4.30.7. MAX1785x

This module supports the MAX17852 and MAX17853 monitoring ICs by Maxim Integrated. Communication with these ICs is handled through a MAX17841B bridge IC. The communication between bridge IC and monitoring IC is encoded as Maxim Battery Management UART, which is a optimized, differential variant of UART with a focus on robustness.

A typical setup consists of one master unit with one to two bridge ICs and several monitoring ICs in a daisy-chain. The current implementation of the driver supports communication through one bridge IC (the “uphost path”), but the hardware supports a redundant approach by adding read only communication on the “downhost path”.

4.30.7.1. Driver structure

The driver is separated into three main modules with additional helper modules. The three main modules are:

  • MAX17841B bridge IC driver that interacts with the interface from Interface MAX17841B v1.0.0 in mxm_17841b.c,

  • Battery Management UART protocol abstraction in max_battery_management.c and

  • MAX1785x device driver in mxm_1785x.c.

  • Implementation of the operation state machine in mxm_17852.c. This implementation is selected by the driver variant in bms.json.

4.30.7.2. Configuration

The driver for the Maxim Integrated monitoring ICs is configurable for some aspects.

4.30.7.2.1. Adaption to new hardware

In order to adapt the driver to new interface hardware, it is necessary to adapt the following:

  • Ensure that the functions that interact with the shutdown pin of the Maxim MAX17841B bridge IC are correct. They are implemented in mxm_cfg.c.

  • Check the SPI configuration of the system and that it aligns with the capabilities of the bridge IC.

  • The driver is intended to be implemented as generic as possible. That means that the main modules should not be necessary to be changed. In order to implement new behavior during the measurement cycle the implementation of the MXM_StateMachineOperation has to be adapted. This can be achieved by implementing a new variant in parallel to the current max17852 variant.

4.30.7.2.2. Startup behavior

During enumeration of the daisy-chain, the monitoring driver can stop and hang if the number of found devices does not match the expected number of devices as reported by the system configuration. In order to select whether the driver should hang or continue, the constant mxm_allowSkippingPostInitSelfCheck can be set.

4.30.7.3. Main flow

The following flow chart describes the main flow of this AFE driver.

  • State Uninitialized: This is the default state of the driver. The driver transitions with the next execution into the Pre-Initialization Self Check state.

  • State Pre-Initialization Self Check: In this state, the driver checks assumptions on its functions. If this step fails, the driver is not able to conclude its work.

  • State Initialization: In this state, the daisy-chain of the driver is initialized and each AFE is assigned an address.

  • State Post-Initialization Self Check: During this state, the number of found AFE is compared to the expected number of devices. If these mismatch the driver can halt in this state depending on the configuration described in Startup behavior.

  • State Idle: The driver rests in this state as long as the system requests the driver to start.

  • State Operation: This is the main state during which the AFE devices are controlled.

    • State Initialization of AFE: In this chain of substates the AFE is initialized and programmed with the expected configuration.

    • State Multiplexer control: In this chain of substates the temperature multiplexer of the BMS-Slave is controlled and switched to the next channel.

    • State Measurement: In this chain of substates the measurement the driver requests a measurement and retrieves each measurement value.

    • State Diagnostic measures: In this chain of substates diagnostic registers are read. When a power on reset condition is recognized (indicating a loss of power and potentially undefined state in one of the AFE devices), a reset of the driver is triggered.

    • State Balancing control: In this chain of substates the balancing switches are controlled. The driver alternates between even and odd switches in order to not create a connection between adjacent switches.

  • If any state except Idle or Uninitialized is stuck and does not advance to the next state during a predefined time frame, the driver resets itself to the Uninitialized state. Requests for operation are preserved during this reset as the system assumes that the driver has been already asked to start.

4.30.7.4. Diagnostic signals generated

This driver uses the following diagnostic messages of the diag-module:

  • DIAG_ID_AFE_CONFIG: This signal is issued when the self check of the driver that is executed before the initialization fails. If this happens, it is very likely due to an implementation error.

  • DIAG_ID_AFE_COMMUNICATION_INTEGRITY: This signal is issued when the CRC of a received message is not correct.

  • DIAG_ID_AFE_SPI: This signal is issued when the SPI API returns an unsuccessful transmission. If this happens, it is very likely due to implementation issues with the SPI interface. Normally, the SPI transmission functions should succeed.

4.30.7.5. Trivia

This section gathers additional information that is worth knowing about the implementation.

4.30.7.5.1. String mapping

The driver assumes, that it has to service all strings in the system and that all strings are connected into one daisy-chain (It expects a chain of strings of modules).

If only one string is defined, the driver maps the modules onto this string as expected. If more than one string is defined, the driver maps the modules by filling the first string, then filling the second and so forth. This behavior is not configurable.

4.30.7.5.2. Automatic reset

When a communication issue occurs, the slave boards might get stuck in a state that is unknown to the foxBMS BMS-Master. In order to prevent this situation, the driver has an error counter. This error counter is automatically reset after MXM_TIMEOUT_RESET_ERROR_COUNTER_ms milliseconds without any new error. If the error threshold of MXM_MAXIMUM_ERROR_COUNT is passed, the driver starts an automatic reset of the whole daisy-chain.

This is achieved by pulling down the shutdown line of the bridge IC, effectively resetting it to default condition. During this shutdown, the BMS-Slave will go into deep sleep and thus loose their non-persistent configuration (and also disable the on-board supply). The driver resets itself to initial condition restores flags indicating whether it may start and then restart the daisy-chain. The user can register this by checking for the freshness of measurement values.