Merge pull request #864 from brentru/fix-mcp-nack

Fix MCP2221 I2C NACK Error

src/adafruit_blinka/microcontroller/mcp2221/mcp2221.py

@@ -248,7 +248,7 @@ def _i2c_write(self, cmd, address, buffer, start=0, end=None):
         for _ in range(MCP2221_RETRY_MAX):
             status = self._i2c_status()
             if status[20] & MASK_ADDR_NACK:
-                raise RuntimeError("I2C slave address was NACK'd")
+                raise OSError("I2C slave address was NACK'd")
             usb_cmd_status = status[8]
             if usb_cmd_status == 0:
                 break
@@ -360,7 +360,8 @@ def i2c_scan(self, *, start=0, end=0x79):
             # try a write
             try:
                 self.i2c_writeto(addr, b"\x00")
-            except RuntimeError:  # no reply!
+            except OSError:  # no reply!
+                # We got a NACK, which could be correct
                 continue
             # store if success
             found.append(addr)