⚡ Electronics Lesson #2

⚡ Electronics #2

Communication, interference,
and how everything fits together

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Analog communication
Digital communication
- Definitions
- Communication protocols
- Interference
Integrating everything

How do we “communicate”?

Senses:

Vision
Hearing
Touch
Balance
Taste
Smell

All of these are “analog”

What is “analog”?

From Wikipedia:

“Analog transmission is a method of conveying […] information using a signal which varies in amplitude, phase, or some other property”

Most “common sense” since it can vary

However, microcontrollers cannot do analog well

Bus capacitance, stray inductance and resistances, all affect the analog signal massively.

For the communication to be reliable,
we need it to be digital.

Terminology

Digital bytes

From Wikipedia:

The byte is a unit of digital information. It is the smallest addressable unit of memory in many computer architectures.

… and bits (binary digit)

From Wikipedia:

A bit is the smallest unit of storage. The modern de facto standard of eight bits in a byte permits binary values 0 through 2⁸=255 for one byte. Modern architectures typically use 32- or 64-bit words, built of four or eight bytes.

What is a “digital” signal?

Duplex

once again from Wikipedia:

A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions.

Half- and full-duplex

Asynchronous

Closest to humans communication, most intuitive (start bit is the “oi”)

But it needs to send a “sync” bit every byte, means it wastes 1 bit in every 9 bits, speed will drop by 1/9. Both sides must also agree to a baud rate beforehand, difficult for “master slave” devices where you have no control over the slave (usually a sensor)

also, asynchronous comms is complex:
since we cannot predict when the “slave” sends info (there’s no master/slave relationship here) and when the uC is doing something else or when it is transmitting it cannot “listen” to the receive (like in human convo: when 2 people talk over each other, cannot hear each other).
Therefore you cannot have more than 1 “slave” on the line, since there is no slave.
UARTs are therefore usually separate from software: something in hardware which writes to a buffer until the uC is ready to access it. Therefore, in a uC UARTs are limited because you can’t just add it effectively into software, it’s a piece of hardware inside the chip.

explain this after the next slide:

bad: Multiplexing is difficult in asynchronous because you cannot “miss” any data, and you cannot predict when the data starts.

good thing about this is: only 1 wire needed, no wasted energy, doesn’t suffer from that much EMI issues, send data only when you need.

Clock & Multiplexing

Basically a metronome for digital synchronous circuits.

	implementation	data rate	configuration
synchronous	simpler	faster	easier
	RX is controlled	no start bit	set 1 baud rate
asynchronous	complex	slower	tougher
	RX is unpredictable	need start bit	set 2 baud rates

	multiplexing	interference	power use
synchronous	easier	bad	more
	RX is controlled	clock signal can be antenna	clock always on
asynchronous	insane	better	less
	RX is unpredictable	data only live when needed, no clock	no clock

Differential Signals

Cyclic Redundancy Check (CRC)

Basically sum of the data and divide it

eg, if the data is 10001100, parity bit is 1:
$1R$ $1$
$2\overline{\smash{)}1+0+0+0+1+1+0+0}$

Network topologies

Bus Network — Different network topologies

Mesh Network — Different network topologies

Protocols

UART

Asynchronous
Full-duplex
Preferred choice

TX 🡒 RX
RX 🡐 TX

I²C

Synchronous
Half-duplex
Prone to interference:
do not use over long wires
Made to use with “sensors”
Most sensors use this

SDA ⟺ SDA
SCL 🡒 SCL

I<sup>2</sup>C Connection Diagram — I²C Connection Diagram

SPI

Synchronous
Pseudo Full-duplex
Fastest, but “Bulkiest”: 4 lines
Easy in hardware, complex in code
Used mostly in display drivers

MOSI 🡒 MOSI
MISO 🡐 MISO
SCK 🡒 SCK
SS 🡒 SS

Miscellaneous

We don’t really need to know the next few for cup, they are mostly used for other industrial purposes

These are all differential protocols. They usually don’t exist for normal microcontrollers since it’s difficult to implement

USB (2.0)

(A)synchronous
Half-duplex (up till USB3)
Differential
Industry standard
Very fast but complex
- Requires accurate clock

D+ ⟺ D+
D- ⟺ D-

CAN, RS485 / LVDS

“all the others”
Differential
CAN is used in cars
The others have industrial applications
- RS485 does not define a protocol

Probably no time to do interference separately, it’s boring anyways and it only really occurs for i2c so:

i2c requires very very precise timing signals
has some wonky pull up system since it is open drain (you pull it low but leave it open and don’t drive it high)
together with a bidirectional data wire, it is very very prone to interference.

tl;dr, if you need to use i2c (most probably will):

keep wire lengths short
maybe don’t use the 400kHz fast mode (probably don’t need it to be that fast in reality anyways)
ensure wires DON’T PASS BY MOTORS
and try to keep buses separate for critical sensors (like BNO055!!!!!!!!!! VERY IMPORTANT!!!!!!)

very good Hackaday article about I2C over long wires

wherever possible (mostly between µC) opt for UART as it’s more stable and the complex hardware implementation is done for us already.

SPI may be preferred but slave mode is not always well implemented in hardware, and software emulation is not that good

But we cannot escape i2c, and it’s not THAT bad anyways, mostly just BNO055’s implementation and wonky pull ups.

I²C	SPI	UART	AIN	PWM	Others
TOF	Mouse	µC	Light	Motor	Ultra
Camera	Camera	RasPi			Neopixel
Color	OpenMV	OpenMV			Color
Compass	Display
	POT

⚡ Electronics #2

Contents

How do we “communicate”?

What is “analog”?

However, microcontrollers cannot do analog well

Terminology

Digital bytes

… and bits (binary digit)

What is a “digital” signal?

Duplex

Half- and full-duplex

Asynchronous

Clock & Multiplexing

Differential Signals

Cyclic Redundancy Check (CRC)

Network topologies

Protocols

UART

I2C

SPI

Miscellaneous

USB (2.0)

CAN, RS485 / LVDS

end

I²C