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/read_vsys.c

@@ -0,0 +1,75 @@
+/**
+ * Copyright (c) 2022 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <stdio.h>
+#include <pico/stdlib.h>
+#include <power_status.h>
+#include "hardware/adc.h"
+#include "pico/float.h"
+
+#if CYW43_USES_VSYS_PIN
+#include "pico/cyw43_arch.h"
+#endif
+
+int main() {
+    stdio_init_all();
+
+    adc_init();
+    adc_set_temp_sensor_enabled(true);
+
+    // Pico W uses a CYW43 pin to get VBUS so we need to initialise it
+    #if CYW43_USES_VSYS_PIN
+    if (cyw43_arch_init()) {
+        printf("failed to initialise\n");
+        return 1;
+    }
+    #endif
+
+    bool old_battery_status;
+    float old_voltage;
+    bool battery_status = true;
+    char *power_str = "UNKNOWN";
+
+    while(true) {
+        // Get battery status
+        if (power_source(&battery_status) == PICO_OK) {
+            power_str = battery_status ? "BATTERY" : "POWERED";
+        }
+
+        // Get voltage
+        float voltage = 0;
+        int voltage_return = power_voltage(&voltage);
+        voltage = floorf(voltage * 100) / 100;
+
+        // Display power if it's changed
+        if (old_battery_status != battery_status || old_voltage != voltage) {
+            char percent_buf[10] = {0};
+            if (battery_status && voltage_return == PICO_OK) {
+                const float min_battery_volts = 3.0f;
+                const float max_battery_volts = 4.2f;
+                int percent_val = ((voltage - min_battery_volts) / (max_battery_volts - min_battery_volts)) * 100;
+                snprintf(percent_buf, sizeof(percent_buf), " (%d%%)", percent_val);
+            }
+
+            // Also get the temperature
+            adc_select_input(4);
+            const float conversionFactor = 3.3f / (1 << 12);
+            float adc = (float)adc_read() * conversionFactor;
+            float tempC = 27.0f - (adc - 0.706f) / 0.001721f;
+
+            // Display power and remember old vales
+            printf("Power %s, %.2fV%s, temp %.1f DegC\n", power_str, voltage, percent_buf, tempC);
+            old_battery_status = battery_status;
+            old_voltage = voltage;
+        }
+        sleep_ms(1000);
+    }
+
+#if CYW43_USES_VSYS_PIN
+    cyw43_arch_deinit();
+#endif
+    return 0;
+}