diff --git a/src/samd21/Kconfig b/src/samd21/Kconfig
index 225eae17..18bd7d81 100644
--- a/src/samd21/Kconfig
+++ b/src/samd21/Kconfig
@@ -6,6 +6,7 @@ config SAMD_SELECT
     bool
     default y
     select HAVE_GPIO
+    select HAVE_GPIO_ADC
     select HAVE_GPIO_I2C
     select HAVE_GPIO_SPI
     select HAVE_GPIO_HARD_PWM
diff --git a/src/samd21/Makefile b/src/samd21/Makefile
index abe21013..2bcf9151 100644
--- a/src/samd21/Makefile
+++ b/src/samd21/Makefile
@@ -20,6 +20,7 @@ src-y += generic/armcm_irq.c generic/timer_irq.c
 src-y += ../lib/samd21/samd21a/gcc/gcc/startup_samd21.c
 src-$(CONFIG_USBSERIAL) += samd21/usbserial.c generic/usb_cdc.c
 src-$(CONFIG_SERIAL) += samd21/serial.c generic/serial_irq.c
+src-$(CONFIG_HAVE_GPIO_ADC) += samd21/adc.c
 src-$(CONFIG_HAVE_GPIO_I2C) += samd21/i2c.c
 src-$(CONFIG_HAVE_GPIO_SPI) += samd21/spi.c
 src-$(CONFIG_HAVE_GPIO_HARD_PWM) += samd21/hard_pwm.c
diff --git a/src/samd21/adc.c b/src/samd21/adc.c
new file mode 100644
index 00000000..38836592
--- /dev/null
+++ b/src/samd21/adc.c
@@ -0,0 +1,121 @@
+// Analog to Digital Converter support
+//
+// Copyright (C) 2018  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "command.h" // shutdown
+#include "gpio.h" // gpio_adc_read
+#include "internal.h" // GPIO
+#include "samd21.h" // ADC
+#include "sched.h" // sched_shutdown
+
+static const uint8_t adc_pins[] = {
+    GPIO('A', 2), GPIO('A', 3), GPIO('B', 8), GPIO('B', 9), GPIO('A', 4),
+    GPIO('A', 5), GPIO('A', 6), GPIO('A', 7), GPIO('B', 0), GPIO('B', 1),
+    GPIO('B', 2), GPIO('B', 3), GPIO('B', 4), GPIO('B', 5), GPIO('B', 6),
+    GPIO('B', 7), GPIO('A', 8), GPIO('A', 9), GPIO('A', 10), GPIO('A', 11)
+};
+
+DECL_CONSTANT(ADC_MAX, 4095);
+
+static void
+adc_init(void)
+{
+    static uint8_t have_run_init;
+    if (have_run_init)
+        return;
+    have_run_init = 1;
+
+    // Enable adc clock
+    enable_pclock(ADC_GCLK_ID, PM_APBCMASK_ADC);
+
+    // Load calibraiton info
+    uint32_t v = *((uint32_t*)ADC_FUSES_BIASCAL_ADDR);
+    uint32_t bias = (v & ADC_FUSES_BIASCAL_Msk) >> ADC_FUSES_BIASCAL_Pos;
+    v = *((uint32_t*)ADC_FUSES_LINEARITY_0_ADDR);
+    uint32_t li0 = (v & ADC_FUSES_LINEARITY_0_Msk) >> ADC_FUSES_LINEARITY_0_Pos;
+    v = *((uint32_t*)ADC_FUSES_LINEARITY_1_ADDR);
+    uint32_t li5 = (v & ADC_FUSES_LINEARITY_1_Msk) >> ADC_FUSES_LINEARITY_1_Pos;
+    uint32_t lin = li0 | (li5 << 5);
+    ADC->CALIB.reg = ADC_CALIB_BIAS_CAL(bias) | ADC_CALIB_LINEARITY_CAL(lin);
+
+    // Setup and enable adc
+    ADC->REFCTRL.reg = ADC_REFCTRL_REFSEL_INTVCC1;
+    ADC->CTRLB.reg = ADC_CTRLB_PRESCALER_DIV128;
+    ADC->SAMPCTRL.reg = 63;
+    ADC->CTRLA.reg = ADC_CTRLA_ENABLE;
+}
+
+struct gpio_adc
+gpio_adc_setup(uint8_t pin)
+{
+    // Find pin in adc_pins table
+    uint8_t chan;
+    for (chan=0; ; chan++) {
+        if (chan >= ARRAY_SIZE(adc_pins))
+            shutdown("Not a valid ADC pin");
+        if (adc_pins[chan] == pin)
+            break;
+    }
+
+    // Enable ADC
+    adc_init();
+
+    // Set pin in ADC mode
+    gpio_peripheral(pin, 'B', 0);
+
+    return (struct gpio_adc){ chan };
+}
+
+enum { ADC_DUMMY=0xff };
+static uint8_t last_analog_read = ADC_DUMMY;
+
+// Try to sample a value. Returns zero if sample ready, otherwise
+// returns the number of clock ticks the caller should wait before
+// retrying this function.
+uint32_t
+gpio_adc_sample(struct gpio_adc g)
+{
+    if (last_analog_read == g.chan) {
+        if (ADC->INTFLAG.reg & ADC_INTFLAG_RESRDY)
+            // Sample now ready
+            return 0;
+        // ADC is still busy
+        goto need_delay;
+    }
+    if (last_analog_read != ADC_DUMMY)
+        // Sample on another channel in progress
+        goto need_delay;
+    last_analog_read = g.chan;
+
+    // Set the channel to sample
+    ADC->INPUTCTRL.reg = (ADC_INPUTCTRL_MUXPOS(g.chan)
+                          | ADC_INPUTCTRL_MUXNEG_GND | ADC_INPUTCTRL_GAIN_DIV2);
+
+    // Start the sample
+    ADC->SWTRIG.reg = ADC_SWTRIG_START;
+
+    // Schedule next attempt after sample is likely to be complete
+need_delay:
+    return 42 * 128 + 200; // 42 == 1 + (63+1)/2 + 1 + 12/2 + 1.5
+}
+
+// Read a value; use only after gpio_adc_sample() returns zero
+uint16_t
+gpio_adc_read(struct gpio_adc g)
+{
+    last_analog_read = ADC_DUMMY;
+    return ADC->RESULT.reg;
+}
+
+// Cancel a sample that may have been started with gpio_adc_sample()
+void
+gpio_adc_cancel_sample(struct gpio_adc g)
+{
+    if (last_analog_read == g.chan) {
+        ADC->SWTRIG.reg = ADC_SWTRIG_FLUSH;
+        ADC->INTFLAG.reg = ADC_INTFLAG_RESRDY;
+        last_analog_read = ADC_DUMMY;
+    }
+}
diff --git a/src/samd21/gpio.h b/src/samd21/gpio.h
index 6c910da8..5bf44454 100644
--- a/src/samd21/gpio.h
+++ b/src/samd21/gpio.h
@@ -27,6 +27,14 @@ struct gpio_pwm {
 struct gpio_pwm gpio_pwm_setup(uint8_t pin, uint32_t cycle_time, uint8_t val);
 void gpio_pwm_write(struct gpio_pwm g, uint8_t val);
 
+struct gpio_adc {
+    uint32_t chan;
+};
+struct gpio_adc gpio_adc_setup(uint8_t pin);
+uint32_t gpio_adc_sample(struct gpio_adc g);
+uint16_t gpio_adc_read(struct gpio_adc g);
+void gpio_adc_cancel_sample(struct gpio_adc g);
+
 struct spi_config {
     uint32_t ctrla, baud;
 };