factor out adc battery measurement

idk if this should be classy or if this is fine. maybe should just pick
a convention at some point?

main/CMakeLists.txt

@@ -1,2 +1,2 @@
-idf_component_register(SRCS "gay-ipod-fw.cpp" "gpio-expander.cpp"
+idf_component_register(SRCS "gay-ipod-fw.cpp" "gpio-expander.cpp" "battery.cpp"
                     INCLUDE_DIRS ".")

main/battery.cpp

@@ -0,0 +1,31 @@
+#include "battery.c"
+
+#include "driver/adc.h"
+#include "esp_adc_cal.h"
+#include "hal/adc_types.h"
+
+namespace gay_ipod {
+
+static esp_adc_cal_characteristics_t calibration;
+
+esp_error_t init_adc(void) {
+  // Calibration should already be fused into the chip from the factory, so
+  // we should only need to read it back out again.
+  esp_adc_cal_characterize(
+      ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 0, &calibration);
+
+  // Max battery voltage should be a little over 2V due to our divider, so
+  // we need the max attenuation to properly handle the full range.
+  adc1_config_width(ADC_WIDTH_BIT_12);
+  adc1_config_channel_atten(ADC1_CHANNEL_6, ADC_ATTEN_DB_11);
+
+  return ESP_OK;
+}
+
+uint32_t read_battery_voltage(void) {
+  // GPIO 34
+  int raw = adc1_get_raw(ADC1_CHANNEL_6);
+  return esp_adc_cal_raw_to_voltage(raw, &calibration);
+}
+
+} // namespace gay_ipod

main/battery.h

@@ -0,0 +1,15 @@
+#ifndef BATTERY_H
+#define BATTERY_H
+
+namespace gay_ipod {
+
+esp_error_t init_adc(void);
+
+/**
+ * Returns the current battery level in millivolts.
+ */
+uint32_t read_battery_voltage(void);
+
+} // namespace gay_ipod
+
+#endif

main/gay-ipod-fw.cpp

@@ -8,7 +8,7 @@
 #include "esp_adc_cal.h"
 #include "esp_log.h"
 #include "gpio-expander.h"
-#include "gpio-expander.h"
+#include "battery.h"
 #include "hal/adc_types.h"
 #include "hal/gpio_types.h"
 #include "hal/spi_types.h"
@@ -97,20 +97,14 @@ extern "C" void app_main(void)
   ESP_ERROR_CHECK(expander.Write());
 
   ESP_LOGI(TAG, "Init ADC");
+  ESP_ERROR_CHECK(init_adc());
 
-  // Attentuate our battery voltage pin to read between 150mV and 2450mV.
-  esp_adc_cal_characteristics_t adc1_chars;
-  esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 0, &adc1_chars);
-  adc1_config_width(ADC_WIDTH_BIT_12);
-  adc1_config_channel_atten(ADC1_CHANNEL_6, ADC_ATTEN_DB_11);
 
   while (1) {
-    int adc_raw = adc1_get_raw(ADC1_CHANNEL_6);
+    uint32_t battery = read_battery_voltage();
-    uint32_t adc_millivolts = esp_adc_cal_raw_to_voltage(adc_raw, &adc1_chars);
-
     expander.Read();
 
-    ESP_LOGI(TAG, "raw adc: %d, millivolts: %d, wall power? %d", adc_raw, adc_millivolts, expander.charge_power_ok());
+    ESP_LOGI(TAG, "millivolts: %d, wall power? %d", battery, expander.charge_power_ok());
 
     vTaskDelay(pdMS_TO_TICKS(1000));
     ESP_ERROR_CHECK(expander.Write());