make ads111x threadsafe

cnc/sensors/ads111x.py

@@ -3,6 +3,7 @@ import fcntl
 import struct
 import time
 import atexit
+import threading
 
 ADS111x_ADDRESS = 0x48
 I2C_SLAVE = 0x0703
@@ -10,6 +11,9 @@ I2C_SLAVE = 0x0703
 # Initialize i2c interface and register it for closing on exit.
 __i2c_dev = os.open("/dev/i2c-1", os.O_SYNC | os.O_RDWR)
 
+# mutex for multi threading requests
+lock = threading.Lock()
+
 
 def __close():
     os.close(__i2c_dev)
@@ -28,12 +32,14 @@ def measure(channel):
     """
     Measure voltage on chip input.
     Raises OSError(Errno 121) "Remote I/O error" on reading error.
+    Thread safe.
     :param channel: chip channel to use.
     :return: Voltage in Volts.
     """
     global __i2c_dev
     if channel < 0 or channel > 3:
         raise ValueError("Wrong channel")
+    lock.acquire()
     # configure
     data = struct.pack(">BH",
                        0x01,  # config register
@@ -50,6 +56,7 @@ def measure(channel):
     # read result
     os.write(__i2c_dev, struct.pack("B", 0x00))  # conversion register
     v = struct.unpack(">h", os.read(__i2c_dev, 2))[0]
+    lock.release()
     return v / 8000.0  # / 32768.0 * 4.096 according to specified range