Merge pull request #18 from wolfmanjm/fix/i2c-timing

bitbang I2C running under Blinka did not work with the peripherals I tested.

adafruit_bitbangio.py

@@ -24,7 +24,7 @@
 """
 
 # imports
-from time import monotonic, sleep
+from time import monotonic
 from digitalio import DigitalInOut
 
 __version__ = "0.0.0-auto.0"
@@ -84,22 +84,27 @@ def __init__(self, scl, sda, *, frequency=400000, timeout=1):
 
         # Set pins as outputs/inputs.
         self._scl = DigitalInOut(scl)
-        self._scl.switch_to_output()
-        self._scl.value = 1
+        # rpi gpio does not support OPEN_DRAIN, so we have to emulate it
+        # by setting the pin to input for high and output 0 for low
+        self._scl.switch_to_input()
 
         # SDA flips between being input and output
         self._sda = DigitalInOut(sda)
-        self._sda.switch_to_output()
-        self._sda.value = 1
+        self._sda.switch_to_input()
 
-        self._delay = 1 / frequency / 2
+        self._delay = (1 / frequency) / 2  # half period
         self._timeout = timeout
 
     def deinit(self):
         """Free any hardware used by the object."""
         self._sda.deinit()
         self._scl.deinit()
 
+    def _wait(self):
+        end = monotonic() + self._delay  # half period
+        while end > monotonic():
+            pass
+
     def scan(self):
         """Perform an I2C Device Scan"""
         found = []
@@ -145,100 +150,100 @@ def writeto_then_readfrom(
         if in_end is None:
             in_end = len(buffer_in)
         if self._check_lock():
-            self.writeto(address, buffer_out, start=out_start, end=out_end)
+            self._write(address, buffer_out[out_start:out_end], False)
             self.readfrom_into(address, buffer_in, start=in_start, end=in_end)
 
     def _scl_low(self):
-        self._scl.value = 0
+        self._scl.switch_to_output(value=False)
 
     def _sda_low(self):
-        self._sda.value = 0
+        self._sda.switch_to_output(value=False)
 
     def _scl_release(self):
         """Release and let the pullups lift"""
         # Use self._timeout to add clock stretching
-        self._scl.value = 1
+        self._scl.switch_to_input()
 
     def _sda_release(self):
         """Release and let the pullups lift"""
         # Use self._timeout to add clock stretching
-        self._sda.value = 1
+        self._sda.switch_to_input()
 
     def _start(self):
         self._sda_release()
         self._scl_release()
-        sleep(self._delay)
+        self._wait()
         self._sda_low()
-        sleep(self._delay)
+        self._wait()
 
     def _stop(self):
         self._scl_low()
-        sleep(self._delay)
+        self._wait()
         self._sda_low()
-        sleep(self._delay)
+        self._wait()
         self._scl_release()
-        sleep(self._delay)
+        self._wait()
         self._sda_release()
-        sleep(self._delay)
+        self._wait()
 
     def _repeated_start(self):
         self._scl_low()
-        sleep(self._delay)
+        self._wait()
         self._sda_release()
-        sleep(self._delay)
+        self._wait()
         self._scl_release()
-        sleep(self._delay)
+        self._wait()
         self._sda_low()
-        sleep(self._delay)
+        self._wait()
 
     def _write_byte(self, byte):
         for bit_position in range(8):
             self._scl_low()
-            sleep(self._delay)
+
             if byte & (0x80 >> bit_position):
                 self._sda_release()
             else:
                 self._sda_low()
-            sleep(self._delay)
+            self._wait()
             self._scl_release()
-            sleep(self._delay)
+            self._wait()
+
         self._scl_low()
-        sleep(self._delay * 2)
+        self._sda.switch_to_input()  # SDA may go high, but SCL is low
+        self._wait()
 
         self._scl_release()
-        sleep(self._delay)
-
-        self._sda.switch_to_input()
-        ack = self._sda.value
-        self._sda.switch_to_output()
-        sleep(self._delay)
+        self._wait()
+        ack = self._sda.value  # read the ack
 
         self._scl_low()
+        self._sda_release()
+        self._wait()
 
         return not ack
 
     def _read_byte(self, ack=False):
         self._scl_low()
-        sleep(self._delay)
-
+        self._wait()
+        # sda will already be an input as we are simulating open drain
         data = 0
-        self._sda.switch_to_input()
         for _ in range(8):
             self._scl_release()
-            sleep(self._delay)
+            self._wait()
             data = (data << 1) | int(self._sda.value)
-            sleep(self._delay)
             self._scl_low()
-            sleep(self._delay)
-        self._sda.switch_to_output()
+            self._wait()
 
         if ack:
             self._sda_low()
-        else:
-            self._sda_release()
-        sleep(self._delay)
+        # else sda will already be in release (open drain) mode
+
+        self._wait()
         self._scl_release()
-        sleep(self._delay)
+        self._wait()
+        self._scl_low()
+        self._sda_release()
+
         return data & 0xFF
 
     def _probe(self, address):