Add code (including various commented experiments)

blink.c

+#include <avr/io.h>
+
+#define switch_pin (1 << PA7)
+#define led_pin (1 << PB2)
+
+int main(void) {
+    // Set the clock prescaler to 1.
+    CLKPR = (1 << CLKPCE);
+    CLKPR = (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0);
+
+    // Set the 8 bit timer's prescaler to 1/1024.
+    TCCR0B |= 0b00000101;
+
+    // Configure led_pin as an output.
+    DDRB |= led_pin;
+
+    // Configure switch_pin as an input.
+    DDRA |= switch_pin;
+
+    // Activate switch_pin's pullup resistor.
+    PORTA |= switch_pin;
+
+    //int count1 = 0;
+    //int count2 = 0;
+
+    int bouncy_switch_state = 0;
+    int debounced_switch_state = 0;
+
+    while (1) {
+        // Turn on the led when the button is pressed
+        /*
+        if (PINA & switch_pin) {
+            PORTB &= ~led_pin;
+        } else {
+            PORTB |= led_pin;
+        }
+        */
+
+        // ~5s blink period based on counting
+        /*
+        if (++count1 == 255) {
+            if (++count2 == 255) {
+                PORTB ^= led_pin;
+                count1 = 0;
+                count2 = 0;
+            }
+        }
+        */
+
+        // ~2.5s second blink period
+        // 255 * 200 * (1024 / 20M) = 2.6112
+        /*
+        if (TCNT0 >= 200) {
+            TCNT0 = 0;
+            if (++count1 == 255) {
+                PORTB ^= led_pin;
+                count1 = 0;
+            }
+        }
+        */
+
+        // the button toggles the led (with debouncing)
+        if ((PINA & switch_pin) != bouncy_switch_state) {
+            bouncy_switch_state = PINA & switch_pin;
+            TCNT0 = 0;
+        }
+        if (TCNT0 >= 195) {
+            // It's been 10ms since the switch changed.
+            if (bouncy_switch_state != debounced_switch_state) {
+                debounced_switch_state = bouncy_switch_state;
+                if (debounced_switch_state == 0) {
+                    PORTB ^= led_pin;
+                }
+            }
+        }
+    }
+}
+