The application can then wait for any enabled alerts to occur by calling :cpp:func:`twai_read_alerts`. The TWAI driver supports the following alerts:
The TWAI controller's **error warning limit** is used to preemptively warn the application of bus errors before the error passive state is reached. By default, the TWAI driver sets the **error warning limit** to **96**. The ``TWAI_ALERT_ABOVE_ERR_WARN`` is raised when the TEC or REC becomes larger than or equal to the error warning limit. The ``TWAI_ALERT_BELOW_ERR_WARN`` is raised when both TEC and REC return to values below **96**.
When enabling alerts, the ``TWAI_ALERT_AND_LOG`` flag can be used to cause the TWAI driver to log any raised alerts to UART. However, alert logging is disabled and ``TWAI_ALERT_AND_LOG`` if the :ref:`CONFIG_TWAI_ISR_IN_IRAM` option is enabled. See :ref:`placing-isr-into-iram`.
The operating bit rate of the TWAI driver is configured using the :cpp:type:`twai_timing_config_t` structure. The period of each bit is made up of multiple **time quanta**, and the period of a **time quantum** is determined by a pre-scaled version of the TWAI controller's source clock. A single bit contains the following segments in the following order:
The **Baud Rate Prescaler (BRP)** is used to determine the period of each time quantum by dividing the TWAI controller's source clock. On the {IDF_TARGET_NAME}, the ``brp`` can be **any even number from 2 to {IDF_TARGET_MAX_BRP}**. Alternatively, you can decide the resolution of each quantum, by setting :cpp:member:`twai_timing_config_t::quanta_resolution_hz` to a non-zero value. In this way, the driver can calculate the underlying ``brp`` value for you. It is useful when you set different clock sources but want the bitrate to keep the same.
The supported clock source for a TWAI controller is listed in the :cpp:type:`twai_clock_source_t` and can be specified in :cpp:member:`twai_timing_config_t::clk_src`.
If the ESP32 is a v2.0 or later chip, the ``brp`` will **also support any multiple of 4 from 132 to 256**, and can be enabled by setting the :ref:`CONFIG_ESP32_REV_MIN` to v2.0 or higher.
The sample point of a bit is located at the intersection of Timing Segments 1 and 2. Enabling **Triple Sampling** causes 3-time quanta to be sampled per bit instead of 1, and extra samples are located at the tail end of Timing Segment 1.
The **Synchronization Jump Width (SJW)** is used to determine the maximum number of time quanta a single bit time can be lengthened/shortened for synchronization purposes. ``sjw`` can **range from 1 to 4**.
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 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 bit rates. The following macro initializers are provided by the TWAI driver.
The TWAI controller contains a hardware acceptance filter which can be used to filter messages of a particular ID. A node that filters out a message **does not receive the message, but will still acknowledge it**. Acceptance filters can make a node more efficient by filtering out messages sent over the bus that are irrelevant to the node. The acceptance filter is configured using two 32-bit values within :cpp:type:`twai_filter_config_t` known as the **acceptance code** and the **acceptance mask**.
The **acceptance code** specifies the bit sequence in which a message's ID, RTR, and data bytes must match in order for the message to be received by the TWAI controller. The **acceptance mask** is a bit sequence specifying which bits of the acceptance code can be ignored. This allows for 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 uses 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 are interpreted under Single Filter Mode. Note: The yellow and blue fields represent standard and extended frame formats respectively.
**Dual Filter Mode** uses the acceptance code and mask to define two separate filters allowing for increased flexibility of IDs 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 are interpreted under **Dual Filter Mode**. Note: The yellow and blue fields represent standard and extended frame formats respectively.
The TX queue can be disabled during configuration by setting the ``tx_queue_len`` member of :cpp:type:`twai_general_config_t` to ``0``. This allows applications that do not require message transmission to save a small amount of memory when using the TWAI driver.
The TWAI driver's ISR (Interrupt Service Routine) can be placed into IRAM so that the ISR can still run whilst the cache is disabled. Placing the ISR into IRAM may be necessary to maintain the TWAI driver's functionality during lengthy cache-disabling operations (such as SPI Flash writes, OTA updates, etc.). Whilst the cache is disabled, the ISR continues to:
- When calling :cpp:func:`twai_driver_install`, the ``intr_flags`` member of :cpp:type:`twai_general_config_t` should set :c:macro:`ESP_INTR_FLAG_IRAM`.
