* optimisation: -C3 compiler flag for optimised size

* bugfix: correct order of setting `DDRn` and `PORTn`
* optimisation: `enum` and `static const` instead of macros
* optimisation: `PT6302_startup` is now `inline`
* optimisation: functions are now static
* bugfix: `data` is now rotated and stored instead of just rotating
* special thanks to @kissadamfkut for pointing all of this out to me

main.c

@@ -1,13 +1,69 @@
 #define F_CPU 8000000UL
 #include <avr/io.h>
 #include <util/delay.h>
-#include "PT6302.h"
 
+/* PT6302 timing constants, all in usec */
+static const uint8_t TCW = 1;     //CLKB pulse width
+static const uint8_t TDOFF = 8;   //Data processing time
+static const uint8_t DTCSH = 16;  //time difference between tDOFF and tCSH
+static const uint8_t TPRZ = 100;  //VDD rise time
+static const uint8_t TWRSTB = 1;  //RSTB pulse width
+
+enum pins {
+  RSTpin = (1 << PC2),
+  CSBpin = (1 << PB0),
+  CLKBpin = (1 << PB6),
+  DINpin = (1 << PB7),
+};
+
+static inline void PT6302_startup (void)
+{
+  DDRB |= (CSBpin | CLKBpin | DINpin);
+  DDRC |= RSTpin;
+  PORTB |= (CSBpin | CLKBpin);
+  PORTB &= ~DINpin;
+  PORTC |= RSTpin;
+  _delay_us (TPRZ);
+  PORTC &= ~RSTpin;
+  _delay_us (TWRSTB);
+}
+
+static void transmit_bytes (const uint8_t *payload, const uint8_t size)
+{
+  PORTB &= ~CSBpin;
+  for (int i = 0; i < size; ++i)
+    {
+      uint8_t data = payload[i];
+      for (int j = 0; j < 8; ++j, data >>= 1)
+        {
+          PORTB &= ~CLKBpin;
+          if ((data & 0x01) == 0)
+            {
+              PORTB |= DINpin;
+            }
+          else
+            {
+              PORTB &= ~DINpin;
+            }
+          _delay_us (TCW);
+          PORTB |= CLKBpin;
+          _delay_us (TCW);
+        }
+      _delay_us (TDOFF);
+    }
+  _delay_us (DTCSH);
+  PORTB |= CSBpin;
+}
 
 int main ()
 {
-  PT6302_startup();
+  PT6302_startup ();
+  DDRC |= (1 << PC4);
   while (1)
     {
+      PORTC |= (1 << PC4);
+      _delay_ms (1000);
+      PORTC &= ~(1 << PC4);
+      _delay_ms (1000);
     }
 }