Introduction to RTOS for ML

Pascal Bodin

Document history

Licenses

Presentation

Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Source code

GNU General Public License v3

Credits

• reveal.js - Copyright (C) 2020 Hakim El Hattab
• Freepik from www.flaticon.com
• Electricity icons created by Dewi Sari - Flaticon

How to navigate

• Use the right and left arrows (in bottom right corner)
• Overview: press "O" letter - navigate with arrow keys - "ESC" or click on slide to exit
• Full screen: press "F" letter - "ESC" to exit

Contents - 1/3

Contents - 2/3

Contents - 3/3

Foreword

Who am I?

• Independent consultant and freelancer - connected devices
       
• Beforehand: software engineer, project leader, team manager, co-founder, technical expert
• Digital Equipement Corporation (DEC), McDonnell Douglas Informations Systems, Orange
• Co-founded (and closed) two companies
• First connected devices project: in 1990

More information

• Systev
• LinkedIn
• GitHub
• Matomo
• X

Goal

• Understand how to develop, run and debug embedded applications
• Understand the benefits (and drawbacks) of Real-Time Operating Systems
• Understand how to get data from sensors and how to store it

Needs of ML applications

Technical domains

Closer look - Computer Science

Closer look - Communication

Closer look - Electronics

Introduction

Hardware progress

1982 - Cray X-MP

• World most powerful computer
• 0.94 GFLOPS (giga floating-point operations per second)
• Price: around US$15 million (would be US$45 million in 2022)

2022 - iPhone 14

• A15 Bionic chip with 5-core GPU: 1.7 TFLOPS
• Price: US$800

In 40 years:

• Processing power multiplied by around 1,700
• Cost divided by around 56,000

What do the years to come have in store?!

Embedded applications

1969 - The computer that made it possible to land on the Moon

• Weight: aroung 32 kg (without the user interface)
• Power supply: 28 V CC - 70 W
• Permanent memory: 36 Kwords
• Erasable memory: 2 Kwords
• Clock: 1 MHz (83 kHz instruction cycle time)
• Simple operating system, with priority handling

Compared to current configurations:

• Very little memory
• Very little processing power

Yet it allowed to bring human beings to the Moon.

It's not because you don't have a lot of memory and processing power that you can't develop great applications 🙂

A modern microcontroller with similar processing power:

• Flash memory: 128 Ko
• RAM: 8 Ko
• Clock: up to 64 MHz
• Power consumption: 30 mW (microcontroller alone)
• Power consumption in sleep mode: 0,1 μW
• Price: around US$3.00

Microcontroller board

Reminders - Memory content retention

• Volatile memory: content is lost when the system turns off - RAM
• Non-volatile memory: content is retained - Flash, EEPROM

Reminders - Memory hierarchy

From most rapid access to slowest access:

• Registers - inside the CPU
• Cache - inside and outside the CPU - usually not present in MCU
• Main memory
• Disk - magnetic or flash

• Registers: RAM
• Cache: RAM
• Main memory: RAM + flash

Microcontroller

• A computer in a chip:
  • Central Processing Unit (CPU)
  • Read-only memory (Flash memory)
  • Read/write memory (RAM - Random Access Memory)
  • Clocks
  • Energy management
  • Peripherals and interfaces:
    • Analog I/O
    • Digital I/O
    • Timers
    • Serial interfaces
    • etc.

An example of MCU architecture:

Board architecture

Important board/microcontroller characteristics?

• Depend on the target application!
• Example:
  • General Purpose Input/Output (GPIO)
  • Serial links
  • Serial buses (SPI, I2C, etc.)
  • Analog to Digital Converters (ADC), Digital to Analog Converters (DAC)
  • Analog blocks
  • Instruction cycle time
  • Memory (size, type, expandable...)
  • Registers size (8/16/32 bits)
  • Packaging
  • etc.

Other important characteristics: hardware tools

• Development board
• Programmer, debugger
• Open source hardware

Other important characteristics: software tools

• Cross-compilation toolchain
• Integrated Development Environment (IDE)
• Open source software

Other important characteristics: support

• Professional support
• Active community (forums, examples...)

Some common microcontroller families

Arm

• UK company created in 1990
• There isn't any Arm microcontroller
• Arm provides Intellectual Property (IP) blocks, under a licensing agreement
• Among these blocks: RISC microcontrollers cores
• For the IoT (low power, low cost): Cortex-M family

Shipped processors

• end 2020: 180 billions
• including 6.7 billions in Q4 2020
• including 4.4 billions of Cortex-M in Q4 2020

• Sep-2022: more than 250 billions

Arm processor architecture is more popular than any other architecture.

