coredevice: add pcf8574a driver

I2C IO expander with 8 quasi-bidirectional pins

artiq/coredevice/pcf8574a.py

@@ -0,0 +1,47 @@
+from artiq.experiment import kernel
+from artiq.coredevice.i2c import (
+    i2c_start, i2c_write, i2c_read, i2c_stop, I2CError)
+
+
+class PCF8574A:
+    """Driver for the PCF8574 I2C remote 8-bit I/O expander.
+
+    I2C transactions not real-time, and are performed by the CPU without
+    involving RTIO.
+    """
+    def __init__(self, dmgr, busno=0, address=0x7c, core_device="core"):
+        self.core = dmgr.get(core_device)
+        self.busno = busno
+        self.address = address
+
+    @kernel
+    def set(self, data):
+        """Drive data on the quasi-bidirectional pins.
+
+        :param data: Pin data. High bits are weakly driven high
+            (and thus inputs), low bits are strongly driven low.
+        """
+        i2c_start(self.busno)
+        try:
+            if not i2c_write(self.busno, self.address):
+                raise I2CError("PCF8574A failed to ack address")
+            if not i2c_write(self.busno, data):
+                raise I2CError("PCF8574A failed to ack data")
+        finally:
+            i2c_stop(self.busno)
+
+    @kernel
+    def get(self):
+        """Retrieve quasi-bidirectional pin input data.
+
+        :return: Pin data
+        """
+        i2c_start(self.busno)
+        ret = 0
+        try:
+            if not i2c_write(self.busno, self.address | 1):
+                raise I2CError("PCF8574A failed to ack address")
+            ret = i2c_read(self.busno, False)
+        finally:
+            i2c_stop(self.busno)
+        return ret

doc/manual/core_drivers_reference.rst

@@ -45,6 +45,12 @@ These drivers are for the core device and the peripherals closely integrated int
 .. automodule:: artiq.coredevice.i2c
     :members:
 
+:mod:`artiq.coredevice.pcf8574a` module
+---------------------------------------
+
+.. automodule:: artiq.coredevice.pcf8574a
+    :members:
+
 :mod:`artiq.coredevice.cache` module
 ------------------------------------