diff --git a/src/avr/gpio.c b/src/avr/gpio.c
index 5dfc34c4..aa39f957 100644
--- a/src/avr/gpio.c
+++ b/src/avr/gpio.c
@@ -258,7 +258,7 @@ static const uint8_t adc_pins[] PROGMEM = {
 
 static const uint8_t ADMUX_DEFAULT = 0x40;
 
-DECL_CONSTANT(ADC_MAX, 1024);
+DECL_CONSTANT(ADC_MAX, 1023);
 
 struct gpio_adc
 gpio_adc_setup(uint8_t pin)
diff --git a/src/sam3x8e/gpio.c b/src/sam3x8e/gpio.c
index d20f3f77..205528d5 100644
--- a/src/sam3x8e/gpio.c
+++ b/src/sam3x8e/gpio.c
@@ -128,7 +128,7 @@ static const uint8_t adc_pins[] = {
 };
 
 #define ADC_FREQ_MAX 20000000
-DECL_CONSTANT(ADC_MAX, 4096);
+DECL_CONSTANT(ADC_MAX, 4095);
 
 struct gpio_adc
 gpio_adc_setup(uint8_t pin)