; filename: "tv_sync_gen.asm"
; title: PIC CCIR TV Sync Generator

; An attempt to generate standards compliant TV sync signal. Uses external 8 MHz crystal.

; 230708 v0.1	First daft
; 230711 v0.2	VSYNC complete pulse train

; GPL Copyleft 2023 pic@polonai.se

	LIST	P=12F683, F=INHX8M
	#include <p12f683.inc>

	__CONFIG _XT_OSC & _WDT_OFF & _MCLRE_ON & _CP_OFF & _BOREN_OFF
	ERRORLEVEL -302			; sod compiler message about using proper bank

; Equates
RESET_V	EQU	0x00			; Address of RESET Vector

; Registers
FLAGS	EQU	0x20		  	; Register containing flags
DELAY	EQU	0x21			; Counter for delays

#define	SYNC	GPIO,0			; COmposite sync signal (active LOW)
#define	HS	GPIO,1			; Horizontal sync (active LOW)
#define	VS	GPIO,2			; Vertical sync (active LOW)
#define FIELD	FLAGS,0			; needed to synchronize V- and HSYNC

	ORG 	0x00
	GOTO 	START
	ORG 	0x04

; Interrupts
	BTFSC	PIR1,TMR1IF		; Test Timer1 interrupt flag (every 20 ms)
	GOTO	VSYNC
	BCF	INTCON,TMR0IF		; Clear TMR0 interrupt (every 64 us)
	MOVLW	0x89
	MOVWF	TMR0			; Preload TMR0 for 64 us
	BCF	SYNC			; 4.5 us
	CALL	p45
	BSF	SYNC
	RETFIE

VSYNC
	BCF	INTCON,TMR0IE		; Disable TMR0 interrupt
	BCF	PIR1,TMR1IF		; Clear Timer1 interrupt flag (every 20 ms)
	MOVLW	0x63			; Preload Timer1 for 20000 µs
	MOVWF	TMR1H
	MOVLW	0xCD
	MOVWF	TMR1L
	BCF	VS
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	SERRATION
	BSF	SYNC
	CALL	p45
	BCF	SYNC
	CALL	SERRATION
	BSF	SYNC
	CALL	p45
	BCF	SYNC
	CALL	SERRATION
	BSF	SYNC
	CALL	p45
	BCF	SYNC
	CALL	SERRATION
	BSF	SYNC
	CALL	p45
	BCF	SYNC
	CALL	SERRATION
	BSF	SYNC
	CALL	p45
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	CALL	EQUALIZING
	BCF	SYNC
	CALL	p25
	BSF	SYNC
	BSF	VS
	BTFSS	FIELD
	GOTO	ODD
	BCF	FIELD
	MOVLW	0x95
	MOVWF	TMR0			; Preload TMR0 for 62.0 us
	GOTO	ENDVSYNC

ODD
	BSF	FIELD
	MOVLW	0xD6
	MOVWF	TMR0			; Preload TMR0 for 30.0 us

ENDVSYNC
	BSF	INTCON,TMR0IE		; Re-enable TMR0 interrupt
	BCF	INTCON,TMR0IF		; Clear TMR0 interrupt (just in case)
	RETFIE

; End interrupt routines

; Subroutines
EQUALIZING
	MOVLW	0x11			; 29.5 us equalizing pulse interval
	MOVWF	DELAY
	DECFSZ	DELAY
	GOTO	$-1
	NOP
	NOP
	RETURN

SERRATION
	MOVLW	0x10			; 27.5 us serration pulse interval
	MOVWF	DELAY
	DECFSZ	DELAY
	GOTO	$-1
	NOP
	RETURN

p25					; Equalizing pulse duration (2.5 us)
	RETURN

p45					; Sync/serration pulse duration (4.5 us)
	NOP
	NOP
	NOP
	NOP
	RETURN

; End subroutines

START
; Init stuff
	CLRF	STATUS			; Do initialization, select Bank 0
	CLRF	INTCON			; Clear flags, disable interrupts
	CLRF	PCLATH			; Keep in lower 2KByte
	BSF	STATUS,RP0	 	; Select Bank 1
	CLRF	OPTION_REG
	BSF	OPTION_REG,PSA		; Prescaler assigned to WDT, TMR0 1:1 (0.5 us)
	CLRF	TRISIO			; All ports output
	CLRF	ANSEL			; All inputs digital
	CLRF	OSCCON			; External 8 MHz XTAL
	CLRF	PIE1
	BSF	PIE1,TMR1IE		; Enable Timer1 interrupt
	BCF	STATUS,RP0		; Select Bank 0
	MOVLW	0x07
	MOVWF	CMCON0			; All inputs digital
	CLRF	T1CON
	BSF	T1CON,TMR1ON		; Enable Timer1
	CLRF	GPIO
	CLRF	FLAGS
	BSF	HS
	BSF	VS
	BSF	SYNC
	BSF	INTCON,TMR0IE		; Enable TMR0
	BSF	INTCON,PEIE		; Enable Peripheral (Timer1) interrupt
	BSF	INTCON,GIE		; Enable global interrupts

	GOTO	$


	END