When the :ref:`CONFIG_TWAI_ISR_IN_IRAM` option is enabled, the TWAI driver will no longer log any alerts, i.e., the ``TWAI_ALERT_AND_LOG`` flag will not have any effect.
The ESP32's TWAI controller contains multiple hardware errata (more details about the errata can be found in the `ESP32's ECO document <https://www.espressif.com/sites/default/files/documentation/eco_and_workarounds_for_bugs_in_esp32_en.pdf>`_). Some of these errata are critical, and under specific circumstances, can place the TWAI controller into an unrecoverable state (i.e., the controller gets stuck until it is reset by the CPU).
The TWAI driver contains software workarounds for these critical errata. With these workarounds, the ESP32 TWAI driver can operate normally, albeit with degraded performance. The degraded performance will affect users in the following ways depending on what particular errata conditions are encountered:
The TWAI 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.
**Stopped**: In this state, the TWAI controller is powered ON and the TWAI driver has been installed. However, the TWAI controller is unable to take part in any bus activities such as transmitting, receiving, or acknowledging messages.
**Running**: In the running state, the TWAI controller is able to take part in bus activities. Therefore messages can be transmitted/received/acknowledged. Furthermore, the TWAI controller is able to transmit error frames upon detection of errors on the bus.
**Bus-Off**: The bus-off state is automatically entered when the TWAI controller's Transmit Error Counter becomes greater than or equal to 256. The bus-off state indicates the occurrence of severe errors on the bus or in the TWAI controller. Whilst in the bus-off state, the TWAI controller is unable to take part in any bus activities. To exit the bus-off state, the TWAI controller must undergo the bus recovery process.
**Recovering**: The recovering state is entered when the TWAI controller undergoes bus recovery. The TWAI controller/TWAI driver remains in the recovering state until 128 occurrences of 11 consecutive recessive bits are observed on the bus.
The TWAI driver distinguishes different types of messages by using the various bit field members of the :cpp:type:`twai_message_t` structure. These bit field members determine whether a message is in standard or extended format, a remote frame, and the type of transmission to use when transmitting such a message.
- Transmit message using Single Shot Transmission, i.e., message will not be retransmitted upon error or loss of arbitration (Unused for the received message)
The following code snippet demonstrates how to configure, install, and start the TWAI driver via the use of the various configuration structures, macro initializers, the :cpp:func:`twai_driver_install` function, and the :cpp:func:`twai_start` function.
The following code snippet demonstrates how to transmit a message via the usage of the :cpp:type:`twai_message_t` type and :cpp:func:`twai_transmit` function.
if (twai_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:`twai_message_t` type and :cpp:func:`twai_receive` function.
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 TWAI driver alerts via the use of the :cpp:func:`twai_reconfigure_alerts` and :cpp:func:`twai_read_alerts` functions.
The following code demonstrates how to stop and uninstall the TWAI driver via the use of the :cpp:func:`twai_stop` and :cpp:func:`twai_driver_uninstall` functions.
The acceptance mask in :cpp:type:`twai_filter_config_t` can be configured such that two or more IDs are 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.
**Network Example:** The TWAI Network example demonstrates communication between two {IDF_TARGET_NAME}s using the TWAI driver API. One TWAI node acts as a network master that initiates and ceases the transfer of data from another node acting as a network slave. The example can be found via :example:`peripherals/twai/twai_network`.
**Alert and Recovery Example:** This example demonstrates how to use the TWAI driver's alert and bus-off recovery API. The example purposely introduces errors on the bus to put the TWAI 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/twai/twai_alert_and_recovery`.
**Self-Test Example:** This example uses the No Acknowledge Mode and Self Reception Request to cause the TWAI controller to send and simultaneously receive a series of messages. This example can be used to verify if the connections between the TWAI controller and the external transceiver are working correctly. The example can be found via :example:`peripherals/twai/twai_self_test`.
