Add I2C bus frequency readback property

pslab/bus/i2c.py

@@ -58,6 +58,7 @@ def __init__(self, device: ConnectionHandler | None = None):
         self._device = device if device is not None else autoconnect()
         self._running = False
         self._mode = None
+        self._frequency = None
 
     def _init(self):
         self._device.send_byte(CP.I2C_HEADER)
@@ -84,6 +85,7 @@ def _configure(self, frequency: float):
             self._device.send_byte(CP.I2C_CONFIG)
             self._device.send_int(brgval)
             self._device.get_ack()
+            self._frequency = self._get_i2c_frequency(brgval)
         else:
             min_frequency = self._get_i2c_frequency(self._MAX_BRGVAL)
             max_frequency = self._get_i2c_frequency(self._MIN_BRGVAL)
@@ -452,6 +454,11 @@ def __init__(self, device: ConnectionHandler | None = None):
         self._init()
         self.configure(125e3)  # 125 kHz is as low as the PSLab can go.
 
+    @property
+    def frequency(self):
+        """float: The I2C bus frequency in Hz."""
+        return self._frequency
+
     def configure(self, frequency: float):
         """Configure bus frequency.
 

tests/test_i2c_frequency.py

@@ -0,0 +1,253 @@
+"""Tests for I2C baudrate readback feature (Issue #195).
+
+These are unit tests that use a mock ConnectionHandler, so they can run
+without a physical PSLab device connected.
+"""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from pslab.bus.i2c import I2CMaster, I2CSlave, _I2CPrimitive
+import pslab.protocol as CP
+
+
+@pytest.fixture
+def mock_device():
+    """Return a mock ConnectionHandler that responds to I2C commands."""
+    device = MagicMock()
+    device.send_byte = MagicMock()
+    device.send_int = MagicMock()
+    device.get_ack = MagicMock(return_value=1)  # ACK
+    device.read = MagicMock(return_value=b"\x01")
+    return device
+
+
+@pytest.fixture
+def master(mock_device):
+    """Return an I2CMaster with a mocked device."""
+    with patch("pslab.bus.i2c.autoconnect", return_value=mock_device):
+        return I2CMaster(device=mock_device)
+
+
+@pytest.fixture
+def primitive(mock_device):
+    """Return a bare _I2CPrimitive with a mocked device."""
+    with patch("pslab.bus.i2c.autoconnect", return_value=mock_device):
+        return _I2CPrimitive(device=mock_device)
+
+
+# ============================================================
+# Test 1: _frequency is None before _configure is called
+# ============================================================
+class TestFrequencyInitialization:
+
+    def test_primitive_frequency_is_none_on_init(self, primitive):
+        """_I2CPrimitive should have _frequency=None before any configure."""
+        # _I2CPrimitive.__init__ does NOT call _configure, so it stays None.
+        assert primitive._frequency is None
+
+    def test_master_frequency_is_set_after_init(self, master):
+        """I2CMaster.__init__ calls configure(125e3), so frequency should be set."""
+        assert master.frequency is not None
+        assert isinstance(master.frequency, float)
+
+
+# ============================================================
+# Test 2: Default frequency is ~125 kHz after I2CMaster init
+# ============================================================
+class TestDefaultFrequency:
+
+    def test_default_frequency_value(self, master):
+        """I2CMaster should default to ~125 kHz (the lowest PSLab supports)."""
+        # The exact value depends on brgval rounding.
+        # brgval = int((1/125000 - 150e-9) * 64e6 - 2) = int(509.39) = 509
+        # actual_freq = 1 / ((509 + 2) / 64e6 + 150e-9)
+        expected_brgval = _I2CPrimitive._get_i2c_brgval(125e3)
+        expected_freq = _I2CPrimitive._get_i2c_frequency(expected_brgval)
+        assert master.frequency == pytest.approx(expected_freq)
+
+
+# ============================================================
+# Test 3: frequency stores ACTUAL hardware freq, not requested
+# ============================================================
+class TestActualFrequency:
+
+    def test_frequency_is_actual_not_requested(self, master):
+        """Stored frequency should be the actual hardware freq, not the requested one."""
+        requested = 400000  # User requests 400 kHz
+        master.configure(requested)
+        # Due to brgval rounding, actual != requested.
+        brgval = _I2CPrimitive._get_i2c_brgval(requested)
+        actual = _I2CPrimitive._get_i2c_frequency(brgval)
+        assert master.frequency == pytest.approx(actual)
+        # Actual will be slightly different from requested.
+        assert master.frequency != requested
+
+    def test_frequency_matches_brgval_computation(self, master):
+        """Verify frequency computation matches the brgval->freq math."""
+        test_frequencies = [125e3, 250e3, 400e3, 1e6, 3.2e6]
+
+        for freq in test_frequencies:
+            brgval = _I2CPrimitive._get_i2c_brgval(freq)
+
+            if _I2CPrimitive._MIN_BRGVAL <= brgval <= _I2CPrimitive._MAX_BRGVAL:
+                master.configure(freq)
+                expected = _I2CPrimitive._get_i2c_frequency(brgval)
+                assert master.frequency == pytest.approx(expected), (
+                    f"Mismatch for requested freq {freq}: "
+                    f"got {master.frequency}, expected {expected}"
+                )
+
+
+# ============================================================
+# Test 4: frequency updates on reconfigure
+# ============================================================
+class TestFrequencyUpdate:
+
+    def test_frequency_updates_after_reconfigure(self, master):
+        """Frequency should update each time configure() is called."""
+        master.configure(125e3)
+        freq1 = master.frequency
+
+        master.configure(400e3)
+        freq2 = master.frequency
+
