Chameleon-PIC Nurve Networks, Chameleon-PIC Datasheet - Page 178

• Manufacturer Part Number: Chameleon-PIC
• Description: MCU, MPU & DSP Development Tools PIC24 & PROPELLER DEV SYSTEM (SBC)
• Manufacturer: Nurve Networks

Specifications of Chameleon-PIC

• Processor To Be Evaluated: PIC24
• Data Bus Width: 16 bit
• Interface Type: USB, VGA, PS/2, I2C, ISP, SPI
• Operating Supply Voltage: 3.3 V, 5 V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The GFX NTSC tile engine is relatively simple as tiles engines go. The single driver file contains both Spin and ASM
language. The driver file name is CHAM_GFX_DRV_001_TB_001.SPIN, I highly recommend you sit down and read it
line by line if you want to understand how it works. However, it has 2 major parts to it; the SPIN message dispatcher and
the ASM code graphics driver. When the driver is started, it loads the ASM code into a Propeller core and that starts
drawing graphics on the NTSC screen immediately. Then the SPIN code has a lot of functionality to print characters,
strings, numbers, and emulation of the crude VT100 like terminal that all the other NTSC and VGA terminal drivers
require. In addition to the terminal functionality, there is a set of “virtual registers” that are used to communicate with the
ASM driver with. These registers control everything from the top and bottom overscan, tile map pointers, fine scrolling,
etc. We will see some of these constants in the following sections when we look at the header files. But, for now, the
whole point is that when the GFX driver is started, the SPIN code launches the ASM driver, it starts immediately drawing
the tiles in the 32x24 tile map. Then we can make changes to the tile map using the high level “terminal” functions or the
low level graphics functions (which is what this section is all about).
Tile Map and Playfield Size
Now, let’s take a look at the specifics of this tile map engine. Referring to Figure 22.1, the tile map starts off 32x24
displayed on the NTSC screen. This is the physical resolution and never changes. That is, we can never display more
than 32x24 tiles on the physical screen. However, the “playfield” can be much larger to facilitate scrolling or page flipping.
The engine supports horizontal tile pitches of 32, 64, 128, and 256. Thus, horizontally, the playfield can be quite large.
Vertically, there are no power of 2 restraints, so you can make the vertical pitch anything you want (as long as you don’t
run out of memory). The tile map itself starts at the declaration tile_maps (make sure to take a look yourself) in the driver
portion of code and is controlled by changing the global variable tile_map_base_ptr_parm in the register interface.
The tile map represents what’s on the screen physically and the entire playfield virtually, which might be much larger than
the physical screen of 32x24. The tile map pointer above points to the start of the tile map, but how the tiles are
interpreted, that is, how many per row is controlled by a register that controls the setup of the tile mode. This variable in
the driver is called tile_map_control_parm and controls a number of interesting things, here’s the initial default setup
code for this variable on driver start up:
tile_map_control_parm := $AC_AC_00_00 ‘ format $bc_tc_hv_ww, tc = top overscan color,
So the format of this 4-byte value is
[bottom overscan color | top overscan color | horizontal and vertical smooth scroll | playfield width]
The top and bottom overscan colors (these are the colors on the top and bottom of your TV screen, usually black) are in a
simple luma/chroma format which we will discuss in the section below on color palettes. The horizontal and vertical
smooth scroll are encoded as 4-bits each, but only vertical smooth scrolling works currently and the valid values are 0..7.
Finally, the playfield width is controlled with this register as well. It can be 32, 64, 128, or 256 tiles wide, a subset of
powers of 2.
Now, this register is accessed via a number of separate “shadow” registers in the global register interface, but I wanted
you to see this in the code, so when we discuss the registers they are familiar.
Tile Map Entry Format
Each tile map entry is a 2-bytes In the following format:
(msb)[palette_index | tile_index](lsb)
The low byte is the actual tile character index 0..255, and the high byte is the palette index 0..255. However, in most
cases you won’t have this many tiles or palettes. In fact, initially there are only 16 palettes 0..15 and there are 144 bitmaps
extracted from file font file c64_font_05_16b.bmp followed by 16 blanks for user definition of course you can overwrite
‘ bc= bottom overscan color,
‘ h=horizontal scroll (0..7), v=vertical scroll (0..7),
‘ ww = width: 0 = 32 tiles, 1 = 64 tiles, 2 = 128 tiles, 3 = 256 tiles
‘ green overscan with $AC
© 2009 NURVE NETWORKS LLC “Exploring the Chameleon PIC 16-Bit”
178