Session 2: Micropython and

Hardware access

Uli Raich

Formally CERN, Geneva, Switzerland

The I2C bus

I2C stands for Inter-Integrated-Circuit. It was invented by Philips Semiconductor

in 1982. Slow, short distance.

Quite a number of sensors in the "WeMos D1 sensor shield collection use the I2C bus

• SHT30 temperature and humidity sensor
• DS1307 real time clock
• The BMP180 barometric pressure and temperature sensor
  
  
• The SSD1306 OLED (Organic Light Emitting Diode) display

The physical bus

The CPU (master) connects to the sensors (slaves) through

2 digital lines:

• SCL: the clock
• SDA: the data

I2C start and stop sequence

When the master want to talk to the slave it issues a start sequence

It terminates the transfer with a stop sequence

I2C addressing

When talking to a slave the master sends a seven bit address

followed by a read/write bit

This allows to access at most 128 devices

I2C data transfer

Data is transmitted 8 bits at a time followed by an acknowledge bit.

If acknowledge is low, transfer ok, otherwise: send stop sequence

I2C write cycle

• Send start sequence
• Send I2C address and R/W bit low
• Send internal register number
• Send data byte
• Optionally send further data bytes
• Send stop sequence

I2C read cycle

• Send start sequence
• Send slave address with R/W low
• Send address of internal register
• Send a start sequence again
• Send slave address with R/W high
• Read data byte
• Send stop sequence

I2C in Micropython

Scanning the I2C bus

The SHT30 digital temperature and relative humidity sensor

The SHT30 is a digital temperature and humidity sensor based on the I2C bus

Here is its data sheet.

Temperature precision: +- 0.3 °C

Relative humidity: +- 3 %

Works on 2.4V – 5.5 V

A look at the SHT30 driver

https://afnog.iotworkshop.africa/pub/AFNOG/HardwareAccessAndMicropython/sht30.py.txt

Reading out the SHT30

Results from the SHT30

WeMos D1 mini sensor and actuator shields (1)

Pushbutton	WS2812 cascadable rgb LED	SSD1306 OLED display
DS118B20 temperature sensor	DHT11 temperature and relative humidity sensor	8x8 LED Matrix

Wemos D1 mini sensor and actuator shields (2)

Passive Buzzer	Data Logger	SHT30 temperature and humidity sensor
triple base		my prototype board: LED + photo resistor

Pinouts of shields

Documentation for the demo programs

Demo programs for all the shields are available on github:

https://github.com/uraich/MicroPython_IoTDemos

A short description of every program can be found in the README

The data logger

The data logger features a DS1307 Real Time Clock (RTC) backed up by a battery An SD card socket also provided. This allows to store large amounts of data locally The RTC keeps the time and measurements can be supplied with a time tag Programs to set and read the RTC are provided. One of the programs setting the RTC gets the time from and NTP server such that manual specification of the current date and time are not needed.

https://github.com/uraich/MicroPython_IoTDemos/tree/master/drivers/ds1307

The push button shield

DS18B20 shield

SSD1306 48x64 OLED display

This one is also an I2C device

It provides 48*64 pixels and allows to

• Write a few characters of text
• Do simple graphics

framebuf class included in Micropython such that all

drawing methods of the framebuf are available for drawing:

https://docs.micropython.org/en/latest/library/framebuf.html

For an example see:

https://github.com/uraich/MicroPython_IoTDemos/tree/master/drivers/oled

The DHT11 shield

	The DHT11 is a digital temperature and relative humidity sensor. It uses a proprietary protocol of communication with its controlling host implemented in the dht.DHT11 class

The LED matrix

The LED matrix has 8x8 LEDs on it, which can individually be switched on or off. In addition to the mled class which takes care of the communication between the host and the device and which has classes to clear the display, set a pixel on or off and to change the brightness, I wrote a class matrix which takes a number 0..64 and lights this number of LEDs starting from bottom left

The buzzer

This shield implements a passive buzzer. A passive buzzer takes a frequency(an active buzzer takes a signal level) and produces a sound at this frequency. This means that the frequency can be changed and a tune can be played. In https://github.com/uraich/MicroPython_IoTDemos/tree/master/drivers/buzzer you will find a program interpreting songs on RTTTL (Ring Tone Text Transfer Language) and playing these on the buzzer A small song library is also provided

The prototype module

This is a home build module featuring a photo-resistor measuring the light intensity impinging on it. It provides an analogue value converted to digital by the ADC on the WeMos D1 CPU Please note that the ESP8266 has a single 10 bit ADC while the ESP32 has 3 12 bit ADCs. A MUX provides up to 18 analogue channels The light intensity can be changed with an LED

The triple base

This module allows to easily stack a rather larger number of shields to a sandwich.

Make sure however that the GPIO lines used by the modules do not clash

Exercises

Now it is up to

You

to do the work!

The exercises are here:

https://afnog.iotworkshop.africa/do/view/AFNOG/Session2