173 lines
4.9 KiB
NASM
173 lines
4.9 KiB
NASM
; VIDEO COMPOSITE PAL IO DEVICE
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; Implemented following timings in http://blog.retroleum.co.uk/electronics-articles/pal-tv-timing-and-voltages/
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; Every line, for 52 times, it loads a byte from memory into PORTA register and then shifts the byte to the left to show another bit (do it 7 times)
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; This also displays byte's MSB pixel "for free", as the video pin is PD7 (last bit of PORTA).
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;
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; INTERFACING WITH PAT80:
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; Use PortB as data port. Before writing anything, issue a read (pin RW HIGH) and check the busy pin on the data port.
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; If the busy pin is high, retry reading until goes low. When the busy pin goes low, we have... TODO
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;
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; ELECTRONICALLY:
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; The data port PB0 is tied to ground with a 1KOhm resistance. When the MC is busy drawing the screen, the data port is in
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; high impedance state, so that avoids causing bus contention, but when read returns a 0bXXXXXXX0 byte. When the MC starts vsync,
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; begins checking the port for data... TODO
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;
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; PINS:
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; Video:
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; Video pin: PA0 (pin 1) (but all PORTA is used)
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; Sync pin: PC0 (pin 22)
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; Communication:
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; Data port is PORTB [INPUT]
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; CLK (clock) signal is on PORTD0 [INPUT]
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; RS (register select) on PORTD1 [INPUT]
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; BUSY signal is on PORTD2 [OUTPUT]
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; Debug:
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; Debug hsync single pulse on pin: PC1 (pin 23) (may be disabled)
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;
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.include "m1284def.inc" ; Atmega 1280 device definition
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; *** reserved registers ***
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; Cursor Position
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; POS_COLUMN (0-103) represents the column on a pair of rows: 0 to 51 is the first row, 52 to 103 the second one
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; POS_ROWP (0-152) represent the pair of rows. POS_ROWP = 5 means the 10th and 11th rows
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; POS_FINE represents fine position (bit inside coarse-position-pointed chunk) in graphic mode.
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.def POS_COLUMN = r21
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.def POS_ROWP = r20
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.def POS_FINE = r24
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; Internal registers
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.def A = r0 ; accumulator
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.def STATUS = r25 ; signal status (see STATUS TABLE)
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;POS_COARSE = Y ; coarse position (aligned to character column)
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;DRAWING_BYTE = X ; current position in framebuffer
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.def LINE_COUNTER = r23
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.def VG_HIGH_ACCUM = r22 ; an accumulator in high registers to be used only by video_generator in interrupt
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.def HIGH_ACCUM = r16 ; an accumulator in high registers to be used outside of interrupts
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; define constant
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.equ VIDEO_PORT_OUT = PORTA ; Used all PORTA, but connected only PA0
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.equ SYNC_PIN = PC0 ; Sync pin (pin 22)
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.equ DEBUG_PIN = PC1 ; DEBUG: Single vertical sync pulse to trigger oscilloscope (pin 23)
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.equ DATA_PORT_IN = PINB
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.equ CLK_PIN = PD0
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.equ RS_PIN = PD1
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.equ BUSY_PIN = PD2
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; memory
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.equ FRAMEBUFFER = 0x0100
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.equ FRAMEBUFFER_END = 0x3EC0
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; start vector
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.org 0x0000
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rjmp main ; reset vector: jump to main label
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.org 0x001E
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rjmp on_tim1_ovf ; interrupt for timer 1 overflow (used by video generation)
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.org 0x40
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; main program
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main:
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; pins setup
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sbi DDRC, SYNC_PIN ; set pin as output
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sbi DDRC, DEBUG_PIN ; set pin as output
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sbi DDRC, BUSY_PIN ; set pin as output
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cbi DDRD, CLK_PIN ; set pin as input
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ldi HIGH_ACCUM, 0xFF
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out DDRA, HIGH_ACCUM ; set port as output (contains video pin)
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ldi HIGH_ACCUM, 0x00
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out DDRB, HIGH_ACCUM ; set port as input (used as data bus)
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; clear ram
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;*** Load data into ram ***
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; Set X to 0x0100
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ldi r27, high(FRAMEBUFFER)
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ldi r26, low(FRAMEBUFFER)
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load_mem_loop:
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clr r17
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st X+, r17
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; if reached the last framebuffer byte, exit cycle
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cpi r27, 0b00111110
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brne load_mem_loop ; if not 0, repeat h_picture_loop
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cpi r26, 0b11000000
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brne load_mem_loop ; if not 0, repeat h_picture_loop
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; test draw character routine
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call cursor_pos_home
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ldi r18, 0x21
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draw_chars:
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mov HIGH_ACCUM, r18
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call draw_char
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inc r18
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cpi r18, 0x7E
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brne draw_chars
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ldi r18, 0x21
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draw_chars2:
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mov HIGH_ACCUM, r18
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call draw_char
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inc r18
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cpi r18, 0x7E
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brne draw_chars2
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ldi r18, 0x21
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draw_chars3:
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mov HIGH_ACCUM, r18
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call draw_char
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inc r18
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cpi r18, 0x7E
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brne draw_chars3
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ldi r18, 0x21
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draw_chars4:
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mov HIGH_ACCUM, r18
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call draw_char
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inc r18
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cpi r18, 0x7E
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brne draw_chars4
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ldi r18, 0x21
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draw_chars5:
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mov HIGH_ACCUM, r18
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call draw_char
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inc r18
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cpi r18, 0x7E
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brne draw_chars5
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ldi r18, 0x21
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draw_chars6:
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mov HIGH_ACCUM, r18
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call draw_char
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inc r18
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cpi r18, 0x7E
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brne draw_chars6
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; *** timer setup (use 16-bit counter TC1) ***
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; The Power Reduction TC1 and TC3 bits in the Power Reduction Registers (PRR0.PRTIM1 and
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; PRR1.PRTIM3) must be written to zero to enable the TC1 and TC3 module.
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ldi HIGH_ACCUM, 0b00000000
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sts PRR0, HIGH_ACCUM
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; Set timer prescaler to 1:1
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LDI HIGH_ACCUM,0b00000001
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sts TCCR1B,HIGH_ACCUM
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; Enambe timer1 overflow interrupt
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LDI HIGH_ACCUM,0b00000001
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STS TIMSK1,HIGH_ACCUM
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; Enable interrupts globally
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SEI
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; Timer setup completed.
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; Wait for data (it never exits)
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jmp comm_init
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forever:
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jmp forever
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.include "video_generator.asm" ; Asyncronous timer-interrupt-based video generation
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.include "character_generator.asm" ; Character generator
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.include "communication.asm" ; Communication with Pat80
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.include "font.asm" ; Font face
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