diff --git a/Makefile b/Makefile
index 15d79641..ac3d4a8b 100644
--- a/Makefile
+++ b/Makefile
@@ -15,6 +15,7 @@ include $(GOSSAMER_PATH)/make.mk
 SRCS += \
   ./app.c \
   ./watch-library/hardware/watch/watch.c \
+  ./watch-library/hardware/watch/watch_adc.c \
   ./watch-library/hardware/watch/watch_buzzer.c \
   ./watch-library/hardware/watch/watch_extint.c \
   ./watch-library/hardware/watch/watch_led.c \
diff --git a/watch-library/hardware/watch/watch_adc.c b/watch-library/hardware/watch/watch_adc.c
new file mode 100644
index 00000000..11cc318c
--- /dev/null
+++ b/watch-library/hardware/watch/watch_adc.c
@@ -0,0 +1,94 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020 Joey Castillo
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include "watch_adc.h"
+#include "adc.h"
+
+void watch_enable_adc(void) {
+    adc_init();
+    adc_enable();
+}
+
+void watch_enable_analog_input(const uint16_t port_pin) {
+    uint8_t port = port_pin >> 8;
+    uint16_t pin = port_pin & 0xF;
+
+    PORT->Group[port].DIRCLR.reg = (1 << pin);
+    PORT->Group[port].PINCFG[pin].reg |= PORT_PINCFG_INEN;
+    PORT->Group[port].PINCFG[pin].reg &= ~PORT_PINCFG_PULLEN;
+    PORT->Group[port].PINCFG[pin].reg |= PORT_PINCFG_PMUXEN;
+    if (pin & 1) {
+        PORT->Group[port].PMUX[pin>>1].bit.PMUXO = HAL_GPIO_PMUX_ADC;
+    } else {
+        PORT->Group[port].PMUX[pin>>1].bit.PMUXE = HAL_GPIO_PMUX_ADC;
+    }
+}
+
+uint16_t watch_get_analog_pin_level(const uint16_t pin) {
+    return adc_get_analog_value(pin);
+}
+
+/// TODO: put reference voltage stuff into gossamer?
+void _watch_set_analog_reference_voltage(uint8_t reference);
+void _watch_set_analog_reference_voltage(uint8_t reference) {
+    ADC->CTRLA.bit.ENABLE = 0;
+
+    if (reference == ADC_REFCTRL_REFSEL_INTREF_Val) SUPC->VREF.bit.VREFOE = 1;
+    else SUPC->VREF.bit.VREFOE = 0;
+
+    ADC->REFCTRL.bit.REFSEL = reference;
+    ADC->CTRLA.bit.ENABLE = 1;
+    while (ADC->SYNCBUSY.reg);
+    // throw away one measurement after reference change (the channel doesn't matter).
+    adc_get_analog_value_for_channel(ADC_INPUTCTRL_MUXPOS_SCALEDCOREVCC);
+}
+
+uint16_t watch_get_vcc_voltage(void) {
+    // 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_REFCTRL_REFSEL_INTREF_Val) _watch_set_analog_reference_voltage(ADC_REFCTRL_REFSEL_INTREF_Val);
+
+    // get the data
+    uint32_t raw_val = adc_get_analog_value_for_channel(ADC_INPUTCTRL_MUXPOS_SCALEDIOVCC_Val);
+
+    // restore the old reference, if needed.
+    if (oldref != ADC_REFCTRL_REFSEL_INTREF_Val) _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 uint16_t port_pin) {
+    uint8_t port = port_pin >> 8;
+    uint16_t pin = port_pin & 0xF;
+
+    PORT->Group[port].DIRSET.reg = (1 << pin);			\
+    PORT->Group[port].PINCFG[pin].reg &= ~(PORT_PINCFG_PULLEN | PORT_PINCFG_INEN);	\
+    PORT->Group[port].PINCFG[pin].reg &= ~PORT_PINCFG_PMUXEN;	\
+}
+
+inline void watch_disable_adc(void) {
+    adc_disable();
+}
diff --git a/watch-library/hardware/watch/watch_private.c b/watch-library/hardware/watch/watch_private.c
index 71461ce2..fbba4059 100644
--- a/watch-library/hardware/watch/watch_private.c
+++ b/watch-library/hardware/watch/watch_private.c
@@ -23,6 +23,8 @@
  */
 
 #include "watch_private.h"
+#include "watch_adc.h"
+#include "adc.h"
 #include "tcc.h"
 #include "tc.h"
 #include "usb.h"
@@ -45,7 +47,6 @@ void _watch_init(void) {
     SUPC->VREG.bit.STDBYPL0 = 1;
     while(!SUPC->STATUS.bit.VREGRDY); // wait for voltage regulator to become ready
 
-    /** TODO: check the battery voltage...
     watch_enable_adc();
     uint16_t battery_voltage = watch_get_vcc_voltage();
     watch_disable_adc();
@@ -56,7 +57,6 @@ void _watch_init(void) {
     } else {
         SUPC->VREG.bit.LPEFF = 0;
     }
- */
 
     // set up the brownout detector (low battery warning)
     NVIC_DisableIRQ(SYSTEM_IRQn);
diff --git a/watch-library/shared/watch/watch_adc.h b/watch-library/shared/watch/watch_adc.h
new file mode 100644
index 00000000..b2685da9
--- /dev/null
+++ b/watch-library/shared/watch/watch_adc.h
@@ -0,0 +1,79 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020 Joey Castillo
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#pragma once
+
+////< @file watch_adc.h
+
+#include "watch.h"
+
+/** @addtogroup adc Analog Input
+  * @brief This section covers functions related to the SAM L22's analog-to-digital converter,
+  *        as well as configuring and reading values from the five analog-capable pins on the
+  *        9-pin connector.
+  */
+/// @{
+/** @brief Enables the ADC peripheral. You must call this before attempting to read a value
+  *        from an analog pin.
+  */
+void watch_enable_adc(void);
+
+/** @brief Configures the selected pin for analog input.
+  * @param pin One of the analog pins, access using the HAL_GPIO_Ax_pin() macro.
+  */
+void watch_enable_analog_input(const uint16_t pin);
+
+/** @brief Reads an analog value from one of the pins.
+  * @param pin One of the analog pins, access using the HAL_GPIO_Ax_pin() macro.
+  * @return a 16-bit unsigned integer from 0-65535 representing the sampled value, unless you
+  *         have changed the number of samples. @see watch_set_num_analog_samples for details
+  *         on how that function changes the values returned from this one.
+  **/
+uint16_t watch_get_analog_pin_level(const uint16_t pin);
+
+/** @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(void);
+
+/** @brief Disables the analog circuitry on the selected pin.
+  * @param pin One of pins A0-A4.
+  */
+void watch_disable_analog_input(const uint16_t pin);
+
+/** @brief Disables the ADC peripheral.
+  * @note You will need to call watch_enable_adc to re-enable the ADC peripheral. When you do, it will
+  *       have the default settings of 16 samples and 1 measurement cycle; if you customized these
+  *       parameters, you will need to set them up again.
+  **/
+void watch_disable_adc(void);
+
+/// @}