🏠 Atari Jaguar Developer Reference ▸ Architecture & Memory ▸ Video & System Clocks, Timing

Video & System Clocks, Timing

System/video/chroma clock generation, NTSC vs PAL video timing, the programmable video timing registers, and the hardware port pinouts (cartridge/expansion, DSP, video) plus the multi-console network and modem ports.

Source: Technical Reference Manual (V10), pp. 8–14. © Atari Corp. 1995.

Clock Generation

In the Jaguar console, the video clock is chosen to allow an inexpensive RF modulator system. This requires slightly different clock speeds for NTSC and PAL systems (but the difference is only about 0.01%). To be cost effective, the GPU/DSP processor clock speed is the same as the video clock speed, and the 68000 runs at 50% of this clock rate.

Clock	NTSC	PAL
Video Clock / GPU/DSP Clock Rate	26.590906 MHz	26.593900 MHz
68000 Clock Rate (50% of Video Clock)	13.295453 MHz	13.296695 MHz

Clock Divider Registers — DO NOT TOUCH

The clock dividers live in Jerry and are set up by the BOOTROM (retail console) or the STUBULATOR (development console). Never write to them. The settings in CLK2, CLK3 and HP in particular must be correct to make the hardware work at all and to prevent dot crawl. The manual is emphatic: “We really mean it: DON’T TOUCH THIS!”

Register	Address	Function
`CLK1`	`$F10010`	Processor clock divider
`CLK2`	`$F10012`	Video clock divider
`CLK3`	`$F10014`	Chroma clock divider (aka CHROMA DIV)

The VMODE register and object processor are initialized and started after reset by the bootcode. The only object then in the object list is a stop object, which displays a blank screen and sends the correct video sync signals to the monitor or TV. This also lets the phase-locked loop settle (about a second at start-up). Do not ever turn video off again (i.e. by writing zero to VMODE).

Pixel (Dot) Clock and Resolution

The video system is programmable within the precision of the supplied video clock. From the video clock the system produces the pixel (or dot) clock. The ratio between the video and pixel clock is set by the high-order bits of the VMODE register. Pixel counts are the same for NTSC and PAL.

For both PAL and NTSC the “safe” video area is about 40µS wide; the area required to guarantee overscan is about 50µS. These numbers are rough guidelines for artwork and object sizes only — they should not be used to calculate values for the video hardware registers.

Pixel Divisor (VMODE)	# Pixels Non-Overscanned	# Pixels Overscanned
1	1046	1330
2	532	655
3	355	443
4	266	332
5	213	266
6	177	222
7	152	190
8	133	166

Atari recommends overscanning both vertically and horizontally for all Jaguar software.

Divisor 1 requires the object processor be started twice per line and produces a ridiculously high TV resolution — ignored.
Divisor 3 gives ~355 overscanned pixels: a good match for 320-pixel-wide computer-style screens.
Divisor 4 gives roughly square pixels (recommended for art creation). Overscanned, this allows ~332 pixels; a 320-pixel-wide bitmap gives <4% error. Only the middle 266 should be counted on as visible, leaving a ~27-pixel border each side that may be visible but must not carry essential game information.
Divisor 5 gives an overscanned count (256) usefully close to a blittable width.

For vertical overscan, use a screen height of 240 lines for NTSC and 288 lines for PAL. The guaranteed visible region for crucial game information is 200 lines for NTSC and 240 lines for PAL. Using 200 lines of critical video for both systems is an acceptable simplification.

To properly initialize your program (including video) you must use the standard Jaguar Start-up Code described in the Jaguar Libraries section.

Programmable Video Timing Registers

These registers define the video raster timing. They are set up at boot and listed for information only — do not modify them or unpredictable results will occur.

Register	Address	Description
`HP`	`$F0002E`	Horizontal Period
`HBB`	`$F00030`	Horizontal Blanking Begin
`HBE`	`$F00032`	Horizontal Blanking End
`HS`	`$F00034`	Horizontal Sync
`HVS`	`$F00036`	Horizontal Vertical Sync
`HDB1`	`$F00038`	Horizontal Display Begin 1
`HDB2`	`$F0003A`	Horizontal Display Begin 2
`HDE`	`$F0003C`	Horizontal Display End
`VP`	`$F0003E`	Vertical Period
`VBB`	`$F00040`	Vertical Blanking Begin
`VBE`	`$F00042`	Vertical Blanking End
`VS`	`$F00044`	Vertical Sync
`VDB`	`$F00046`	Vertical Display Begin
`VDE`	`$F00048`	Vertical Display End
`VEB`	`$F0004A`	Vertical Equalization Begin
`VEE`	`$F0004C`	Vertical Equalization End
`VI`	`$F0004E`	Vertical Interrupt
`HEQ`	`$F00054`	Horizontal Equalization End

Video Ports

Informational only — do not attempt to change these timings or unpredictable results will occur.

There are four versions of the Jaguar console:

Video Standard	Where Used
NTSC	USA / Canada
PAL-I	United Kingdom
PAL-B	Germany / other European countries
Peritel/Scart	France

The console has an external video connector supporting Composite video, S-Video, and RGB. An RF Modulator output is present on all versions except the French Peritel/Scart version. The Peritel/Scart version is identical to PAL-B except that it has no RF modulator; Composite, S-Video, and RGB are all available on it with the same timings and characteristics as PAL-B.

RF and Composite

Informational only — do not attempt to change these timings.

	Chroma Clock (MHz)	Subcarrier	Sound Carrier (MHz)
PAL-I	4.43361875	591.250	6
PAL-B	4.43361875	591.250	5.5
NTSC Channel 3	3.579545	61.25	4.5
NTSC Channel 4	3.579545	61.25	4.5

Video Timings

Informational only — do not attempt to change these timings.

