tests/extmod_hardware: Add a test for machine.PWM freq and duty.

This adds a hardware test for `machine.PWM`.  It requires a jumper wire
between two pins, uses `machine.PWM` to output on one of them, and
`machine.time_pulse_us()` to time the PWM on the other pin (some boards
test more than one pair of pins).

It times both the high and low duty cycle (and hence the frequency) for a
range of PWM frequencies and duty cycles (including full on and full off).

Currently supported on:
- esp32 (needs a minor hack for initialisation, and some tests still fail)
- esp8266 (passes for frequencies 1kHz and less)
- mimxrt / Teensy 4.0 (passes)
- rp2 (passes)
- samd21 (passes for frequencies 2kHz and less)

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod_hardware/machine_pwm.py

@@ -0,0 +1,166 @@
+# Test machine.PWM, frequncy and duty cycle (using machine.time_pulse_us).
+#
+# IMPORTANT: This test requires hardware connections: the PWM-output and pulse-input
+# pins must be wired together (see the variable `pwm_pulse_pins`).
+
+import sys
+import time
+
+try:
+    from machine import time_pulse_us, Pin, PWM
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+import unittest
+
+pwm_freq_limit = 1000000
+freq_margin_per_thousand = 0
+duty_margin_per_thousand = 0
+timing_margin_us = 5
+
+# Configure pins based on the target.
+if "esp32" in sys.platform:
+    pwm_pulse_pins = ((4, 5),)
+    freq_margin_per_thousand = 2
+    duty_margin_per_thousand = 1
+    timing_margin_us = 20
+elif "esp8266" in sys.platform:
+    pwm_pulse_pins = ((4, 5),)
+    pwm_freq_limit = 1_000
+    duty_margin_per_thousand = 3
+    timing_margin_us = 50
+elif "mimxrt" in sys.platform:
+    if "Teensy" in sys.implementation._machine:
+        # Teensy 4.x
+        pwm_pulse_pins = (
+            ("D0", "D1"),  # FLEXPWM X and UART 1
+            ("D2", "D3"),  # FLEXPWM A/B
+            ("D11", "D12"),  # QTMR and MOSI/MISO of SPI 0
+        )
+    else:
+        pwm_pulse_pins = (("D0", "D1"),)
+elif "rp2" in sys.platform:
+    pwm_pulse_pins = (("GPIO0", "GPIO1"),)
+elif "samd" in sys.platform:
+    pwm_pulse_pins = (("D0", "D1"),)
+    if "SAMD21" in sys.implementation._machine:
+        # MCU is too slow to capture short pulses.
+        pwm_freq_limit = 2_000
+else:
+    print("Please add support for this test on this platform.")
+    raise SystemExit
+
+
+# Test a specific frequency and duty cycle.
+def _test_freq_duty(self, pulse_in, pwm, freq, duty_u16):
+    print("freq={:<5} duty_u16={:<5} :".format(freq, duty_u16), end="")
+
+    # Check configured freq/duty_u16 is within error bound.
+    freq_error = abs(pwm.freq() - freq) * 1000 // freq
+    duty_error = abs(pwm.duty_u16() - duty_u16) * 1000 // (duty_u16 or 1)
+    print(" freq={} freq_er={}".format(pwm.freq(), freq_error), end="")
+    print(" duty={} duty_er={}".format(pwm.duty_u16(), duty_error), end="")
+    print(" :", end="")
+    self.assertLessEqual(freq_error, freq_margin_per_thousand)
+    self.assertLessEqual(duty_error, duty_margin_per_thousand)
+
+    # Calculate expected timing.
+    expected_total_us = 1_000_000 // freq
+    expected_high_us = expected_total_us * duty_u16 // 65535
+    expected_low_us = expected_total_us - expected_high_us
+    expected_us = (expected_low_us, expected_high_us)
+    timeout = 2 * expected_total_us
+
+    # Wait for output to settle.
+    time_pulse_us(pulse_in, 0, timeout)
+    time_pulse_us(pulse_in, 1, timeout)
+
+    if duty_u16 == 0 or duty_u16 == 65535:
+        # Expect a constant output level.
+        no_pulse = (
+            time_pulse_us(pulse_in, 0, timeout) < 0 and time_pulse_us(pulse_in, 1, timeout) < 0
+        )
+        self.assertTrue(no_pulse)
+        if expected_high_us == 0:
+            # Expect a constant low level.
+            self.assertEqual(pulse_in(), 0)
+        else:
+            # Expect a constant high level.
+            self.assertEqual(pulse_in(), 1)
+    else:
+        # Test timing of low and high pulse.
+        n_averaging = 10
+        for level in (0, 1):
+            t = 0
+            time_pulse_us(pulse_in, level, timeout)
+            for _ in range(n_averaging):
+                t += time_pulse_us(pulse_in, level, timeout)
+            t //= n_averaging
+            expected = expected_us[level]
+            print(" level={} timing_er={}".format(level, abs(t - expected)), end="")
+            self.assertLessEqual(abs(t - expected), timing_margin_us)
+
+    print()
+
+
+# Test a specific frequency with multiple duty cycles.
+def _test_freq(self, freq):
+    print()
+    self.pwm.freq(freq)
+    for duty in (0, 10, 25, 50, 75, 90, 100):
+        duty_u16 = duty * 65535 // 100
+        if sys.platform == "esp32":
+            # TODO why is this bit needed to get it working on esp32?
+            self.pwm.init(freq=freq, duty_u16=duty_u16)
+            time.sleep(0.1)
+        self.pwm.duty_u16(duty_u16)
+        _test_freq_duty(self, self.pulse_in, self.pwm, freq, duty_u16)
+
+
+# Given a set of pins, this test class will test multiple frequencies and duty cycles.
+class TestBase:
+    @classmethod
+    def setUpClass(cls):
+        print("set up pins:", cls.pwm_pin, cls.pulse_pin)
+        cls.pwm = PWM(cls.pwm_pin)
+        cls.pulse_in = Pin(cls.pulse_pin, Pin.IN)
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.pwm.deinit()
+
+    def test_freq_50(self):
+        _test_freq(self, 50)
+
+    def test_freq_100(self):
+        _test_freq(self, 100)
+
+    def test_freq_500(self):
+        _test_freq(self, 500)
+
+    def test_freq_1000(self):
+        _test_freq(self, 1000)
+
+    @unittest.skipIf(pwm_freq_limit < 2000, "frequency too high")
+    def test_freq_2000(self):
+        _test_freq(self, 2000)
+
+    @unittest.skipIf(pwm_freq_limit < 5000, "frequency too high")
+    def test_freq_5000(self):
+        _test_freq(self, 5000)
+
+    @unittest.skipIf(pwm_freq_limit < 10000, "frequency too high")
+    def test_freq_10000(self):
+        _test_freq(self, 10000)
+
+
+# Generate test classes, one for each set of pins to test.
+for pwm, pulse in pwm_pulse_pins:
+    cls_name = "Test_{}_{}".format(pwm, pulse)
+    globals()[cls_name] = type(
+        cls_name, (TestBase, unittest.TestCase), {"pwm_pin": pwm, "pulse_pin": pulse}
+    )
+
+if __name__ == "__main__":
+    unittest.main()