Project repo: https://github.com/UWARG/efs-can-power-module

Specifications (from the EE Team)

Overall Hardware Architecture (taken from https://uwarg-docs.atlassian.net/wiki/x/AoCsmQ )

Background

Each group of 6 cells is connected to a battery monitoring circuit (shortened as BMC later).

This circuit measures:

• The voltage of each cell

• The current delivered by the group

  • This is done by forcing the battery to deliver current through a resistor with a tiny resistance. This resistor, called a shunt/shunt resistor, follows Ohm’s law: V=IR

  • This implies that I=V/R, which means that the current passing through a shunt is equal to its voltage (which can be measured easily), divided by its resistance (which is a known constant).

And it sends:

• Battery-related information relating to the voltages and currents

• The voltage and current information are sent as 2 analog signals, which our microcontroller converts to digital ones via an Analog-to-Digital Converter (ADC).

• Some other information is sent via the I²C protocol

Description

Specific Hardware Information (taken from https://uwarg-docs.atlassian.net/wiki/x/HYBXq)

Image Modified

\uD83E\uDD14 Current Roadblocks

Tasks Break Down:

✅  Action Items

• Initialize the Battery Monitoring Circuit using I²C
•  Read battery data using the specified ports

\uD83D\uDEA9 Milestones

•  Understand project specifications
•  Understand the I²C protocol
•  Understand the CAN protocol
•  Understand DroneCAN
•  etc.

Open Questions

• How should the MCU beacon itself to the FC (i.e. so that the FC is aware that the MCU on the power module is there)?

• What should be sent over CAN? Possible options:

  • uavcan.equipment.power.BatteryInfo (ID:1092)

  • ardupilot.equipment.power.BatteryInfoAux (ID:20004)

  • uavcan.equipment.power.CircuitStatus (ID:1091) (less interested in)

---

\uD83D\uDD17 Reference materials

Relevant protocols

• I²C: I2C - Inter-Integrated Circuit

  • I²C is all about registers (i.e. memory that can be accessed by an address)

  • Each I²C peripheral has its own register map (i.e. what the registers are used for, such as storing the voltage of a cell)

  • To read data from a peripheral, you send an address to it, and it will respond with data

  • To write data to a peripheral, you tell it which address to write to, and what data to write

• CAN:

  • A general description of CAN: https://uwarg-docs.atlassian.net/wiki/x/BgDPlw

  • DroneCAN: a protocol built on top of CAN to communicate information between drone parts and the FC: https://dronecan.github.io/

    • Libcanard: an implementation of DroneCAN for low-end microcontrollers. An example code on how to use it: https://github.com/dronecan/libcanard/blob/master/examples/SimpleNode/simple_node.c

      • EFS-Canard: a portable WARG version of the Libcanard library

    • AP-Periph has some example code on how to send DroneCAN messages when you have a specific type of device (e.g. a battery)

  • A helpful presentation on CAN: https://docs.google.com/presentation/d/12i6TiUuXQw5mO2u3pq8DMXTD1bCRe8rJtzVZRk6pDz0/edit?usp=sharing

  • A helpful document on CAN applications: https://uwarg-docs.atlassian.net/wiki/x/AwA8q

Project Resources

Battery Monitoring Circuit

Datasheet: https://www.ti.com/product/BQ76925?qgpn=bq76925#software-development

• Addressing: all I²C addresses are calculated using the formula

  Code Block
  language c
  ADDRESS = 0x20 + REG_ADDR

Where ADDRESS is the real address to read/write to, and REG_ADDR is the register address mentioned on the datasheet, from 0x00 to 0x1F.
Remember this distinction between the real and register addresses. All I²C addresses you see after this point will be register addresses.

Source: Datasheet, 8.5.1.1 I²C Addressing

• Selecting the cell for the VCOUT pin:
  To select the cell to measure the voltage of:

  Code Block
  language c
  // Write to the register address: REG_ADDR = 0x01 // With data: DATA = 0x10 + CELL

Where CELL is the cell number. CELL=0x0 refers to cell 1, CELL=0x1 refers to cell 2, and so on, up to CELL=0x5referring to cell 6.

Source: Datasheet, 8.6.1 Register Descriptions (Tables 6 to 8)

• Reference voltage selection:
  

  Code Block
  language c
  // Write to the register address: REG_ADDR = 0x04 // With data: DATA = REF_SEL

Where REF_SEL can be set to 0 or 1, with the following effects on the BMC pins:

Table 14. Reference Voltage Selection

Source: Datasheet, 8.6.1 Register Descriptions (Tables 13 and 14)

• Cell Voltage Monitoring
  

  Code Block
  language c
  // The voltage of the currently selected cell, VCn, // is calculated with the following formula: VCn = ((VCOUT * GC_VREF + OC_VCOUT) / G_VCOUT) * (1 + GC_VCOUT) // Where: VCOUT = ADC_count / Full_scale_count * VREF_NOMINAL // This is the voltage of the VCOUT pin, as measured by the MCU GC_VCOUT = ((VCn_GC_4 << 4) + VCn_GAIN_CORR) * 1E-3 OC_VCOUT = ((VCn_OC_4 << 4) + VCn_OFFSET_CORR) * 1E-3 GC_VREF = (1 + ((VREF_GC_4 << 4) + VREF_GAIN_CORR) * 1E-3) + (((VREF_OC_5 << 5) + (VREF_OC_4 << 4) + VREF_OFFSET_CORR) * 1E-3)/VREF_NOMINAL

Several values can be accessed through the I²C interface:

Source: Datasheet, 8.3.2.2 Cell Voltage Monitoring

Up-to-date electrical schematics from the EE team

Project 6s Power Module (Ask the EE team for access)

Receiving incoming messages (Polling/Interrupt/DMA)

We are going to receive messages from an external device (the BMC), but we can’t predict when we will receive them. There are 3 solutions to this:

• Polling: repeatedly checking whether a message was received using a loop. If a message gets detected, it gets processed, then the loop continues. This is a purely software solution.

  • Advantages:

    • Very easy to implement

    • You don’t need to worry about race conditions (when multiple pieces of code unintentionally access and modify some variable at the same time)

  • Disadvantages:

    • Extremely inefficient. The loop eats up most of the microcontroller’s computational resources, which it needs for many other tasks.

• Interrupt: when a message gets received, it triggers a hardware interrupt. This causes all other processes to stop. Then the message gets processed, and the other processes resume after that. STM32 microcontrollers support many internal interrupts (for common events like receiving an I²C message), and external interrupts (triggered by a GPIO pin).

  • Advantages:

    • More efficient than polling

  • Disadvantages:

    • You need to be careful about race conditions

    • Still needs the microcontroller for data reception

• Direct Memory Access (DMA): Similar to an interrupt, except the peripheral writes its message directly to the microcontroller’s memory. Then it signals the microcontroller that a new message came.

  • Advantages:

    • Most efficient for large messages

  • Disadvantages:

    • You need to be careful about race conditions

    • Less efficient than regular interrupts for small messages