drivers: sensor: Add STM32 VREF+ sensor

Add VREF+ sensor driver and DT node definition.

This driver allows determining the actual voltage applied to an SoC's
VREF+ pin, by comparing the VREFINT internal bandgap voltage reference
with its factory calibration data.

In packages where VREF+ is bonded to VDDA, this permits direct measurement
of VDDA voltage.

Signed-off-by: Kenneth J. Miller <ken@miller.ec>

drivers/sensor/CMakeLists.txt

@@ -108,6 +108,7 @@ add_subdirectory_ifdef(CONFIG_SM351LT		sm351lt)
 add_subdirectory_ifdef(CONFIG_HAS_STMEMSC	stmemsc)
 add_subdirectory_ifdef(CONFIG_STM32_TEMP	stm32_temp)
 add_subdirectory_ifdef(CONFIG_STM32_VBAT	stm32_vbat)
+add_subdirectory_ifdef(CONFIG_STM32_VREF	stm32_vref)
 add_subdirectory_ifdef(CONFIG_STTS751		stts751)
 add_subdirectory_ifdef(CONFIG_SX9500		sx9500)
 add_subdirectory_ifdef(CONFIG_TH02		    th02)

drivers/sensor/Kconfig

@@ -257,6 +257,8 @@ source "drivers/sensor/stm32_temp/Kconfig"
 
 source "drivers/sensor/stm32_vbat/Kconfig"
 
+source "drivers/sensor/stm32_vref/Kconfig"
+
 source "drivers/sensor/stts751/Kconfig"
 
 source "drivers/sensor/sx9500/Kconfig"

drivers/sensor/stm32_vref/CMakeLists.txt

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
+zephyr_library()
+
+zephyr_library_sources(stm32_vref.c)

drivers/sensor/stm32_vref/Kconfig

@@ -0,0 +1,12 @@
+# STM32 vref sensor configuration options
+
+# Copyright (c) 2023 Kenneth J. Miller <ken@miller.ec>
+# SPDX-License-Identifier: Apache-2.0
+
+config STM32_VREF
+	bool "STM32 VREF Sensor"
+	default y
+	depends on DT_HAS_ST_STM32_VREF_ENABLED
+	select ADC
+	help
+	  Enable driver for STM32 Vref sensor.

