Project repo: GitHub - UWARG/efs-can-power-module: WARG 6s power module firmware with DroneCAN interface supported

Table of Contents

1 Specifications (from the EE Team)
- 1.1 Overall Hardware Architecture
  - 1.1.1 Background
  - 1.1.2 Description
- 1.2 Specific Hardware Information
- 1.3 Current Roadblocks
2 Tasks Break Down:
- 2.1 Action Items
- 2.2 Milestones
3 Open Questions
4 Reference materials
- 4.1 Relevant protocols
- 4.2 Project Resources
  - 4.2.1 Analog-to-Digital Converter (ADC) Explained
    - 4.2.1.1 Reference Voltage
    - 4.2.1.2 Counting and Resolution
  - 4.2.2 Battery Monitoring Circuit
    - 4.2.2.1 Addressing
    - 4.2.2.2 Selecting the cell for the VCOUT pin
    - 4.2.2.3 Reference voltage selection
    - 4.2.2.4 Cell Voltage Monitoring
    - 4.2.2.5 BMC Temperature monitoring
  - 4.2.3 Up-to-date electrical schematics from the EE team
- 4.3 Receiving incoming messages (Polling/Interrupt/DMA)

Specifications (from the EE Team)

Overall Hardware Architecture

Taken from https://uwarg-docs.atlassian.net/wiki/x/AoCsmQ

Background

The drone uses a battery pack containing 12 cells, where 2 groups of 6 cells are each connected in series (hence 6S).

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:

We are not exactly sure of everything yet.

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

MCU (STM32xX) in the top right is the microcontroller that we are writing the code to
Its job is to translate the messages it receives from the BQ76925 6s LiPo Battery Monitor boards and send the corresponding messages to the flight controller (FC) using the CAN protocol
- The flight controller we are using is a Pixhawk 4, which runs the Ardupilot firmware
- Because battery voltages and currents will change over time, the “translation” is continuous

The FC needs this information to make other decisions, e.g. if a part is consuming too much power, or when to land

Specific Hardware Information

Taken from https://uwarg-docs.atlassian.net/wiki/x/HYBXq

MCU Schematics

The ports we are interested in:

PA9: SCL_1, Serial Clock line for I²C 1, to configure BMC 1
PA10: SDA_1, Serial Data line for I²C 1, to configure BMC 1
PA7: SCL_2, Serial Clock line for I²C 2, to configure BMC 2
PB4: SDA_2, Serial Data line for I²C 2, to configure BMC 2
PA12: CAN_TX, Transmit line for CAN
PA11: CAN_RX, Receive line for CAN
PA3: OVERCURRENT, receives 5.5 V when an overcurrent occurs, and 0 V otherwise
PA4: VIOUT_FILT, receives 0.25-1.25 V (REF_SEL=0) / 0.5-2.5 V (REF_SEL=1), representing the cell current for cells 1-6, as measured by a 1-milliohm shunt
PA5: VCOUT1_FILT, receives 1.47-1.53 V (REF_SEL=0) / 2.94-3.06 V (REF_SEL=1), representing the cell voltages for cells 1-6
PB0: ADC1_FILT, receives VREF, the ADC reference voltage
PB1: VCOUT2_FILT, receives 1.47-1.53 V (REF_SEL=0) / 2.94-3.06 V (REF_SEL=1), representing the cell voltages for cells 7-12
PA2: ADC2_FILT, receives VREF, the ADC reference voltage

Current Roadblocks

Libcanard has a high learning curve. Neither the DroneCAN website nor the Libcanard repo has beginner-friendly tutorials. YouTube videos also won’t help directly

Tasks Break Down:

Action Items

Initialize the Battery Monitoring Circuit using I²C

Read battery data using the specified ports

Fix suspected I²C timing issue during communication to the BMC

Milestones

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:

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

Analog-to-Digital Converter (ADC) Explained

There are many physical quantitles in the world that we may want to measure, such as voltage, current, or temperature. These are analog quantities, which means that can change continuously and have infinitely many values (*).

Technically, quantum mechanics tells us that some quantities we think of as continuous are actually discrete, with extremely tiny but finitely tiny steps, called “quanta“. In the context of WARG, these effects are negligible.

In WARG EFS, we work with microcontrollers quite regularly. They use digital signals to work, which means it works with a finite set of numbers; in particular, they use something like 0 V to represent a logic ‘0', and 5 V to represent a logic ‘1’. By doing math with these ‘0’ and '1’ signals, we can program the microcontrollers to do anything we want.

An Analog-to-Digital Converter (ADC)'s job is to connect between these two worlds.

Specifically, it converts an analog voltage into a digital signal that the microcontroller can understand.

Reference Voltage

There’s just one issue: voltages have a basically infinite range, but our ADC has a finite range. Therefore, we have to tell the ADC what voltage range it should expect. By convention, the lowest voltage an ADC can understand is usually 0 V.

On the other hand, you can specify the highest voltage it should expect.

This is the so-called ADC reference voltage.

Some ADCs can measure both positive and negative voltages. In these boards, it may require multiple reference voltages, or it may set the lower range to be a function of the higher range.

Counting and Resolution

Now our ADC is ready to do its job. To make its output easy to understand, the output of an ADC is directly proportional to its voltage input.

For example, an ADC may be told to sense an input voltage from 0-8 V into a natural number from 0 to 31. This would mean that an input of 4.25 V will correspond to a number of around 16.

To put this into jargon, since there are 32 numbers between 0 and 31, and 32 is 2 to the 5th power, we say that the ADC’s resolution is 5 bits, and it counted a value of 16.

An ADC’s resolution is the number of bits that are used to represent its outputs,
and its count is the number it’s currently giving.

