Lesson 03 - I2C
The lecture
This is the first of two lecturs about I2C and components on the I2C bus
I2C (or wire) is a very common local network or fieldbus within robotics and controlsystems.
It is widely used in many areas and types of sensors.
The goal for today is get insight and get hands on in using I2C.
So today we will develop our own drivers for I2C as well as I2C slaves using a standard Arduino platform.
The Inter-integrated Circuit (I2C) Protocol is a protocol intended to allow multiple “slave” digital integrated circuits (“chips”)
to communicate with one or more “master” chips.
Like the Serial Peripheral Interface (SPI), it is only intended for short distance communications within a single device.
Like Asynchronous Serial Interfaces (such as RS-232 or UARTs), it only requires two signal wires to exchange information.
Preparation for the lecture
Read litterature
See movies
Find two Arduinos and three wires
Identify I2C pins (SDA , SCL) and ground
Maybe do it together with one more from your studygroup
Exercise 1
Implement and test an I2C scanner: (See lecti2c01-rawcode) just to get started
Exercise 2
Design and implement a simple protocol which can
carry out ADC on channel 0,1,2 and return the conversion as 16 bit signed int (which is default)
hint: decide a master code for ADC and a minor code for channel selection
hint: design return telegram so master can see what is contained
hint: dont forget we can have more than one slave so ID maight be interesting in response.
or maybe not
Exercise 3
Design and code an arduino based I2C slave with following functonallity
Run the examples on two Arduinos connected by sda,slk and gnd
If you are runing them from one PC remember set set Arduino type and USB port every time your are reprogramming
Instead of the master try to run the scanner to see if you can see your slave
Construct a I2C slave device
Here is the spec seen from master side
has address 8 or whatever you want
has two digital outputs (pin 12,13)
has one digital input (11) known as no 11 pin
Command to set a digital pin: <id><0 or 1)
id 12,13,14
Command to read digital pin
pin nr 11 has id 11
And of course a master that can test it
hint attach a button to pin 11 so you can see high or low
hint 2 let master running cyclic at 1 Hz
Exercise 4
Performance and Quality
Measure signal on I2c by use of oscilloscope. Is the builtin pull-up resistors suifficient
In exercise 2 configure it to run as fast as possible
Do you loose packages - might be observed by the system locks up or is hanging
What is max speed (pkg pr sec)
Observe on oscilloscope and identify busy as wells as non busy periodes (no traffic)
Litterature and slides
Slides
Litt
Reading hints and order
First Priority
To get an overview: See more here.
The real I2C primer is here
Secondary priority litt
A simple Arduino to Arduino setup for a single master and single slave experiment
You need
Two Arduinos
Three wires to interconnect them (sda, sdl and ground)
Movies
After today - you should (at least) be able to …
Understand I2C protocol
Be able to design a master slave protocol on top of I2C
Code it and get it running
Understand timing and performance on the I2C bus
Tips
3.3V versus 5V
Some devices like Arduino UNO, Mega,… is powered by 5V.
See more here (a practical viewpoint)
Other devices like esp32, m5stack etc is running on 3.3V
This might give problems when having a mix of 3.3V and 5V devices on the bus
You might damage 3.3V devices by having a 5V device on the canbus.
meaning you might be lucky or … not.
citing from element14.com
2C devices have open-drain drivers, which only pull the SCL and SDA lines low to 0V.
Each I2C line should have a single pull-up resistor: make sure they pull up to 3.3V and not 5V.
Also make sure that each 5V part has a reasonable Vih (Input voltage high) threshold.
Many 5V chips use TTL Vih which is 2.0V, which works.
Others may have Vih = 0.7 x 5.0 = 3.5V, which means you'll have to do something else.
A hack
disable internal pullup resistors in the 5V based device and use eternal pullup resistors connected to a 3.3V source. You might be lucky because
citing hackaday (link above)
3.3V TTL logic shares the 0.8V and 2V thresholds for logic 0 and logic 1 transitions with 5V TTL logic,
so a 3.3V TTL output can drive a 5V TTL input without any extra hardware required.
But I dont give any guarantee
Ground
When wiring up always start with ground so all devices has same reference
I2C recover
If your I2C locks up this code might help you lect03-i2c-recover