Add pio onewire example (#334)

* Adds PIO onewire example

Co-authored-by: martin <admin@crossleys.biz>

pio/onewire/README.adoc

@@ -0,0 +1,57 @@
+= Interfacing 1-Wire devices to the Pico
+
+This example demonstrates how to use link:https://www.analog.com/en/technical-articles/guide-to-1wire-communication.html[1-Wire] devices with the Raspberry Pi Pico (RP2040).
+1-Wire is an interface that enables a master to control several slave devices over a simple shared serial bus.
+
+The example provides a 1-Wire library that is used to take readings from a set of connected link:https://www.analog.com/media/en/technical-documentation/data-sheets/ds18b20.pdf[DS18B20] 1-Wire temperature sensors. The results are sent to the default serial terminal connected via USB or UART as configured in the SDK.
+
+The library uses a driver based on the RP2040 PIO state machine to generate accurate bus timings and control the 1-Wire bus via a GPIO pin.
+
+_1-Wire(R) is a registered trademark of Maxim Integrated Products, Inc._
+
+== Wiring information
+
+Connect one or more DS18B20 sensors to the Pico as shown in the diagram and table below.
+
+Connect GPIO 15 to 3V3(OUT) with a pull-up resistor of about 4k ohms.
+
+[[pio_onewire_wiring-diagram]]
+[pdfwidth=75%]
+.Wiring diagram
+image::pio_onewire.png[]
+
+[[pio_onewire_connections-table]]
+.Connections table
+[options="header,footer"]
+|==================================================
+|Pico    |pin         |DS18B20    |pin / sensor wire
+|GND     |38 or equivalent   |GND        |1 / Black
+|GPIO 15 |20          |DQ         |2 / Yellow
+|3V3(OUT)|36          |VDD        |3 / Red
+|==================================================
+
+== Bill of materials
+
+.A list of materials for the example circuit
+[[pio_onewire_bom-table]]
+[cols=3]
+|===
+| *Item*  | *Quantity* | *Details*
+| Breadboard | 1 | generic part
+| Raspberry Pi Pico | 1 | https://www.raspberrypi.com/products/raspberry-pi-pico/
+| DS18B20 | 3 | chip or wired sensor
+| 3900 ohm resistor | 1 | generic part
+| M/M jumper wire | 13 | generic part
+|===
+
+== List of files
+[[pio_onewire_file-list]]
+CMakeLists.txt:: CMake file to incorporate the example in the build tree.
+onewire.c:: Source code for the example program.
+ow_rom.h:: Header file with generic ROM command codes for 1-Wire devices. 
+ds18b20.h:: Header file with function command codes for the DS18B20 device.
+onewire_library/:: Subdirectory containing the 1-Wire library and driver.
+onewire_library/CMakeLists.txt:: CMake file to build the 1-Wire library and driver.
+onewire_library/onewire_library.c:: Source code for the 1-Wire user functions.
+onewire_library/onewire_library.h:: Header file for the 1-Wire user functions and types.
+onewire_library/onewire_library.pio:: PIO assembly code for the 1-Wire driver.