examples: add an example PCF8574 script

examples/i2c-pcf8574.py

@@ -0,0 +1,28 @@
+"""
+this serves as an example script for the run-script mode. connect a PCF8574 to the
+glasgow, and run the following command:
+
+    glasgow run-script i2c-pcf8574.py i2c-initiator -V 3.3 --pin-scl 0 --pin-sda 1
+
+this will read the pin state, and then output a pattern, before powering off the
+device
+"""
+
+from asyncio import sleep
+
+ADDR = 0x20
+
+# set all pins high, so that their inputs may be read
+await iface.write(ADDR, [ 0xFF ], stop=False)
+p = await iface.read(ADDR, 1, stop=True)
+print('p: 0x{:02X}'.format(p))
+
+await sleep(0.5)
+
+# set a pattern on the pins
+await iface.write(ADDR, [ 0x55 ], stop=True)
+
+await sleep(0.5)
+
+# power down the device after use
+await device.set_voltage("AB", 0)