Initial Release

pwm/measure_duty_cycle/measure_duty_cycle.c

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+/**
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <stdio.h>
+
+#include "pico/stdlib.h"
+#include "hardware/pwm.h"
+#include "hardware/clocks.h"
+
+// This example drives a PWM output at a range of duty cycles, and uses
+// another PWM slice in input mode to measure the duty cycle. You'll need to
+// connect these two pins with a jumper wire:
+const uint OUTPUT_PIN = 2;
+const uint MEASURE_PIN = 5;
+
+float measure_duty_cycle(uint gpio) {
+    // Only the PWM B pins can be used as inputs.
+    assert(pwm_gpio_to_channel(gpio) == PWM_CHAN_B);
+    uint slice_num = pwm_gpio_to_slice_num(gpio);
+
+    // Count once for every 100 cycles the PWM B input is high
+    pwm_config cfg = pwm_get_default_config();
+    pwm_config_set_clkdiv_mode(&cfg, PWM_DIV_B_HIGH);
+    pwm_config_set_clkdiv(&cfg, 100);
+    pwm_init(slice_num, &cfg, false);
+    gpio_set_function(gpio, GPIO_FUNC_PWM);
+
+    pwm_set_enabled(slice_num, true);
+    sleep_ms(10);
+    pwm_set_enabled(slice_num, false);
+    float counting_rate = clock_get_hz(clk_sys) / 100;
+    float max_possible_count = counting_rate * 0.01;
+    return pwm_get_counter(slice_num) / max_possible_count;
+}
+
+const float test_duty_cycles[] = {
+        0.f,
+        0.1f,
+        0.5f,
+        0.9f,
+        1.f
+};
+
+int main() {
+    stdio_init_all();
+    printf("\nPWM duty cycle measurement example\n");
+
+    // Configure PWM slice and set it running
+    const uint count_top = 1000;
+    pwm_config cfg = pwm_get_default_config();
+    pwm_config_set_wrap(&cfg, count_top);
+    pwm_init(pwm_gpio_to_slice_num(OUTPUT_PIN), &cfg, true);
+
+    // Note we aren't touching the other pin yet -- PWM pins are outputs by
+    // default, but change to inputs once the divider mode is changed from
+    // free-running. It's not wise to connect two outputs directly together!
+    gpio_set_function(OUTPUT_PIN, GPIO_FUNC_PWM);
+
+    // For each of our test duty cycles, drive the output pin at that level,
+    // and read back the actual output duty cycle using the other pin. The two
+    // values should be very close!
+    for (int i = 0; i < count_of(test_duty_cycles); ++i) {
+        float output_duty_cycle = test_duty_cycles[i];
+        pwm_set_gpio_level(OUTPUT_PIN, output_duty_cycle * (count_top + 1));
+        float measured_duty_cycle = measure_duty_cycle(MEASURE_PIN);
+        printf("Output duty cycle = %.1f%%, measured input duty cycle = %.1f%%\n",
+               output_duty_cycle * 100.f, measured_duty_cycle * 100.f);
+    }
+}