[nrf fromlist] drivers: adc: nrfx: enable negative values for single-ended ADC readings

The ADC driver API already supports ADC readings which can return signed
values, these are differential readings. In Nordic's datasheet, we have
a mode called "single ended", but its just a name. "Single ended"
is a differential reading, with the negative channel tied to GND. This is
not compatible with zephyrs definition of a single ended reading.

To support Nordic's "single ended" mode, the user must configure
a differential reading, with the negative input tied to ground, which the
saadc driver can then use to configure the reading as Nordic SAADC
"single ended", and return negative values as expected.

Upstream PR #: 94069

Signed-off-by: Jakub Zymelka <[email protected]>
(cherry picked from commit 84ab620)

Author: jaz1-nordic

drivers/adc/adc_nrfx_saadc.c

@@ -191,22 +191,24 @@ static int input_assign(nrf_saadc_input_t *pin_p,
 
 	if (channel_cfg->differential) {
 		if (channel_cfg->input_negative > ARRAY_SIZE(saadc_psels) ||
-		    channel_cfg->input_negative < NRF_SAADC_AIN0 ||
 		    (IS_ENABLED(CONFIG_NRF_PLATFORM_HALTIUM) &&
 		     (channel_cfg->input_positive > NRF_SAADC_AIN7) !=
 		     (channel_cfg->input_negative > NRF_SAADC_AIN7))) {
 			LOG_ERR("Invalid analog negative input number: %d",
 				channel_cfg->input_negative);
 			return -EINVAL;
 		}
-		*pin_n = saadc_psels[channel_cfg->input_negative];
+		*pin_n = channel_cfg->input_negative == NRF_SAADC_GND ?
+			 NRF_SAADC_INPUT_DISABLED :
+			 saadc_psels[channel_cfg->input_negative];
 	} else {
 		*pin_n = NRF_SAADC_INPUT_DISABLED;
 	}
 #else
 	*pin_p = channel_cfg->input_positive;
-	*pin_n = channel_cfg->differential ? channel_cfg->input_negative
-					   : NRF_SAADC_INPUT_DISABLED;
+	*pin_n = (channel_cfg->differential && (channel_cfg->input_negative != NRF_SAADC_GND))
+			 ? channel_cfg->input_negative
+			 : NRF_SAADC_INPUT_DISABLED;
 #endif
 	LOG_DBG("ADC positive input: %d", *pin_p);
 	LOG_DBG("ADC negative input: %d", *pin_n);
@@ -356,8 +358,14 @@ static int adc_nrfx_channel_setup(const struct device *dev,
 	 * after ADC sequence ends.
 	 */
 	if (channel_cfg->differential) {
-		ch_cfg->mode = NRF_SAADC_MODE_DIFFERENTIAL;
-		m_data.single_ended_channels &= ~BIT(channel_cfg->channel_id);
+		if (channel_cfg->input_negative == NRF_SAADC_GND) {
+			ch_cfg->mode = NRF_SAADC_MODE_SINGLE_ENDED;
+			/* Do not mark as single-ended to not correct negative values. */
+			m_data.single_ended_channels &= ~BIT(channel_cfg->channel_id);
+		} else {
+			ch_cfg->mode = NRF_SAADC_MODE_DIFFERENTIAL;
+			m_data.single_ended_channels &= ~BIT(channel_cfg->channel_id);
+		}
 	} else {
 		ch_cfg->mode = NRF_SAADC_MODE_SINGLE_ENDED;
 		m_data.single_ended_channels |= BIT(channel_cfg->channel_id);

include/zephyr/dt-bindings/adc/nrf-saadc.h

@@ -7,6 +7,30 @@
 #ifndef ZEPHYR_INCLUDE_DT_BINDINGS_ADC_NRF_SAADC_H_
 #define ZEPHYR_INCLUDE_DT_BINDINGS_ADC_NRF_SAADC_H_
 
+/**
+ * @brief Short ADC negative input to ground
+ *
+ * @details The nRF SAADC hardware only supports differential readings.
+ * The nRF SAADC SE (single ended) mode is differential with the negative
+ * input shorted to GND (ground). To use the nRF SAADC SE mode, set the
+ * negative input to NRF_SAADC_GND:
+ *
+ * @code{.dts}
+ *   zephyr,input-positive = <NRF_SAADC_AIN3>;
+ *   zephyr,input-negative = <NRF_SAADC_GND>;
+ * @endcode
+ *
+ * The nRF SAADC driver also supports using the nRF SAADC SE mode in
+ * emulated "single-ended" mode, as defined by zephyr. In this mode,
+ * negative readings will be clamped to 0 by software to emulate the
+ * behavior of an ADC in "single-ended" mode, as defined by zephyr. To
+ * do this, only define the positive input:
+ *
+ * @code{.dts}
+ *   zephyr,input-positive = <NRF_SAADC_AIN3>;
+ * @endcode
+ */
+#define NRF_SAADC_GND  0
 #define NRF_SAADC_AIN0 1
 #define NRF_SAADC_AIN1 2
 #define NRF_SAADC_AIN2 3