examples: add an example PCF8574 script

examples/i2c-pcf8574.py

@@ -0,0 +1,59 @@
+"""
+this serves as an example script. 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 configure the I/O pins as Outputs, and will present a walking 1-bit logic
+low, with the other pins high... connect 8x LEDs with resistor from Vcc to
+demonstrate
+"""
+
+import asyncio
+
+class PCF8574:
+    """
+    a quasi-bidirectional I/O expander
+    """
+    def __init__(self, iface, addr=0):
+        self.iface = iface
+
+        assert addr in range(8)
+        self.addr = 0x20 | ( addr & 0x7 )
+
+    async def _read(self):
+        p = await self.iface.read(self.addr, 1, stop=True)
+        if p is None:
+            raise Exception("I2C NAK")
+        return p[0]
+
+    async def read(self, mask=0xff):
+        # in order to read pins, we must set them to "high"... but don't disturb
+        # other pins that may have a set value if we're not interested in them
+        p = await self._read()
+        await self.write(p | mask)
+
+        p = await self._read()
+        return p & mask
+
+    async def write(self, p):
+        # "high" bits have acceleration on their rising edge, then a ~100uA pullup
+        # "low"  bits are driven low
+        success = await self.iface.write(self.addr, [ p ], stop=True)
+        if not success:
+            raise Exception("I2C NAK")
+
+
+io = PCF8574(iface, 0)
+
+# read the I/O pin values
+print("0x{:02X}".format(await io.read()))
+
+# write a walking 1-bit low pattern
+for i in range(8):
+    p = ~(1 << i)
+    await io.write(p)
+    await asyncio.sleep(0.5)
+
+# power down the device after use
+await device.set_voltage("AB", 0)