+        assert freq1 != freq2
+        assert freq2 > freq1  # 400 kHz > 125 kHz
+
+    def test_frequency_updates_multiple_times(self, master):
+        """Frequency should correctly track through multiple reconfigurations."""
+        frequencies = [125e3, 250e3, 400e3, 250e3, 125e3]
+
+        for requested in frequencies:
+            master.configure(requested)
+            brgval = _I2CPrimitive._get_i2c_brgval(requested)
+            expected = _I2CPrimitive._get_i2c_frequency(brgval)
+            assert master.frequency == pytest.approx(expected)
+
+
+# ============================================================
+# Test 5: frequency unchanged on invalid configure (ValueError)
+# ============================================================
+class TestFrequencyOnError:
+
+    def test_frequency_unchanged_on_invalid_frequency(self, master):
+        """If configure() raises ValueError, frequency should remain unchanged."""
+        master.configure(125e3)
+        original_freq = master.frequency
+
+        # Too low frequency should raise ValueError.
+        with pytest.raises(ValueError):
+            master.configure(1)  # Way too low
+
+        # Frequency should be unchanged.
+        assert master.frequency == original_freq
+
+    def test_frequency_unchanged_on_too_high_frequency(self, master):
+        """If configure() raises ValueError for too-high freq, frequency unchanged."""
+        master.configure(125e3)
+        original_freq = master.frequency
+
+        # Too high frequency should raise ValueError.
+        with pytest.raises(ValueError):
+            master.configure(100e6)  # Way too high (100 MHz)
+
+        assert master.frequency == original_freq
+
+
+# ============================================================
+# Test 6: frequency property is read-only
+# ============================================================
+class TestFrequencyReadOnly:
+
+    def test_frequency_property_is_read_only(self, master):
+        """Setting frequency directly should raise AttributeError."""
+        with pytest.raises(AttributeError):
+            master.frequency = 500000
+
+
+# ============================================================
+# Test 7: frequency property returns float
+# ============================================================
+class TestFrequencyType:
+
+    def test_frequency_is_float(self, master):
+        """Frequency should always be a float."""
+        assert isinstance(master.frequency, float)
+
+    def test_frequency_is_positive(self, master):
+        """Frequency should always be positive."""
+        assert master.frequency > 0
+
+
+# ============================================================
+# Test 8: _configure sends correct bytes to hardware
+# ============================================================
+class TestConfigureHardwareInteraction:
+
+    def test_configure_sends_correct_protocol_bytes(self, master, mock_device):
+        """Verify that _configure sends the correct I2C header and config bytes."""
+        mock_device.reset_mock()
+        master.configure(400e3)
+
+        brgval = _I2CPrimitive._get_i2c_brgval(400e3)
+        calls = mock_device.send_byte.call_args_list
+
+        # Should send I2C_HEADER then I2C_CONFIG.
+        assert any(c.args[0] == CP.I2C_HEADER for c in calls)
+        assert any(c.args[0] == CP.I2C_CONFIG for c in calls)
+
+        # Should send brgval as int.
+        mock_device.send_int.assert_called_with(brgval)
+
+
+# ============================================================
+# Test 9: Boundary values — min and max valid frequencies
+# ============================================================
+class TestBoundaryFrequencies:
+
+    def test_min_valid_frequency(self, master):
+        """Test with the minimum valid frequency (brgval = MAX_BRGVAL = 511)."""
+        min_freq = _I2CPrimitive._get_i2c_frequency(_I2CPrimitive._MAX_BRGVAL)
+        master.configure(min_freq)
+        assert master.frequency == pytest.approx(min_freq)
+
+    def test_max_valid_frequency(self, master):
+        """Test with a high frequency whose brgval equals MIN_BRGVAL."""
+        # _get_i2c_frequency(MIN_BRGVAL) gives the theoretical max, but due
+        # to int() truncation in _get_i2c_brgval, that value may not
+        # round-trip. Instead, find a frequency that computes to MIN_BRGVAL.
+        max_brgval_freq = _I2CPrimitive._get_i2c_frequency(
+            _I2CPrimitive._MIN_BRGVAL + 1
+        )
+        master.configure(max_brgval_freq)
+        assert master.frequency is not None
+        assert master.frequency > 0
+
+    def test_just_below_min_frequency_raises(self, master):
+        """Frequency just below minimum should raise ValueError."""
+        min_freq = _I2CPrimitive._get_i2c_frequency(_I2CPrimitive._MAX_BRGVAL)
+
+        with pytest.raises(ValueError):
+            master.configure(min_freq * 0.5)  # Half of minimum
+
+    def test_just_above_max_frequency_raises(self, master):
+        """Frequency just above maximum should raise ValueError."""
+        max_freq = _I2CPrimitive._get_i2c_frequency(_I2CPrimitive._MIN_BRGVAL)
+
+        with pytest.raises(ValueError):
+            master.configure(max_freq * 2)  # Double of maximum
+
+
+# ============================================================
+# Test 10: I2CSlave inherits _frequency from _I2CPrimitive
+# ============================================================
+class TestSlaveInheritsFrequency:
+
+    def test_slave_has_frequency_attribute(self, mock_device):
+        """I2CSlave should also have _frequency from _I2CPrimitive."""
+        with patch("pslab.bus.i2c.autoconnect", return_value=mock_device):
+            slave = I2CSlave(address=0x48, device=mock_device)
+            # Slave doesn't call _configure, so it should be None.
+            assert slave._frequency is None