Signetics 2650 & 2636 programming/2636 PVI

The Signetics 2636 Programmable Video Interface (PVI) is a 40-pin DIL chip that handles a number of functions within the console:

• Video generation
  • Screen colour
  • Background grid
  • Score digits
  • Four objects (sprites)
  • Object completion and collision detection
• Square wave audio
• A/D conversion for the analogue joysticks
• Chip select signals for memory and input/output devices
• 37 bytes of scratch memory

These functions are all implemented by reading and writing to various registers within the PVI.

Graphical features of the PVI

Screen colour      Score digits      Objects      Background grid      Object duplicates

PVI registers

Internally the PVI registers are located at addresses $F00 to $FFF. Within the console's memory map they are located at addresses $1F00 to $1FFF. Programmers only need to be concerned with these latter addresses, so those will be the ones we use throughout this book. For the same reason we will also ignore the areas where the memory map is duplicated.

Note that some control registers are write-only. If a program needs to know the contents of any of them, a duplicate copy must be saved elsewhere. Tests on some of these registers on one console indicate that if a read operation is attempted the contents will be corrupted.

PVI registers 1F00 Object descriptor 1 1F0E 2 bytes of scratch 1F10 Object descriptor 2 1F1E 2 bytes of scratch 1F20 Object descriptor 3 1F2E XXXXXXXXXXXXXXXXX 1F40 Object descriptor 4 1F4E 32 bytes of scratch 1F6E XXXXXXXXXXXXXXXXX 1F80 Background vertical bar definition 1FA8 Background horizontal bar extensions 1FAD 1 byte of scratch 1FAE XXXXXXXXXXXXXXXXX 1FC0 Control registers 1FD0 Duplicate 1FC0 1FE0 Duplicate 1FC0 1FF0 Duplicate 1FC0		Object descriptor 1,2,3 or 4 0 Shape of object, 10 lines of 8 bits 1 2 3 4 5 6 7 8 9 A Horizontal coordinate of object B Horizontal coordinate of duplicate C Vertical coordinate of object D Vertical offset of duplicate

Control registers

Byte	7	6	5	4	3	2	1	0	Read/Write	Function
1FC0	Size object 4		Size object 3		Size object 2		Size object 1		Write only	Sizes of objects
1FC1			R1	G1	B1	R2	G2	B2	Write only	Colours of objects
1FC2			R3	G3	B3	R4	G4	B4	Write only
1FC3							Format	Position	Write only	Score format and position
1FC4
1FC5
1FC6		Grid colour R G B			Grid enable	Screen colour R G B			Write only	Grid enable and colours
1FC7	Sound								Write only	Tone period
1FC8	Score digit 1				Score digit 2				Write only	Values of the four score digits
1FC9	Score digit 3				Score digit 4
1FCA	Object/grid collision 1 2 3 4				Object display complete 1 2 3 4				Read only	Collision status. Object display completion. VRLE set at leading edge of VRST. All bits reset when read or at trailing edge of VRST.
1FCB		VRLE	Inter-object collision 1/2 1/3 1/4 2/3 2/4 3/4
1FCC	A/D pot 1								Read only	A/D value, valid during VRST only.
1FCD	A/D pot 2
1FCE
1FCF

Objects

The PVI has four, programmable, two-dimensional objects that can be positioned anywhere on screen in a single 3-bit colour in one of four different sizes. It is said that the PVI was the first device to have this capability. This style of graphic was later termed a sprite, but we will stick with 'object' in this book as it is the term used in the Signetics datasheet.

Object shape

The shape of the object is set in a ten-byte array. Bits set to zero are transparent, while bits set to a one are displayed as the selected colour.

Object colours

Object colours are set by registers $1FC1 and $1FC2. See Programming colours for details.

Object size

Objects can be displayed in four different sizes as set by two-bits in register $1FCO.

Magnification	Size	MSB	LSB
x1	8 x 10	0	0
x2	16 x 20	0	1
x4	32 x 40	1	0
x8	64 x 80	1	1

Object position

Positioning of objects is determined by reference to the origin of the tv scan at the top-left of the screen.

The horizontal coordinate (i.e. $1F0A) is set as the number of horizontal clocks to skip after the start of the raster line before the object is displayed. It maybe changed while the object is being displayed in which case the remainder of the object will be displaced. The vertical coordinate (i.e. $1F0C) of an object is set with an eight-bit unsigned value equal to the number of lines to skip before the object is displayed. This value must be set before the trailing edge of VRST.

