From 0f03257ee9ef46ac61e20f7201e2c50dedff01b2 Mon Sep 17 00:00:00 2001
From: Joey Castillo <jose.castillo@gmail.com>
Date: Sat, 6 Nov 2021 23:52:00 -0400
Subject: movement: add voltage monitor watch face
---
watch-library/watch/watch_adc.c | 29 +++++++++++++++++++++++++
watch-library/watch/watch_adc.h | 47 +++++++++++++++++++++++++++++++++++++++++
2 files changed, 76 insertions(+)
(limited to 'watch-library')
diff --git a/watch-library/watch/watch_adc.c b/watch-library/watch/watch_adc.c
index 90980a88..4aff86e6 100644
--- a/watch-library/watch/watch_adc.c
+++ b/watch-library/watch/watch_adc.c
@@ -138,6 +138,35 @@ void watch_set_analog_sampling_length(uint8_t cycles) {
_watch_sync_adc();
}
+void watch_set_analog_reference_voltage(watch_adc_reference_voltage reference) {
+ ADC->CTRLA.bit.ENABLE = 0;
+
+ if (reference == ADC_REFERENCE_INTREF) SUPC->VREF.bit.VREFOE = 1;
+ else SUPC->VREF.bit.VREFOE = 0;
+
+ ADC->REFCTRL.bit.REFSEL = reference;
+ ADC->CTRLA.bit.ENABLE = 1;
+ _watch_sync_adc();
+ // throw away one measurement after reference change (the channel doesn't matter).
+ _watch_get_analog_value(ADC_INPUTCTRL_MUXPOS_SCALEDCOREVCC);
+}
+
+uint16_t watch_get_vcc_voltage() {
+ // stash the previous reference so we can restore it when we're done.
+ uint8_t oldref = ADC->REFCTRL.bit.REFSEL;
+
+ // if we weren't already using the internal reference voltage, select it now.
+ if (oldref != ADC_REFERENCE_INTREF) watch_set_analog_reference_voltage(ADC_REFERENCE_INTREF);
+
+ // get the data
+ uint32_t raw_val = _watch_get_analog_value(ADC_INPUTCTRL_MUXPOS_SCALEDIOVCC_Val);
+
+ // restore the old reference, if needed.
+ if (oldref != ADC_REFERENCE_INTREF) watch_set_analog_reference_voltage(oldref);
+
+ return (uint16_t)((raw_val * 1000) / (1024 * 1 << ADC->AVGCTRL.bit.SAMPLENUM));
+}
+
inline void watch_disable_analog_input(const uint8_t pin) {
gpio_set_pin_function(pin, GPIO_PIN_FUNCTION_OFF);
}
diff --git a/watch-library/watch/watch_adc.h b/watch-library/watch/watch_adc.h
index c9b6ad2a..a4f9edad 100644
--- a/watch-library/watch/watch_adc.h
+++ b/watch-library/watch/watch_adc.h
@@ -94,6 +94,53 @@ void watch_set_analog_num_samples(uint16_t samples);
**/
void watch_set_analog_sampling_length(uint8_t cycles);
+typedef enum {
+ ADC_REFERENCE_INTREF = ADC_REFCTRL_REFSEL_INTREF_Val,
+ ADC_REFERENCE_VCC_DIV1POINT6 = ADC_REFCTRL_REFSEL_INTVCC0_Val,
+ ADC_REFERENCE_VCC_DIV2 = ADC_REFCTRL_REFSEL_INTVCC1_Val,
+ ADC_REFERENCE_VCC = ADC_REFCTRL_REFSEL_INTVCC2_Val,
+} watch_adc_reference_voltage;
+
+
+/** @brief Selects the reference voltage to use for analog readings. Default is ADC_REFERENCE_VCC.
+ * @param reference One of ADC_REFERENCE_VCC, ADC_REFERENCE_VCC_DIV1POINT6, ADC_REFERENCE_VCC_DIV2
+ * or ADC_REFERENCE_INTREF.
+ * @details In order to turn an analog voltage into a 16-bit integer, the ADC needs to compare the
+ * measured voltage to a reference point. For example, if you were powering the watch with
+ * VCC == 3.0V and you had two 10K resistors connected in series from 3V to GND, you could
+ * expect to get 3 volts when you measure the top of the voltage divider, 0 volts at the
+ * bottom, and 1.5 volts in the middle. If you read these values uising a reference voltage
+ * of ADC_REFERENCE_VCC, the top value would be about 65535, the bottom about 0, and the
+ * middle about 32768. However! If we used ADC_REFERENCE_VCC_DIV2 as our reference, we would
+ * expect to get 65535 both at the top and the middle, because the largest value the ADC can
+ * measure in this configutation is 1.5V (VCC / 2).
+ *
+ * By changing the reference voltage from ADC_REFERENCE_VCC to ADC_REFERENCE_VCC_DIV1POINT6
+ * or ADC_REFERENCE_VCC_DIV2, you can get more resolution when measuring small voltages (i.e.
+ * a phototransistor circuit in low light).
+ *
+ * There is also a special reference voltage called ADC_REFERENCE_INTREF. The SAM L22's
+ * Supply Controller provides a selectable voltage reference (by default, 1.024 V) that you
+ * can select as a reference voltage for ADC conversions. Unlike the three references we
+ * talked about in the last paragraph, this reference voltage does not depend on VCC, which
+ * makes it very useful for measuring the battery voltage (since you can't really compare
+ * VCC to itself). You can change the INTREF voltage to 2.048 or 4.096 V by poking at the
+ * supply controller's VREF register, but the watch library does not support this use case.
+ **/
+void watch_set_analog_reference_voltage(watch_adc_reference_voltage reference);
+
+/** @brief Returns the voltage of the VCC supply in millivolts (i.e. 3000 mV == 3.0 V). If running on
+ * a coin cell, this will be the battery voltage.
+ * @details Unlike other ADC functions, this function does not return a raw value from the ADC, but
+ * rather scales it to an actual number of millivolts. This is because the ADC doesn't let
+ * us measure VCC per se; it instead lets us measure VCC / 4, and we choose to measure it
+ * against the internal reference voltage of 1.024 V. In short, the ADC gives us a number
+ * that's complicated to deal with, so we just turn it into a useful number for you :)
+ * @note This function depends on INTREF being 1.024V. If you have changed it by poking at the supply
+ * controller's VREF.SEL bits, this function will return inaccurate values.
+ */
+uint16_t watch_get_vcc_voltage();
+
/** @brief Disables the analog circuitry on the selected pin.
* @param pin One of pins A0-A4.
*/
--
cgit v1.2.3