Some Cortex-M licensees:

• Infineon
• Microchip
• Nordic Semiconductor
• NXP Semiconductors
• Renesas
• Silicon Labs
• STMicroelectronics
• Texas Instruments
• etc.

Arm cores

Instruction sets

Provided elements

Example: STMicroelectronics - STM32 family

STM32L073RZ

• Price: US$6.95 (unit) - US$2.66 (10,000)

Development board

Price: US$13

STM32WL55

• Price: US$9.25 (unit) - US$4.54 (10,000)

Development board

US$42

Example: Silicon Labs - EFR32 family

• Bluetooth: EFR32BG21, EFR32BG22, EFR32BG24, EFR32BG26, EFR32BG27
• Wi-Fi: SiWx915, SiWx917, RS9116, WF200
• Thread: EFR32MG12, EFR32MG13, EFR32MG21, EFR32MG24
• Etc.

EFR32MG24

• Price: around US$7 (unit)

Development board

US$79

Espressif - ESP family

Espressif family

• ESP32-P Series - high-power RISC-V dual-core + low-power single-core
• ESP32-S Series - Xtensa LX7 dual-cor or single-core - Wi-Fi, BLE
• ESP32-C Series - RISC-V dual-core or single-core - Wi-Fi (2.4 GHz / 5 GHz), BLE, IEEE 802.15.4
• ESP32-H Series - RISC-V single-core
• ESP32 Series - Xtensa LX6 dual-core or single-core - Wi-Fi, Bluetooth, BLE

ESP32-C6FH4

Some of the features

• High-power RISC-V processor - clock up to 160 MHz
• Low-power RISC-V processor - clock up to 20 MHz
• L1 cache: 32 KB
• ROM: 320 KB
• Hig-power SRAM: 512 KB
• Low-power SRAM: 16 KB
• Flash: 4 MB

• Price: US$2.05

Development board

• Price: US$7.96

Software development - part 1

Cross development

Reminders

Usual development

Then you run the executable file.

Toolchain: compiler + linker + other utilities

Cross development for microcontroller board

Transferring the executable file to the microcontroller board can be done in several ways:

• Over a debug interface
• Over a serial link while the microcontroller runs a bootloader
• Over a wireless connection while the microcontroller runs a bootloader
• Etc.

Debugging

• The development computer can control the execution of the application running in the microcontroller
• Requires a debug interface on the microcontroller board and a serial link

• Every microcontroller manufacturer: full development environnement
• Often free
• Often based on Open Source software (Eclipse, gcc, openOCD, etc.)

Exercise 01

Build and run an Hello World application for the EFR32xG24 Dev Kit

EFR32MG24

EF32xG24 Dev Kit

Reference documentation

• Microcontroller:
  • EF32MG24 Wireless SoC Family Data Sheet (132 pages)
  • EFR32xG24 Wireless SoC Reference Manual (1152 pages)
• Board:
  • xG24 Dev Kit User's Guide
  • Schematic - Revision A01
• Sofware:
  • Developer Documentation:
    • Simplicity Studio 5 User's Guide
    • Platform (SDK)

Development environments

Several possibilities:

• Simplicity Studio v5 (SSv5) - based upon Eclipse IDE
• VS Code + extension
• ...

We will use SSv5.

SSv5

Exercise 01

• Using git, clone the RTOS-presentation GitHub repository
• Follow the instructions provided by the RTOS-presentation/exercises/01-HelloWorld/README.md file.

Peripherals

Peripheral: a piece of equipment that can be connected to the microcontroller or to the microcontroller board.

Sensors

• Pressure
• Temperature
• Light level
• Magnetic field
• Gas flow
• Tilt
• Acceleration
• Contact
• etc.

Actuators

• Relay
• Motor
• Stepper motor
• Servomotor
• etc.

Other peripherals

• Printer
• Display
• OBD connector (On-Board Diagnostics)
• RFID tag reader
• Computer (!)
• etc.

Interfaces

Interface: a means (hardware and software) of connecting a peripheral to the microcontroller or to the microcontroller board.

GPIO (General Purpose digital Input Output)

• Microcontroller pin which can handle a digital value: 0 or 1
• Signal voltage for bit value 0 is 0 V
• Signal voltage for bit value 1 depends on microcontroller. Usually: supply voltage (e.g. 3.3 V)
• Can be configured either as an input or as an output (with different modes)

• On a microcontroller: usually several "ports" of GPIOs
• Ports are labelled, by a letter or by a number
• In a given port, each GPIO is numbered

For instance: PA00 to PA07, PB00 to PB03, PC00 to PC09 and PD00 to PD05.

