Add embedded programming page

content/08_embedded_programming.md

++++
+title = "Embedded Programming"
+date = "2018-10-19"
+menu = "main"
+weight = 9
++++
+
+## Embedded Programming
+
+Files: [blink.c](/designs/07_blink.c)
+
+This week we need to take a board we've made and give it some new functionality. No more flashing with pre-written code.
+
+Two weeks ago I added an LED and a button to Neil's [hello world](http://academy.cba.mit.edu/classes/embedded_programming/index.html#echo) board, so I'll start by making the LED turn on. I attached my LED to port PB2, and my switch to port PA7. To turn on the LED, we only need to do two things: enable PB2 as an output, and set it high. Here's code that does just that.
+
+```
+#include <avr/io.h>
+
+#define led_pin (1 << PB2)
+
+int main(void) {
+    // Configure led_pin as an output.
+    DDRB |= led_pin;
+
+    // Set led_pin high.
+    PORTB |= led_pin;
+
+    // Nothing left to do, so just spin.
+    while (1) {}
+
+    return 0;
+}
+```
+
+Next let's use the button. The easiest approach is to use the button as a contact switch, turning on the LED only for the duration of the button press. To do this we'll need to configure pin PA7 as an input. I didn't add a pullup resistor on my board, so I also have to enable PA7's internal one. This will ensure that PA7 reads high when the button isn't pressed.
+
+```
+#include <avr/io.h>
+
+#define led_pin (1 << PB2)
+#define switch_pin (1 << PA7)
+
+int main(void) {
+    // 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;
+
+    while (1) {
+        // Turn on the LED when the button is pressed.
+        if (PINA & switch_pin) {
+            // Turn off the LED.
+            PORTB &= ~led_pin;
+        } else {
+            // Turn on the LED.
+            PORTB |= led_pin;
+        }
+    }
+
+    return 0;
+}
+```
+
+![](/img/05_print_3.jpg#c)

static/designs/08_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;
+                }
+            }
+        }
+    }
+}
+