Parameter	PAL	NTSC	Notes
Video master clock	26.593900 MHz	26.590906 MHz
Horizontal period	64.0 µS	63.5555 µS
Hsync width	4.7 µS	4.76 µS
Hback porch	5.7 µS	4.45 µS
Hfront porch	1.65 µS	1.27 µS
Equalization pulse width	2.35 µS	2.54 µS
Vertical sync pulse width	27.3 µS	29.26 µS
Vertical lines (interlaced)	625	525
Vertical lines (non-interlaced)	624	524
Vertical sync pulses	5	6	Non-interlaced
Vertical eq pulses before sync	5	6	Non-interlaced
Vertical eq pulses after sync	6	6	Non-interlaced
Vertical front porch	12 lines	12 lines	Non-interlaced
Vertical back porch	17 lines	12 lines	Non-interlaced

Jaguar Console Hardware Ports

Cartridge / Expansion Port

A custom 50-pin (54-position with key), double-row PCB-mounting edge connector. The far (back) row is row A, the near row is row B, when looking at the console from the front. Pins 13 and 14 are the KEY (no signal). Pin numbering runs through 54.

Pin	A	B	Pin	A	B
1	EA10	GND	28	ED6	NC
2	EA9	GND	29	ED9	GND
3	EA11	EA23	30	ED13	UART1
4	EA8	EA22	31	ED2	UART0
5	EA12	EA21	32	ED5	GND
6	EA7	EA20	33	ED10	RESET1L
7	EA13	EA19	34	ED12	CART_IN
8	EA6	GND	35	ED3	CART_OUT
9	EA14	NC	36	ED4	VCC
10	EA5	GND	37	ED11	VCC
11	EA15	NC	38	VCC	PLL
12	EA4	GND	39	ED31	NC
13	KEY	KEY	40	ED16	E2DATA
14	KEY	KEY	41	ED23	NC
15	EA16	EA1	42	ED24	GPIO0
16	EA3	EA0	43	ED30	GPIO1
17	EA17	WAITL	44	ED17	GPIO2
18	EA2	RESETL	45	ED22	GPIO3
19	EA18	EWE0L	46	ED25	GPIO4
20	ROM1	EWE2L	47	ED29	SCK
21	GND	ERW	48	ED18	WS
22	ED15	EOE1L	49	ED21	TXD
23	ED0	EOE0L	50	ED26	RXD
24	ED7	GND	51	ED28	GND
25	ED8	EINT0	52	ED19	ECPUCLK
26	ED14	EINT1	53	ED20	GND
27	ED1	9V	54	ED27	GND

DSP Port

A custom 12-pin, two-row edge connector. The top row is row A, the bottom row is row B. Pin 1 is on the left, pin 6 on the right when looking at the console from the rear.

Pin	Name	Description
1A	GND	Ground
2A	SCK	Synchronous serial clock
3A	WS	Synchronous serial word strobe
4A	TXD	Synchronous serial transmit data (data out)
5A	RXD	Synchronous serial receive data (data in)
6A	GND	Ground
1B	+5V	+5V, 50mA maximum load
2B	UART_TXD	Asynchronous transmit data (data out)
3B	UART_RXD	Asynchronous receive data (data in)
4B	Reserved	Do not connect
5B	Reserved	Do not connect
6B	GND	Ground

All active signals have 5V TTL levels. The SCK, WS, TXD and RXD signals are also connected to the cartridge expansion connector. They are used on the CD-ROM peripheral, so care must be taken to avoid contention (see the audio subsystem section).

Video Connector

A custom 24-pin, two-row edge connector. The top row is row A, the bottom row is row B. Pin 1 is on the left, pin 12 on the right when looking at the console from the rear.

Pin	Name	Description
1A	Audio_Left	EIAJ Line Level, left audio
2A	Audio_Gnd	Audio Return (ground)
3A	Reserved
4A	Video_Gnd	Video Return (ground)
5A	Blue	Blue video, 75 Ohm, 0.7V peak-to-peak
6A	HSync	Horizontal Sync, 75 Ohm, 3.0V peak-to-peak
7A	Green	Green video, 75 Ohm, 0.7V peak-to-peak
8A	Chroma	S-Video Chroma, 75 Ohm, 1.0V peak-to-peak
9A	Reserved
10A	Reserved
11A	9V	9V DC, 100mA maximum load
12A	Reserved
1B	Audio_Right	EIAJ Line Level, right audio
2B	Audio_Gnd	Audio Return (ground)
3B	Video_Gnd	Video Return (ground)
4B	Red	Red video, 75 Ohm, 0.7V peak-to-peak
5B	VSL	Composite Sync, +5V, TTL Levels
6B	Reserved
7B	Video_Gnd	Video Return (ground)
8B	Luma	S-Video Luma, 75 Ohm, 1.0V peak-to-peak
9B	Reserved
10B	Video_Gnd	Video Return (ground)
11B	Composite	Composite Video, 75 Ohm, 1.0V peak-to-peak
12B	Reserved

Reserved signals should be left unconnected — they may be used in future console versions and should be passed through on video adaptors. Terminate the active signals correctly; do not load the 75 Ohm outputs with more than 75 Ohms.

Multi-Console Games

There are two types of multi-console game:

Jaguar Network — a special Local-Area-Network of multiple Jaguar consoles connected together via the consoles’ asynchronous serial port. The low-level networking drivers were in development at the time of writing; contact Jaguar Development Support for further information.
Jaguar Modem — connects two Jaguar consoles over telephone lines. The specification for using the Jaguar modem is described in the document titled Jaguar Voice Modem.