GPIO as an output

• When PD02 is at 1 (i.e. VMCU), the LED is off, as no current can flow through it
• When PD02 is at 0 (i.e. 0 V), the LED is on, as some current can flow through it

Easy, isn't it? 🙂

Physical world has some constraints - 1/2

• Current through the LED should not be greater than a value depending on the LED (for instance: 1.5 mA)
• A resistor in series with the LED can limit the current. Resistor value is given by Ohm's law: R137 = VMCU / Imax
• We have VMCU = 3.3 V and decide on Imax = 1.5 mA
• ⇒ R137 = 2.2 kΩ

Physical world has some constraints - 2/2

The LED current flows into the microcontroller. A GPIO can't accept more than a given value (provided by the microcontroller data sheet).

For the EFR32MG24, maximum value is 50 mA. As R137 limits the current to 1.5 mA, we are OK.

Another limit: total maximum value for all GPIO pins. For the EFR32MG24: 200 mA.

Note: the GPIO is configured as an output, but the current flows INTO the microcontroller.

GPIO as an input - 1/5

• When the switch is closed, PB02 is set to 0 (GND = 0 V)
• When the switch is open, PB02 is not set. It is "floating"

How to set PB02 to 1 when the switch is open?

GPIO as an input - 2/5

• R140 ensures that PB02 is set to 1 when the switch is open
• It is named pullup resistor
• The resistor value is large enough to get a low current through it (but beware: it is not null)
• If the switch was connected to VMCU instead of GND, we would use a pulldown resistor

GPIO as an input - 3/5

• The board schematic shows an additional resistor, R141. What is its purpose?
• If the GPIO was configured as an output (bug), set to 1, and the switch was closed: short-circuit
• R141 limits the current in such a case

GPIO as an input - 4/5

• The board schematic shows a capacitor, C142. What is its purpose?

What really happens when closing a switch

Vertical: 1 V / division - Horizontal: 1 ms / division

GPIO as an input - 5/5

• C142 "fills in" the gaps / filters out the highest frequencies
• ⇒ debouncing
• Debouncing can also be done in software

Voltage and current adaptation

If peripheral voltage is different from microcontroller voltage, or if it requires more current than what the microcontroller can provide, or if regulations require some specific protections, adaptation is required:

• Optocoupler
• Relay
• Etc.

GPIO and ML application

• Start/stop a task on a contact closure:
  • Start processing a video stream when a door opens
  • Start converting and processing analog data when an engine starts
  • Etc.
• Activate a peripheral:
  • Start an alarm siren when an unknown person enters a room
  • Stop a conveyor belt when a problem is detected
  • Etc.

Exercise 02

Display a message when a button is pushed or released

• Follow the instructions provided by the RTOS-presentation/exercises/02-GpioInput/README.md file.

Exercise 03

Make a LED blink

• Follow the instructions provided by the RTOS-presentation/exercises/03-GpioOutput/README.md file.

ADC (Analog to Digital Converter)

• In the physical world: an analog signal is continuous in time and in amplitude
• In a computer: data is discrete in time and in amplitude
• ADC: transforms an analog signal into data which is used by the computer to create its perception of the physical world

Quantization: maps a continuous set of values to a finite set of values

• With 4 bits: 16 values
• With 12 bits: 4096 values
• With 15 bits: 32,768 values
• With 24 bits: 16,777,216 values
• With 32 bits: around 4.3 billions of values

Whatever the number of bits, there are always continuous values which are not part of the finite set of values. Quantization error: difference between the continuous value and the quantized value.

Resolution: number of bits.

Sampling: periodically quantizing the continuous value

Sample rate: sampling frequency

Nyquist–Shannon sampling theorem: the sample rate must be at least twice the bandwidth of the signal.

• Sufficient condition to capture all the information from a continuous-time signal of finite bandwidth

Some sampling rates

• Legacy analog telephone: 8 kHz (good human sounding voice: 300 - 3400 Hz)
• Audio CD: 44.1 kHz

Usually: samples/s - sps, ksps, Msps

Reference voltage

The ADC needs a reference voltage to which it compares the incoming analog voltage.

The ADC cannot measure voltages larger than the reference voltage.

Depending on the microcontroller, the reference voltage may be internal or external. Several references may be available.

Input selection

For some microcontrollers: the ADC block can support measurement on a number of internal and external signals.

Voltage adaptation

Depending on the signal to be converted, amplification or attenuation can be required.

The microcontroller can provide some adaptation means.

ADC and ML application

• Check battery voltage, and reduce the number of tasks when too low
• Convert microphone signal, to perform wake word detection
• Convert pressure sensor signal, to perform local weather forecast
• Etc.

Exercise 04

Convert an analog signal

• Follow the instructions provided by the RTOS-presentation/exercises/04-Adc/README.md file.