Add an example to read VBUS and VSYS (#331)

The process is different on Pico and Pico W so demonstrate how to do it.

Fixes #324

adc/read_vsys/power_status.c

@@ -0,0 +1,78 @@
+/**
+ * Copyright (c) 2023 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include "stdbool.h"
+#include "hardware/adc.h"
+#include "power_status.h"
+
+#if CYW43_USES_VSYS_PIN
+#include "pico/cyw43_arch.h"
+#endif
+
+#ifndef PICO_POWER_SAMPLE_COUNT
+#define PICO_POWER_SAMPLE_COUNT 3
+#endif
+
+// Pin used for ADC 0
+#define PICO_FIRST_ADC_PIN 26
+
+int power_source(bool *battery_powered) {
+#if defined CYW43_WL_GPIO_VBUS_PIN
+    *battery_powered = !cyw43_arch_gpio_get(CYW43_WL_GPIO_VBUS_PIN);
+    return PICO_OK;
+#elif defined PICO_VBUS_GPIO_PIN
+    gpio_set_function(PICO_VBUS_GPIO_PIN, GPIO_FUNC_SIO);
+    *battery_powered = !gpio_get(PICO_VBUS_GPIO_PIN);
+    return PICO_OK;
+#else
+    return PICO_ERROR_NO_DATA;
+#endif
+}
+
+int power_voltage(float *voltage_result) {
+#ifndef PICO_VSYS_PIN
+    return PICO_ERROR_NO_DATA;
+#endif
+#if CYW43_USES_VSYS_PIN
+    cyw43_thread_enter();
+    // Make sure cyw43 is awake
+    cyw43_arch_gpio_get(CYW43_WL_GPIO_VBUS_PIN);
+#endif
+
+    // setup adc
+    adc_gpio_init(PICO_VSYS_PIN);
+    adc_select_input(PICO_VSYS_PIN - PICO_FIRST_ADC_PIN);
+ 
+    adc_fifo_setup(true, false, 0, false, false);
+    adc_run(true);
+
+#if CYW43_USES_VSYS_PIN
+    // We seem to read low values from cyw43 sometimes - this seems to fix it
+    int ignore_count = PICO_POWER_SAMPLE_COUNT;
+    while (!adc_fifo_is_empty() || ignore_count-- > 0) {
+        (void)adc_fifo_get_blocking();
+    }
+#endif
+
+    // read vsys
+    uint32_t vsys = 0;
+    for(int i = 0; i < PICO_POWER_SAMPLE_COUNT; i++) {
+        uint16_t val = adc_fifo_get_blocking();
+        vsys += val;
+    }
+
+    adc_run(false);
+    adc_fifo_drain();
+
+    vsys /= PICO_POWER_SAMPLE_COUNT;
+#if CYW43_USES_VSYS_PIN
+    cyw43_thread_exit();
+#endif
+    // Generate voltage
+    const float conversion_factor = 3.3f / (1 << 12);
+    *voltage_result = vsys * 3 * conversion_factor;
+    return PICO_OK;
+}