Battery Monitoring Circuit

Datasheet: BQ76925

Addressing

All I²C addresses are calculated using the formula

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:

// 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

// 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

REF_SEL	VREF (V)	VCOUT Gain (V)	VIOUT Voltage Range (V)

REF_SEL	VREF (V)	VCOUT Gain (V)	VIOUT Voltage Range (V)
0	1.5	0.3	0.25 - 1.25
1	3.0	0.6	0.5 - 2.5

Yes, it’s true that you could also enable CRC with this memory address. To do that, add 0x80 to DATA above.

We haven’t decided whether we want CRC though, so right now we’ll use the default option… which is “disabled“.

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

Cell Voltage Monitoring

// 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:

VC1_GC_4
- I²C register address: 0x17
- Data bit location: 6
VC2_GC_4
- I²C register address: 0x17
- Data bit location: 4
VC3_GC_4
- I²C register address: 0x18
- Data bit location: 6
VC4_GC_4
- I²C register address: 0x18
- Data bit location: 4
VC5_GC_4
- I²C register address: 0x18
- Data bit location: 2
VC6_GC_4
- I²C register address: 0x18
- Data bit location: 0

VC1_OC_4
- I²C register address: 0x17
- Data bit location: 7
VC2_OC_4
- I²C register address: 0x17
- Data bit location: 5
VC3_OC_4
- I²C register address: 0x18
- Data bit location: 7
VC4_OC_4
- I²C register address: 0x18
- Data bit location: 5
VC5_OC_4
- I²C register address: 0x18
- Data bit location: 3
VC6_OC_4
- I²C register address: 0x18
- Data bit location: 1

VC1_OFFSET_CORR

I²C register address: 0x11
Data bit locations: 7 to 4
VC2_OFFSET_CORR
- I²C register address: 0x12
- Data bit locations: 7 to 4
VC3_OFFSET_CORR
- I²C register address: 0x13
- Data bit locations: 7 to 4
VC4_OFFSET_CORR
- I²C register address: 0x14
- Data bit locations: 7 to 4
VC5_OFFSET_CORR
- I²C register address: 0x15
- Data bit locations: 7 to 4
VC6_OFFSET_CORR
- I²C register address: 0x16
- Data bit locations: 7 to 4

VC1_GAIN_CORR
- I²C register address: 0x11
- Data bit locations: 3 to 0
VC2_GAIN_CORR
- I²C register address: 0x12
- Data bit locations: 3 to 0
VC3_GAIN_CORR
- I²C register address: 0x13
- Data bit locations: 3 to 0
VC4_GAIN_CORR
- I²C register address: 0x14
- Data bit locations: 3 to 0
VC5_GAIN_CORR
- I²C register address: 0x15
- Data bit locations: 3 to 0
VC6_GAIN_CORR
- I²C register address: 0x16
- Data bit locations: 3 to 0

VREF_OC_5
- I²C register address: 0x1b
- Data bit location: 2
VREF_OC_4
- I²C register address: 0x1b
- Data bit location: 1
VREF_GC_4
- I²C register address: 0x1b
- Data bit location: 0

Source: Datasheet, 8.3.2.2 Cell Voltage Monitoring

BMC Temperature monitoring

The internal temperature of the battery management circuit can be monitored. To do that, configure the VCOUT pin to output that temperature:

// Write to the register address:
REG_ADDR = 0x01

// With data:
DATA = 0x16

Then, the temperature can be deduced as follows:

// TEMP: temperature (degrees C)
// V: voltage measured at the VCOUT pin (mV)

TEMP = 5/22 * (V - 1090)

Note that TEMP has an uncertainty of plus/minus 7/22 ≈ 0.227 °C.

Also note that TEMP will be between -25 to 85 °C in normal operating conditions, which corresponds to 980 to 1464 mV.

These are visualized in the Desmos graph below.

Not with the board that we have now. Monitoring that requires connecting the BMC to an external thermister network, which the EE team didn’t include.

Sources: Datasheet, 8.3.2.5.1 Internal Temperature Monitoring;
Datasheet, 7.12 Internal Temperature Measurement

Up-to-date electrical schematics from the EE team

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

Oftentimes you’ll want to connect to the board to download a program. Here’s how you’ll find how to wire your connector, in case you forgot:

How to find programmer pins on the Altium design file

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

Embedded Flight Software

6S Power Module

Specifications (from the EE Team)

Overall Hardware Architecture

Background

Description

Specific Hardware Information

Current Roadblocks

Tasks Break Down:

Action Items

Milestones

Open Questions

Reference materials

Relevant protocols

Project Resources

Analog-to-Digital Converter (ADC) Explained

Reference Voltage

Counting and Resolution

Battery Monitoring Circuit

Addressing

Selecting the cell for the `VCOUT` pin

Reference voltage selection

Cell Voltage Monitoring

BMC Temperature monitoring

Up-to-date electrical schematics from the EE team

Receiving incoming messages (Polling/Interrupt/DMA)

Related content

6S Power Module

Specifications (from the EE Team)

Overall Hardware Architecture

Background

Description

Specific Hardware Information

Current Roadblocks

Tasks Break Down:

Action Items

Milestones

Open Questions

Reference materials

Relevant protocols

Project Resources

Analog-to-Digital Converter (ADC) Explained

Reference Voltage

Counting and Resolution

Battery Monitoring Circuit

Addressing

Selecting the cell for the VCOUT pin

Reference voltage selection

Cell Voltage Monitoring

BMC Temperature monitoring

Up-to-date electrical schematics from the EE team

Receiving incoming messages (Polling/Interrupt/DMA)

Related content

Selecting the cell for the `VCOUT` pin