read twice the first time and immediately after changing channels, on…

…ce otherwise. removed the samples constructor parameter

quick2wire/parts/pcf8591.py

@@ -59,15 +59,6 @@
 because the PCF8591 chip can only detect voltage differences of half
 that between its reference voltage and ground.
 
-The PCF8591 uses the successive approximation conversion technique.
-The initial sample returned from the chip will be inaccurate and
-successive samples will return increasingly accurate measurements.
-Thus there is a trade-off between speed and accuracy.  The PCF8591
-class lets you tune this trade off by setting the number of samples
-that are made every time the `value` property is queried by specifying
-the `samples` parameter when instantiating the PCF8591 and by
-assigning to the `samples` property after construction.
-
 The output channel must be opened before use, to turn on the chip's
 D/A converter, and closed when no longer required, to turn it off
 again and conserve power. It's easiest to do this with a context
@@ -104,7 +95,7 @@ class PCF8591(object):
     See module documentation for details on how to use this class.
     """
 
-    def __init__(self, master, mode, address=BASE_ADDRESS, samples=3):
+    def __init__(self, master, mode, address=BASE_ADDRESS):
         """Initialises a PCF8591.
         
         Parameters:
@@ -114,12 +105,9 @@ def __init__(self, master, mode, address=BASE_ADDRESS, samples=3):
                 THREE_DIFFERENTIAL or SINGLE_ENDED_AND_DIFFERENTIAL.
         address -- the I2C address of the PCF8591 chip.
                    (optional, default = BASE_ADDRESS)
-        samples -- the number of samples made per query.
-                   (optional, default = 3)
         """
         self.master = master
         self.address = address
-        self.samples = samples
         self._control_flags = (mode << 4)
         self._last_channel_read = None
         self._output = _OutputChannel(self)
@@ -197,12 +185,12 @@ def read_differential(self, channel):
 
     def read_raw(self, channel):
         if channel != self._last_channel_read:
-            self.master.transaction(writing_bytes(self.address, self._control_flags|channel))
+            self.master.transaction(
+                writing_bytes(self.address, self._control_flags|channel),
+                reading(self.address, 1))
             self._last_channel_read = channel
 
-        for i in range(self.samples):
-            results = self.master.transaction(reading(self.address, 1))
-
+        results = self.master.transaction(reading(self.address, 1))
         return results[0][0]
 
 

quick2wire/parts/test_pcf8591.py

@@ -114,20 +114,24 @@ def test_cannot_be_created_with_an_invalid_mode():
         PCF8591(i2c, 999)
 
 def test_can_read_a_single_ended_pin():
-    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED, samples=1)
-
+    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED)
+    
     pin = adc.single_ended_input(2)
 
-    i2c.add_response(bytes([64]))
+    i2c.add_response(bytes([0x80]))
+    i2c.add_response(bytes([0x40]))
 
     sample = pin.value
 
     assert i2c.request_count == 2
 
-    m1, = i2c.request(0)
-    assert is_write(m1)
-    assert m1.len == 1
-    assert m1.buf[0][0] == 0b00000010
+    m1a,m1b = i2c.request(0)
+    assert is_write(m1a)
+    assert m1a.len == 1
+    assert m1a.buf[0][0] == 0b00000010
+
+    assert is_read(m1b)
+    assert m1b.len == 1
 
     m2, = i2c.request(1)
     assert is_read(m2)
@@ -137,21 +141,27 @@ def test_can_read_a_single_ended_pin():
 
 
 def test_can_read_a_differential_pin():
-    adc = create_pcf8591(i2c, THREE_DIFFERENTIAL, samples=1)
+    adc = create_pcf8591(i2c, THREE_DIFFERENTIAL)
 
     pin = adc.differential_input(1)
 
+
+    i2c.add_response(bytes([0x80]))
+
     # -64 in 8-bit 2's complement representation
     i2c.add_response(bytes([0xC0]))
 
     sample = pin.value
 
     assert i2c.request_count == 2
 
-    m1, = i2c.request(0)
-    assert is_write(m1)
-    assert m1.len == 1
-    assert m1.buf[0][0] == 0b00010001
+    m1a,m1b = i2c.request(0)
+    assert is_write(m1a)
+    assert m1a.len == 1
+    assert m1a.buf[0][0] == 0b00010001
+
+    assert is_read(m1b)
+    assert m1b.len == 1
 
     m2, = i2c.request(1)
     assert is_read(m2)
@@ -200,44 +210,43 @@ def test_sends_correct_mode_bits_for_single_ended_and_differential_mode():
     assert i2c.request(0)[0].buf[0][0] == 0b00100001
 
 
-def test_can_be_configured_to_perform_multiple_samples_and_reports_last_one():
-    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED, samples=3)
+def test_does_not_switch_channel_and_only_reads_once_for_subsequent_reads_from_same_pin():
+    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED)
 
-    pin = adc.single_ended_input(1)
+    pin0 = adc.single_ended_input(0)
 
-    i2c.add_response(bytes([32]))
-    i2c.add_response(bytes([64]))
-    i2c.add_response(bytes([128]))
+    sample = pin0.value
+    assert i2c.request_count == 2
 
-    sample = pin.value
+    sample = pin0.value
+    assert i2c.request_count == 3
 
-    assert_is_approx(0.5, sample)
+    sample = pin0.value
+    assert i2c.request_count == 4
 
 
-def test_does_not_write_control_byte_to_switch_channel_if_multiple_reads_from_same_pin():
-    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED, samples=1)
+def test_switches_channel_and_reads_twice_when_reading_from_different_pin():
+    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED)
 
     pin0 = adc.single_ended_input(0)
     pin1 = adc.single_ended_input(1)
 
     sample = pin0.value
     assert i2c.request_count == 2
-    ctl1, = i2c.request(0)
-    assert is_write(ctl1)
-    assert ctl1.len == 1
-    assert ctl1.buf[0][0] == 0b00000000
 
     sample = pin0.value
     assert i2c.request_count == 3
 
     sample = pin1.value
     assert i2c.request_count == 5
 
-    ctl2, = i2c.request(3)
-    assert is_write(ctl2)
-    assert ctl2.len == 1
-    assert ctl2.buf[0][0] == 0b00000001
-
+    ma,mb = i2c.request(3)
+    assert is_write(ma)
+    assert ma.len == 1
+    assert ma.buf[0][0] == 0b00000001
+    assert is_read(mb)
+    assert mb.len == 1
+
 
 def test_opening_and_closing_the_output_pin_turns_the_digital_to_analogue_converter_on_and_off():
     adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED)
@@ -267,7 +276,7 @@ def test_output_pin_opens_and_closes_itself_when_used_as_a_context_manager():
 
 
 def test_setting_value_of_output_pin_sends_value_as_second_written_byte():
-    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED, samples=1)
+    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED)
 
     with adc.output as pin:
         pin.value = 0.5
@@ -289,7 +298,7 @@ def test_setting_value_of_output_pin_sends_value_as_second_written_byte():
 
 
 def test_setting_value_of_output_pin_does_not_affect_currently_selected_input_pin():
-    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED, samples=1)
+    adc = create_pcf8591(i2c, FOUR_SINGLE_ENDED)
 
     with adc.output as opin:
         assert i2c.request_count == 1