Embedded systems for ML

Interfaces - part 2

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 (a sequence of) data

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.

Practice session 04

Convert an analog signal

Follow the instructions provided by the embedded-systems-for-ML/practice-sessions/04-Adc/README.md file.

UART (Universal Asynchronous Receiver/Transmitter)

GPIO: set/get 0 or 1
ADC: get the approximate value of a signal
UART: exchange data

Universal: can be used in many different contexts
Asynchronous: no common clock required between the receiver and the transmitter

Data is transmitted bit by bit.

Physical layer

At least 3 wires: Transmit, Receive, Ground
Additional wires possible: Request to send, Ready for sending, etc.
Wires definition: V.24 (TIA-232 in the USA)

Electrical levels:

V.28: -15 V to -3 V and +3 V to +15 V
0 and 5 V
0 and 3.3 V
...

Distance: < 15 m
Maximum speed: 20 kb/s (according to V.28)
On short distances: 115 kb/s or more

Connectors: DB9 or DB25 D-subminiature connectors

Source: Mouser

20 years ago: every PC had one UART port at least, named serial port or COM port

For today's PCs: serial-USB adapters

Source: FTDI

Source: Mouser

Data transfer:

Important:

Each side must be configured in the same way (speed, parity, etc.)

UART and ML application

Collect data from a sensor
Update the embedded application with a retrained model
Etc.

Practice session 05

Get acquainted with the UART

EFR32MG24

Functional diagram:

Source: EFR32MG24 Datasheet

The UART is actually a USART: Universal Synchronous/Asynchronous Receiver/Transmitter.

Synchronous part presented in later sections.

EUSART: an additional UART which can be used in low-energy mode.

Block diagram:

Source: EFR32MG24 Datasheet

Several blocks between USART signals and µC pins
Each block must be configured

xG24 Dev Kit

No more serial connectors on modern PCs
USB allows to transport virtual serial ports
On Dev Kit: USB on PC side, UART on µC side
On-board debugger is used as a gateway

Block diagram:

Source: xG24 User's Guide

Practice session 05 - step 1

Purpose: send characters to the PC
What to do: follow the instructions provided by the embedded-systems-for-ML/practice-sessions/05-Uart-step1-board/README.md file

Practice session 05 - step 2

Purpose: echo characters sent to the board
What to do: follow the instructions provided by the embedded-systems-for-ML/practice-sessions/05-Uart-step2-board/README.md file

Some serial port deficiencies

About 20% of overhead (1 start bit + 1 stop bit)
Both sides must be configured in the same way
TX must be connected to RX. Or not
Connects two devices only (for V.24)

Interfaces presented in next sections provide other kinds of solutions.

SPI (Serial Peripheral Interface)

SPI is an a synchronous protocol with:

Two roles: controller and peripheral
A clock is provided by the controller
Four wires:
- SCK (Serial Clock)
- PICO (Peripheral In Controller Out)
- POCI (Peripheral Out Controller In)
- CS (Chip Select)

(Power lines not shown)

Multiple peripherals:

(Power lines not shown)

Note: wires were previously known as MOSI (Master Out Slave In), MISO, SS (Slave Select).

You may still come across these names.

SDO (Serial Data Out) and SDI (Serial Data In) may also be used.

Data is synchronized with the clock. Several possible configurations:

Source: EFR32xG24 Reference Manual

The protocol allows full-duplex operations.

The controller controls the clock.

When multiple peripherals are connected, the controller uses the CS signal to activate the peripheral it wants to interact with.

Maximum speed: up to 10 Mb/s.
Maximum distance: a few tens of cm.

How data is exchanged

The data protocol is defined by the peripheral.

Quite often: the peripheral interacts through registers.

Registers can be read and/or written.

A generic example

Source: Bosch BME280 data sheet

Read operation

The controller selects the peripheral (CS low) and starts the clock
On PICO: the controller sends MSB set to 1 + register address on 7 bits
On POCI: the peripheral sends data over 8 bits
If clock goes on and CS is not raised, the peripheral sends next register
...

Write operation

The controller selects the peripheral and starts the clock
On PICO: the controller sends MSB set to 0 + register address on 7 bits
On PICO: the controller sends data over 8 bits
The controller stops the clock and raise CS

To summarize:

Serial port: allows for more than several meters
SPI: short distances, higher speed

Practice session 06

Get acquainted with the SPI bus

On the EFR32MG24

The SPI interface is implemented over the (E)USART interface
This explains the "S" in the USART acronym

On the xG24 Dev Kit

The IMU (Inertial Measurement Unit) can use SPI
3-axis gyroscope, 3-axis accelerometer, temperature sensor

Block diagram:

Source: xG24 User's Guide

A switch controls sensor power
Pullup resistor on CS ⇒ sensor not selected by default
SPI signals are connected to PC and PA ports

Practice session 06

Purpose: get accelerometer data
What to do: follow the instructions provided by the embedded-systems-for-ML/practice-sessions/06-Spi-accel/README.md file

Reducing the number of wires

SPI with multiple peripherals:

Many wires...

I²C (Inter-Integrated Circuit)

I²C is another synchronous protocol with:

Two roles: controller and peripheral (target)
Multiple controllers can be present
Two wires:
- SCL (Serial Clock line)
- SDA (Serial Data line)

(Power lines not shown)

Multiple peripherals:

(Power lines not shown)

Each device has a unique 7-bit address (no CS signal)
The controller provides the clock
Every byte is acknowledged by a bit

Source: I²C-bus specification and user manual

When multiple controllers are connected:

Clock synchronization
Arbitration

Done thanks to wired-AND connections of SCL and SDA and pullup resistors.

For more information: I²C-bus specification and user manual

Standard-mode speed: 100 kb/s
High-speed mode: 3.4 Mb/s
Maximum distance: a few tens of cm.