Voltage divider driver initial implementation

app/boards/arm/nice_nano/nice_nano.dts

@@ -29,6 +29,16 @@
 		};
 	};
 
+	vbatt {
+		compatible = "zmk,battery-voltage-divider";
+		io-channels = <&adc 2>;
+		output-ohms = <2000000>;
+		full-ohms = <(2000000 + 806000)>;
+	};
+};
+
+&adc {
+	status = "okay";
 };
 
 &gpio0 {

app/boards/arm/nice_nano/nice_nano_defconfig

@@ -10,6 +10,10 @@ CONFIG_ARM_MPU=y
 # enable GPIO
 CONFIG_GPIO=y
 
+CONFIG_ADC=y
+
+CONFIG_NEWLIB_LIBC=y
+
 CONFIG_USE_DT_CODE_PARTITION=y
 
 CONFIG_MPU_ALLOW_FLASH_WRITE=y

app/drivers/zephyr/CMakeLists.txt

@@ -5,6 +5,7 @@ if(CONFIG_ZMK_KSCAN_GPIO_DRIVER)
   zephyr_library_sources(
     kscan_gpio_matrix.c
     kscan_gpio_direct.c
+    battery_voltage_divider.c
     )
 
   zephyr_library_sources_ifdef(CONFIG_EC11 ec11.c)

app/drivers/zephyr/battery_voltage_divider.c

@@ -0,0 +1,115 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define DT_DRV_COMPAT zmk_battery_voltage_divider
+
+#include <device.h>
+#include <drivers/kscan.h>
+#include <drivers/gpio.h>
+#include <drivers/adc.h>
+#include <drivers/sensor.h>
+#include <logging/log.h>
+#include <math.h>
+
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#if DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT)
+
+#define VBATT DT_PATH(vbatt)
+
+struct battery_config {
+  struct device *adc;
+  struct adc_channel_cfg acc;
+  struct adc_sequence as;
+  int16_t adc_raw;
+};
+
+static struct battery_config battery_config;
+
+static int lithium_ion_mv_to_pct(int16_t bat_mv) {
+  // Magic function that maps mV to this discharge graph from adafruit: https://learn.adafruit.com/li-ion-and-lipoly-batteries/voltages
+ return round(106.818 + (-0.032685 - 106.818) / pow(1 + pow(bat_mv/3679.35, 58.979), 0.347386));
+}
+
+static void battery_read(struct k_work *workd) {
+  struct battery_config *cfg = &battery_config;
+  struct adc_sequence *as = &cfg->as;
+
+  int rc = adc_read(cfg->adc, as);
+  as->calibrate = false;
+  if (rc == 0) {
+    int32_t val = cfg->adc_raw;
+
+    adc_raw_to_millivolts(adc_ref_internal(cfg->adc), cfg->acc.gain, as->resolution, &val);
+
+    rc = val * (uint64_t)DT_PROP(VBATT, full_ohms) / DT_PROP(VBATT, output_ohms);
+    LOG_DBG("ADC raw %d ~ %d mV => %d mV\n", cfg->adc_raw, val, rc);
+    int percent = lithium_ion_mv_to_pct(rc);
+    LOG_DBG("Percent: %d", percent);
+  } else {
+    LOG_DBG("Failed to read ADC: %d", rc);
+  }
+}
+
+K_WORK_DEFINE(battery_work, battery_read);
+
+static void battery_handler(struct k_timer *timer)
+{
+    k_work_submit(&battery_work);
+}
+
+K_TIMER_DEFINE(battery_tick, battery_handler, NULL);
+
+static int battery_setup(struct device *_arg) {
+  struct battery_config *cfg = &battery_config;
+  struct adc_sequence *as = &cfg->as;
+  struct adc_channel_cfg *acc = &cfg->acc;
+
+  cfg->adc = device_get_binding(DT_IO_CHANNELS_LABEL(VBATT));
+
+  if (cfg->adc == NULL) {
+		LOG_ERR("ADC %s failed to retrieve", DT_IO_CHANNELS_LABEL(VBATT));
+		return -ENOENT;
+	}
+
+  *as = (struct adc_sequence){
+		.channels = BIT(0),
+		.buffer = &cfg->adc_raw,
+		.buffer_size = sizeof(cfg->adc_raw),
+		.oversampling = 4,
+		.calibrate = true,
+	};
+
+#ifdef CONFIG_ADC_NRFX_SAADC
+	*acc = (struct adc_channel_cfg){
+		.gain = ADC_GAIN_1_5,
+		.reference = ADC_REF_INTERNAL,
+		.acquisition_time = ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 40),
+    .input_positive = SAADC_CH_PSELP_PSELP_AnalogInput0 + DT_IO_CHANNELS_INPUT(VBATT),
+	};
+
+	as->resolution = 12;
+#else
+#error Unsupported ADC
+#endif
+
+	int adc_rc = adc_channel_setup(cfg->adc, acc);
+	LOG_DBG("AIN%u setup returned %d", DT_IO_CHANNELS_INPUT(VBATT), adc_rc);
+
+  if (adc_rc != 0) {
+    return adc_rc;
+  }
+
+  k_timer_start(&battery_tick, K_NO_WAIT, K_SECONDS(5));
+
+  return 0;
+}
+
+SYS_INIT(battery_setup,
+        APPLICATION,
+        CONFIG_APPLICATION_INIT_PRIORITY);
+
+#endif /* DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT) */

app/drivers/zephyr/dts/bindings/zmk,battery-voltage-divider.yaml

@@ -0,0 +1,9 @@
+# Copyright (c) 2020 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Battery SoC monitoring using voltage divider
+
+compatible: "zmk,battery-voltage-divider"
+
+include: voltage-divider.yaml
+