Object duplicates

One or more duplicate objects can be displayed further down the screen. The horizontal coordinate of the duplicate (i.e. $1F0B) is set in the same way as the original object. The vertical coordinate of the duplicate is set as the "number of lines to skip - 1" after displaying the last line of its predecessor.

The horizontal coordinate of the duplicate is read by the PVI on each line during HRST, so if it is changed during display of a duplicate, the rest of the duplicate will appear at a different position. The vertical coordinate of the duplicate must be programmed before the object completion status bit is set by the previous occurrence of that object.

Object completion

When the last line of an object has been displayed an object complete bit is set in register $1FCA and the PVI generates an interrupt signal for the microprocessor.

Object collision

If an object overlaps the background grid an object/grid collision bit is set in register $1FCA. If two objects overlap an inter-object collision bit is set in register $1FCB.

Score

The four score digits may be displayed as either two separate 2-digit numbers or as a single 4-digit number. They may be displayed at the top or bottom of the screen, or by reprogramming during the vertical scan between lines 40 and 199 they may appear at both top and bottom. The mode of display is determined by the format and position bits of register $1FC3.

$1FC3	7	6	5	4	3	2	1 Format	0 Position
0							two 2-digit	top
1							one 4-digit	bottom

Each score digit is 12 clocks wide and 20 lines high.

The top position corresponds to lines 0 to 19 of the background grid, or vertical coordinates 20 to 39. The bottom position corresponds to lines 180 to 199, or vertical coordinates 200 to 219.

The colour of the score is the inverse of the colour programmed for the background grid. See: Programming colours

Horizontal position of score digits

	1	2	3	4	Appearance
Format = 0	28	44	76	92	12  34
Format = 1	28	44	60	76	1234

The four score digits in $1FC8 and $1FC9 may be set to any hexadecimal value 0 to F. Values 0-9 display as the number, A-F display a blank.

Background grid

The PVI has a programmable background grid comprised of an array of elements, 16 wide and 20 deep.

These elements are vertical bars, one pixel wide, arranged in pairs of horizontal rows; the first row of each pair is two raster lines deep and the second row is 18 raster lines deep. Each of these 320 vertical bars may be switched on or off by setting a bit in a 40-byte array located at memory addresses $1F80 - $1FA7.

The vertical bars may be extended horizontally, controlled by five registers $1FA8 - $1FAC. Each of the five extension registers control four consecutive rows of vertical bars.

Extension registers Register 7 6 5 4 3 2 1 0   Extend group x1,2,4 Extend row x8 $1FA8 Rows 1-4 4B 4A 3 2B 2A 1 $1FA9 Rows 5-8 8B 8A 7 6B 6A 5 $1FAA Rows 9-12 12B 12A 11 10B 10A 9 $1FAB Rows 13-16 16B 16A 15 14B 14A 13 $1FAC Rows 17-20 20B 20A 19 18B 18A 17

Group extension Extension    7        6     x1 0 0 x2 0 1 x1 1 0 x4 1 1

Bits 5-0 of the extension register enable all of the vertical bars in a horizontal row to be extended to 8 pixels. The 18-line vertical bars are divided into two parts (A and B) each of 9 lines making it possible to extend the top and bottom part of the bars by 1, 2 or 4 pixels.

Bits 7-6 of the extension register enable all of the vertical bars in a group of four rows to be extended by to either 1, 2 or 4 pixels.

The colour of the background grid is set by bits 2-0 in register $1FC6. The background grid is enabled by setting bit 3 in register $1FC6. If the background grid is not enabled, registers $1F80-$1FAC maybe used as scratch memory, a whopping 45 bytes, in addition to the 37 bytes of scratch memory

Sound

The PVI can generate a single audio square wave output. Its frequency is set by register $1FC7. If this value is 0, the square wave is inhibited, otherwise its period is 2(n+1)T_H, where n is the value set in $1FC7 and TH is the horizontal reset period. For a PAL system this simplifies to 128(n+1)μs.

If the sound register is changed while audio is being output, the change will not become effective until the next negative or positive transition of the audio signal.

See PVI audio frequency chart for a table of frequencies attainable.

Analogue to digital conversion

Two analogue to digital converters in the PVI are used to determine the position of the analogue joysticks. External circuitry driven from the processor's flag output is used to select input from either the horizontal or the vertical potentiometers.

The conversion process occurs during the active part of the video scan. The resulting digital values must be read from $1FCC and $1FCD during the vertical reset period to get a valid result.