drivers/sensor/stm32_vref/stm32_vref.c

@@ -0,0 +1,142 @@
+/*
+ * Copyright (c) 2023 Kenneth J. Miller <ken@miller.ec>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#define DT_DRV_COMPAT st_stm32_vref
+
+#include <zephyr/device.h>
+#include <zephyr/devicetree.h>
+#include <zephyr/drivers/sensor.h>
+#include <zephyr/drivers/adc.h>
+#include <zephyr/logging/log.h>
+#if defined(CONFIG_SOC_SERIES_STM32H5X)
+#include <stm32_ll_icache.h>
+#endif /* CONFIG_SOC_SERIES_STM32H5X */
+
+LOG_MODULE_REGISTER(stm32_vref, CONFIG_SENSOR_LOG_LEVEL);
+
+struct stm32_vref_data {
+	const struct device *adc;
+	const struct adc_channel_cfg adc_cfg;
+	struct adc_sequence adc_seq;
+	struct k_mutex mutex;
+	int16_t sample_buffer;
+	int16_t raw; /* raw adc Sensor value */
+};
+
+struct stm32_vref_config {
+	uint16_t *cal_addr;
+	int cal_mv;
+};
+
+static int stm32_vref_sample_fetch(const struct device *dev, enum sensor_channel chan)
+{
+	struct stm32_vref_data *data = dev->data;
+	struct adc_sequence *sp = &data->adc_seq;
+	int rc;
+
+	if (chan != SENSOR_CHAN_ALL && chan != SENSOR_CHAN_VOLTAGE) {
+		return -ENOTSUP;
+	}
+
+	k_mutex_lock(&data->mutex, K_FOREVER);
+
+	rc = adc_channel_setup(data->adc, &data->adc_cfg);
+	if (rc) {
+		LOG_DBG("Setup AIN%u got %d", data->adc_cfg.channel_id, rc);
+		goto unlock;
+	}
+
+	rc = adc_read(data->adc, sp);
+	if (rc == 0) {
+		data->raw = data->sample_buffer;
+	}
+
+unlock:
+	k_mutex_unlock(&data->mutex);
+
+	return rc;
+}
+
+static int stm32_vref_channel_get(const struct device *dev, enum sensor_channel chan,
+				  struct sensor_value *val)
+{
+	struct stm32_vref_data *data = dev->data;
+	const struct stm32_vref_config *cfg = dev->config;
+	float vref;
+
+	if (chan != SENSOR_CHAN_VOLTAGE) {
+		return -ENOTSUP;
+	}
+
+	if (data->raw == 0) {
+		LOG_ERR("Raw ADC value is zero");
+		return -ENODATA;
+	}
+
+/*
+ * ERRATA: STM32H5X: bus fault errors occur when reading engineering bytes with
+ * icache enabled.
+ * See https://github.com/zephyrproject-rtos/zephyr/commit/065a8f2
+ */
+#if defined(CONFIG_SOC_SERIES_STM32H5X)
+	LL_ICACHE_Disable();
+#endif /* CONFIG_SOC_SERIES_STM32H5X */
+
+	/* Calculate VREF+ using VREFINT bandgap voltage and calibration data */
+	vref = cfg->cal_mv * (*cfg->cal_addr) / data->raw;
+	/* millivolt to volt */
+	vref /= 1000;
+
+#if defined(CONFIG_SOC_SERIES_STM32H5X)
+	LL_ICACHE_Enable();
+#endif /* CONFIG_SOC_SERIES_STM32H5X */
+
+	return sensor_value_from_double(val, vref);
+}
+
+static const struct sensor_driver_api stm32_vref_driver_api = {
+	.sample_fetch = stm32_vref_sample_fetch,
+	.channel_get = stm32_vref_channel_get,
+};
+
+static int stm32_vref_init(const struct device *dev)
+{
+	struct stm32_vref_data *data = dev->data;
+	struct adc_sequence *asp = &data->adc_seq;
+
+	k_mutex_init(&data->mutex);
+
+	if (!device_is_ready(data->adc)) {
+		LOG_ERR("Device %s is not ready", data->adc->name);
+		return -ENODEV;
+	}
+
+	*asp = (struct adc_sequence){
+		.channels = BIT(data->adc_cfg.channel_id),
+		.buffer = &data->sample_buffer,
+		.buffer_size = sizeof(data->sample_buffer),
+		.resolution = 12U,
+	};
+
+	return 0;
+}
+
+static struct stm32_vref_data stm32_vref_dev_data = {
+	.adc = DEVICE_DT_GET(DT_INST_IO_CHANNELS_CTLR(0)),
+	.adc_cfg = {.gain = ADC_GAIN_1,
+		    .reference = ADC_REF_INTERNAL,
+		    .acquisition_time = ADC_ACQ_TIME_MAX,
+		    .channel_id = DT_INST_IO_CHANNELS_INPUT(0),
+		    .differential = 0},
+};
+
+static const struct stm32_vref_config stm32_vref_dev_config = {
+	.cal_addr = (uint16_t *)DT_INST_PROP(0, vrefint_cal_addr),
+	.cal_mv = DT_INST_PROP(0, vrefint_cal_mv),
+};
+
+SENSOR_DEVICE_DT_INST_DEFINE(0, stm32_vref_init, NULL, &stm32_vref_dev_data, &stm32_vref_dev_config,
+			     POST_KERNEL, CONFIG_SENSOR_INIT_PRIORITY, &stm32_vref_driver_api);

dts/bindings/sensor/st,stm32-vref.yaml

@@ -0,0 +1,23 @@
+# Copyright (c) 2023 Kenneth J. Miller <ken@miller.ec>
+# SPDX-License-Identifier: Apache-2.0
+
+description: STM32 family VREF+ node
+
+compatible: "st,stm32-vref"
+
+include: sensor-device.yaml
+
+properties:
+  vrefint-cal-addr:
+    type: int
+    required: true
+    description: |
+      Device engineering bytes address containing the factory-measured
+      calibration bandgap voltage (VREFINT_CAL).
+
+  vrefint-cal-mv:
+    type: int
+    required: true
+    description: |
+      VDDA/VREF+ voltage in millivolts applied during manufacturing to determine
+      the internal bandgap voltage reference VREFINT.