From a94a046ca269a05672cb8b825f24056ce2e39397 Mon Sep 17 00:00:00 2001
From: Jon Langseth <jon.langseth@lilug.no>
Date: Sun, 22 Jul 2012 13:51:12 +0200
Subject: [PATCH] Timer-code. This works fine with my video-grabber. My LCD
 struggles, but that may be hardware, not software.

---
 syncgen/main.c | 86 +++++++++++++++++++++++++-------------------------
 1 file changed, 43 insertions(+), 43 deletions(-)

diff --git a/syncgen/main.c b/syncgen/main.c
index 8ad9cb3..e67d582 100644
--- a/syncgen/main.c
+++ b/syncgen/main.c
@@ -5,17 +5,17 @@
 #define ZERO  PORTB &= 0xFC
 #define BLACK PORTB |= 0x01
 
-//       -----..-----   ^
-//      |            |  |
-//     -| RST    VCC |-`
-//      |            |
-//   .--| X1     PB2 |-
-//  [ ] |            |  470R
-//   '--| X2     PB1 |--^v^v-.
-//      |            |       |----> VidOut
-//   .--| GND    PB0 |--^v^v-'
-//   |  |            |  1kR
-//-----  ------------
+//          -----..-----   ^
+//          | ___        |  |
+// Com RST -| RST    VCC |-`
+//          |            |
+// MegaClk -| CLKI  ADC1 |---< VidDetect
+//          |            |  470R
+// VSwithc -| PB4    PB1 |--^v^v-.
+//          |            |       |----> VidOut
+//       .--| GND    PB0 |--^v^v-'
+//       |  |            |  1kR
+//      ---  ------------
 
 #include "avrosdlogo.h"
 
@@ -82,7 +82,7 @@ void asm_render_line( void ) {
 
 void hsync(void)
 { 
-	line++;
+	line++; // Bumping the line-counter as part of hsync gives just the right front-porch ;D
 	ZERO; _delay_us(4); // sync
 	BLACK; _delay_us(8);  // Back porch
 }
@@ -94,38 +94,31 @@ void vsync(uint8_t odd_even)
 
 	// Pre-equalization pulses
 	b = odd_even ? 6 : 5;
-	for(a=0; a<b ; a++) // 6x short sync
+	for(a=0; a<b ; a++) // short sync
 	{
 		ZERO;
 		_delay_us(2);
-
 		BLACK;
 		_delay_us(30);
-
 	}
 
 	// VSYNC
-	for(a=0; a<5 ; a++) // 5x long sync
+	for(a=0; a<5 ; a++) // long sync
 	{
-
 		ZERO;
 		_delay_us(30);
-
 		BLACK;
 		_delay_us(2);
-
 	}
 
 	// Post-equalization pulses
 	b = odd_even ? 5 : 4;
-	for(a=0; a<b ; a++) // 5x short sync
+	for(a=0; a<b ; a++) // short sync
 	{
 		ZERO;
 		_delay_us(2);
-
 		BLACK;
 		_delay_us(30);
-
 	}
 	line = odd_even ? 5 : 317;
 	renderLine = 0;
@@ -135,23 +128,27 @@ void vsync(uint8_t odd_even)
 
 ISR (TIMER0_COMPA_vect) 
 {
-	if(line == 310) vsync(0);
-	else if(line == 622) vsync(1);
-	else
+	if(line == 310) vsync(0); // End of ODD fields, start vsync'ing
+	else if(line == 622) vsync(1); // End of EVEN, start vsync'ing
+	else // In frame. Generate a hsync pulse, then do some drawing :)
 	{
 		hsync();
-		int t_line = line;
+
+		int t_line = line; // Reducing line-count complexity by half ;)
 		t_line = ( line > 312 ) ? line - 312 : line;
 
+		// Draw the logo-image centered vertically
 		if ( t_line > 118 && t_line < 118 + (2*testimg_height) )
 		{
 			_delay_us(12);
 			asm_render_line();
 
+			// Stretch the image vertically by repeating the same line twice
 			if( !stretch )
 			{
 				stretch = 1;
 				renderLine += hres;
+				// Make sure we don't draw anything from RAM after the end of the image
 				if ( renderLine > ((testimg_height*hres)-1) ) renderLine = 0;
 			} else stretch--;
 		}
@@ -159,13 +156,17 @@ ISR (TIMER0_COMPA_vect)
 	return;
 }
 
-/*
-ISR (INT0_vect) 
+ISR (TIMER1_OVF_vect) 
 {
-	//PORTB ^= 0x10;
+	// If this interrupt-vector is run, Timer1 has overflowed, and that means
+	// we have not seen any signal of significant level for quite a while.
+	// This means Sync-generation is needed...
+
+	// Make sure sync-gen is running...
+	TIMSK |= (1<<OCIE0A); // Enable compare-mach-a interrupt for Timer0
+	// Open up the switch blocking attiny-generated video:
 	PORTB |= 0x10;
 }
-*/
 
 int main(void)
 {  
@@ -192,6 +193,10 @@ int main(void)
 	ADCSRA |= (1 << ADPS2)|(1 << ADPS1)|(0<<ADPS0); // set prescaler to 64
 	DIDR0 |= (1<<ADC1D);   // Disable logic input for PB2
 
+	TCCR1 |= (1<<CS13)|(0<<CS12)|(0<<CS11)|(1<<CS10); // Prescale Timer1 to CK/256
+	// CK/32 @ 20MHz gives 0.05us*256*256 = 3276.8us per Overflow, or 51 lines.
+	TIMSK |= (1<<TOIE1); // Enable Timer Overflow interrupts for Timer1
+
 	sei(); // We are ready to fire interrupts.
 
 	// Here, we continue to do nothing...
@@ -204,23 +209,18 @@ int main(void)
 			// greater than some minimal value on ADC1/PB2.
 			// This approach requires a weak pulldown on the relevant
 			// video input to the circuit, but it beats not detecting anything.
-			if (ADCH > 10)
+			// 255/50*4=20, thus 0.4V
+			if (ADCH > 20)
 			{
-				// Dummy code. 
-				// This needs to be replaced with something that triggers a timer ...
-				/// Simply indicate on PB4 that we are seeing somethin.
-				PORTB |= 0x10;
+				// Make sure any signal generated by us does not leak back
+				PORTB &= 0xEF; // Switch OFF VOut-switch for ATtiny
 				// The next statement stops sync-generation, thus also image-generation..
 				TIMSK &= ~(1<<OCIE0A); // Enable compare-mach-a interrupt
+				// Reset Timer1 so we don't generate a TOVF..
+				TCNT1 = 0;
 			}
-			else
-			{
-				// Dummy code. Simply indicate on PB4 that we are below the threshold.
-				PORTB &= 0xEF;
-
-				// Make sure sync-gen is running...
-				TIMSK |= (1<<OCIE0A); // Enable compare-mach-a interrupt
-			}
+			// If we did not see a signal above the treshold, TCNT1 will continue
+			// to count up, until a TOV1 interrupt is generated.
 		}
 	};
 }
-- 
2.39.2