Add pcf8574 example

i2c/pcf8574/README.adoc

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+= Using a PCF8574 remote 8-Bit I/O expander over I2C to get more IO pins
+:xrefstyle: short
+
+This example demonstrates how to use the PCF8574 Remote 8-Bit I/O Expander with the Raspberry Pi Pico over the I2C bus. The code toggles the pins of the PCF8574 sequentially, turning them on and off one by one.
+
+== Wiring information
+
+See <<pcf8574-wiring-diagram>> for wiring instructions.
+
+[[pcf8574-wiring-diagram]]
+[pdfwidth=75%]
+.Wiring the PCF8574 to Pico using I2C
+image::pico-and-io-expander.png[]
+
+== List of Files
+
+A list of files with descriptions of their function;
+
+i2c_pcf8574_blink.py:: The example code.
+i2c_get_address.py:: Some code to detect the I2C address of the expander board.
+
+== Bill of Materials
+
+.A list of materials required for the example
+[[pcf8574-bom-table]]
+[cols=3]
+|===
+| *Item* | *Quantity* | Details
+| Breadboard | 1 | generic part
+| Raspberry Pi Pico | 1 | https://www.raspberrypi.com/products/raspberry-pi-pico/
+| PCF8574 I/O expander board | 1 | 
+| LEDs or other things to control | 8 | 
+|===

i2c/pcf8574/i2c_get_address.py

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+import machine
+
+# Initialize I2C bus on pins 16 (SDA) and 17 (SCL) with a frequency of 400kHz
+i2c = I2C(0, sda=Pin(16), scl=Pin(17), freq=400000)
+
+# print the I2C address
+print('I2C address:')
+print(hex(i2c.scan()[0]), ' (hex)')

i2c/pcf8574/i2c_pcf8574_blink.py

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+# Add more IO Pins via a PCF8574 Remote 8-Bit I/O Expander
+
+from machine import Pin, I2C
+import utime
+
+# Initialize I2C bus on pins 16 (SDA) and 17 (SCL) with a frequency of 400kHz
+i2c = I2C(0, sda=Pin(16), scl=Pin(17), freq=400000)
+
+# Address of the PCF8574 I/O expander on the I2C bus
+address = 0x20
+
+def toggle_pins():
+    try:
+        for i in range(8):
+            # Create a bitmask to set each pin high one at a time
+            pin_state = 1 << i
+
+            # Write the bitmask to the PCF8574
+            i2c.writeto(address, bytearray([pin_state]))
+
+            # Sleep for 200ms to keep the pin high
+            utime.sleep(0.2)
+
+            # Reset all pins to low
+            i2c.writeto(address, bytearray([0x00]))
+    except OSError as e:
+        # Print an error message if there is an issue accessing the I2C device
+        print("Error accessing the I2C device:", e)
+
+# Continuously toggle the pins
+while True:
+    toggle_pins()