pbio/drv/battery/battery_ev3.c: Implement EV3 battery driver

This reads the values from the ADC and scales them according to
the ideal scaling factors.

Author: ArcaneNibble

lib/pbio/drv/battery/battery_ev3.c

@@ -7,6 +7,7 @@
 
 #include <stdbool.h>
 
+#include <pbdrv/adc.h>
 #include <pbdrv/battery.h>
 #include <pbio/error.h>
 #include <pbdrv/gpio.h>
@@ -15,22 +16,40 @@
 #include <tiam1808/hw/hw_syscfg0_AM1808.h>
 
 static const pbdrv_gpio_t battery_type_gpio = PBDRV_GPIO_EV3_PIN(19, 7, 4, 8, 8);
+static const pbdrv_gpio_t battery_voltage_measure_en_gpio = PBDRV_GPIO_EV3_PIN(1, 7, 4, 0, 6);
+
+#define PBDRV_EV3_BATTERY_VOLTAGE_ADC_CH    4
+#define PBDRV_EV3_BATTERY_CURRENT_ADC_CH    3
 
 void pbdrv_battery_init(void) {
     pbdrv_gpio_alt(&battery_type_gpio, SYSCFG_PINMUX19_PINMUX19_7_4_GPIO8_8);
+    pbdrv_gpio_out_high(&battery_voltage_measure_en_gpio);
 }
 
 pbio_error_t pbdrv_battery_get_voltage_now(uint16_t *value) {
-    // TODO. Calculate battery voltage based on ADC channel 4.
-    // For now, return nominal value for now so we don't trigger low battery shutdown.
-    *value = 7200;
+    pbio_error_t err;
+    uint16_t value_raw;
+    if ((err = pbdrv_adc_get_ch(PBDRV_EV3_BATTERY_VOLTAGE_ADC_CH, &value_raw)) != PBIO_SUCCESS) {
+        return err;
+    }
+    // Battery voltage is read through a voltage divider consisting of two
+    // 100 K resistors, which halves the voltage. The ADC returns 10 LSBs,
+    // where full scale is 5 V.
+    *value = ((uint32_t)(value_raw) * 2 * 5000) / 1023;
     return PBIO_SUCCESS;
 }
 
 pbio_error_t pbdrv_battery_get_current_now(uint16_t *value) {
-    // TODO. Calculate battery current based on ADC channel 3.
-    *value = 0;
-    return PBIO_ERROR_NOT_IMPLEMENTED;
+    pbio_error_t err;
+    uint16_t value_raw;
+    if ((err = pbdrv_adc_get_ch(PBDRV_EV3_BATTERY_CURRENT_ADC_CH, &value_raw)) != PBIO_SUCCESS) {
+        return err;
+    }
+    // Battery current is read across a 0.05 ohm equivalent shunt resistor
+    // which is then connected to an op-amp configured with a gain of 16
+    // (non-inverting). This yields 1 A = 0.8 V.
+    *value = ((uint32_t)(value_raw) * 5000 * 10) / (1023 * 8);
+    return PBIO_SUCCESS;
 }
 
 pbio_error_t pbdrv_battery_get_type(pbdrv_battery_type_t *value) {