add main.cpp header comments describing expected operation

fw/HackSpaceCon/src/main.cpp

@@ -1,3 +1,29 @@
+/*
+
+    hackspacecon wand firmware
+
+    initially written by true
+    rgb programs and further hacking by 000000widow
+  
+  operation workflow:
+  - at poweron, set up GPIO and used peripherals, then go into standby mode.
+  - when button interrupt happens, wake up and process. if button is pushed,
+    prepare to run RGB program.
+  - enable the TCB0 periodic timer, which will interrupt every ~61Hz.
+  - initialize the rgb program. idle the CPU.
+  - timer will interrupt every ~61Hz and the program function will be called
+    again, until prog function returns 0.
+  - run the RGB program. if program returns 0, disable periodic timer, and
+    wait for the next button event to wake us up and run a program again.
+  
+  todo:
+  - test the code
+  - actually process the RGB data in the sample rainbow puke program
+  - add more programs
+  - support incrementing or randomly selecting the next program on each button push
+  - run TCB0 in standby mode to save a little more power
+*/
+
 #include <Arduino.h>
 #include <avr/io.h>
 #include <avr/interrupt.h>
@@ -43,11 +69,12 @@ void loop() {
 		case 1: {	// just started running a program
 			digitalWrite(PIN_PA6, HIGH);	// enable LED power supply,
 			delay(10);						// wait a moment for LEDs to stabilize,
-			enable_rgb_timer();				// then start the RGB program timebase.
 
 			rgb_program[0](1);				// initialize the program,
 			run_rgb_program++;				// and set to running mode.
 
+			enable_rgb_timer();				// then start the RGB program timebase.
+
 			// we can idle the CPU after running the program
 			SLPCTRL.CTRLA = SLPCTRL_SMODE_IDLE_gc | SLPCTRL_SEN_bm;
 			__asm("sleep");