diff --git a/examples/digital_io.c b/examples/digital_io.c
new file mode 100644
index 0000000..dc9e9f1
--- /dev/null
+++ b/examples/digital_io.c
@@ -0,0 +1,37 @@
+/*
+This example showcases digital input and output using the AVR chip's GPIO
+pins. For all pins, we first have to configure them as either input or output.
+We do this by writing to the DDRx register (data direction register) either a 1
+(for output) or 0 (for input).
+
+We can set the output of a pin by writing to PORTx, using bitwise operators.
+
+Reading can be done by reading from PINx, using bitwise operators.
+*/
+#include <avr/io.h>
+#include <util/delay.h>
+
+int main() {
+    // Pin 12 == Port B4, aka 4th bit on the B register.
+    // Set pin 12 as output.
+    DDRB |= 1 << PORTB4;
+    // Set pin 5 as input.
+    DDRD &= ~(1 << PORTD5);
+    // Write a 0 to pin 12 initially.
+    PORTB &= ~(1 << PORTB4);
+    uint8_t led = 0;
+    while (1) {
+        // Write current led status to pin 12.
+        if (led) {
+            PORTB |= 1 << PORTB4;
+        } else {
+            PORTB &= ~(1 << PORTB4);
+        }
+        led = !led;
+        _delay_ms(1000.0);
+        // Read input from pin 5. If it's HIGH, keep the LED on.
+        uint8_t pin5Value = (PIND & (1 << PIND5)) >> PIND5;
+        if (pin5Value) led = 1;
+    }
+    return 0;
+}
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diff --git a/src/gympal.c b/src/gympal.c
index a0d766b..634dba8 100644
--- a/src/gympal.c
+++ b/src/gympal.c
@@ -1,19 +1,33 @@
 #include <avr/io.h>
+#include <avr/interrupt.h>
 #include <util/delay.h>
 
+volatile uint8_t a = 0;
+
 void updatePin12(uint8_t status) {
     PORTB = status << PORTB4;
 }
 
+ISR(PCINT2_vect) {
+    uint8_t value = (PIND & (1 << PIND5)) >> PIND5;
+    PORTB = value << PORTB4;
+}
+
 int main() {
+    PCICR |= 1 << PCIE2;
+    PCMSK2 |= 1 << PCINT21;
+    sei();
     // Pin 12 == Port B4, aka 4th bit on the B register.
     // Set pin 12 as output.
-    DDRB = 1 << PORTB4;
-    uint8_t ledOn = 1;
+    DDRB |= 1 << PORTB4;
+    // Set pin 5 as input.
+    DDRD &= ~(1 << PORTD5);
+    PORTB &= ~(1 << PORTB4);
     while (1) {
-        updatePin12(ledOn);
-        ledOn = !ledOn;
-        _delay_ms(1000.0);
+        // updatePin12(ledOn);
+        // ledOn = !ledOn;
+        // _delay_ms(1000.0);
+        // DO NOTHING! rely on interrupt.
     }
     return 0;
 }
\ No newline at end of file