Nuts and Volts - April 2010

■ FIGURE 2. EA DOGM internal memory and pixel position map.

Control of the display is through Synchronous Peripheral Interface (SPI) for initialization, setting pixel contrast, pixel addressing, and pixel on/off. The use of SPI really simplifies interconnection between the Experimenter and the display. Only a few control lines plus power and ground are needed.

To support the internal workings of the display, a total of nine 1 µF external capacitors are needed as shown in Figure 3. The backlight which mounts under the display requires three 100 ohm resistors to current-limit the three integrated LEDS within the backlight. Make sure when mounting the display on the backlight to remove all the protective films, labels, and papers that are on the components. If left on (and the units are soldered together), they will form a permanent shadow and will block the light. This takes away from a clean display.

A timing diagram for the SPI is also shown. The SPI data out is clocked eight bits at a time. All communication between the Experimenter to the display is one way — only the Experimenter talks. The command or data indication (A0) must be valid during transfer. The EA DOGM brochure lists a table of all the programming commands to properly initialize the display. The ‘C’ Graphics library does this as well as handles all communications with the display.

Introducing the Graphics Library

To help use the capabilities of the DOGM, a Graphics library is included. This library provides an Application Programming Interface (API) that is really just a bunch of C functions and rules. We will discuss each of these with examples. The library lets you draw lines, bars, circles, triangles, rectangles, and bitmaps, plot data, and output text. These parts of the library API are the graphic primitives. However, in order to use these primitives a lot of low level drivers are required.

Most of these low level drivers function when the primitives are actually drawing, with the exception of initializing the SPI and display — this has to be done by you. The library was written using the Microchip PIC24F C compiler and occupies about 6,018 bytes of Flash and

■ FIGURE 3. The Experimenter and display hardware interface.

1,264 bytes of RAM. The library is fully contained in three files: Font.h (which contains the bit images for the different character fonts); Graphics.c (the library source code); and Graphics.h (a list of library functions for main code reference).

These must be included in the project and the FONT.H and GRAPHICS.H must appear in the MAIN code using the compiler directives #include FONT.H and #include GRAPHICS.H. A GRAPHICDEMO project is available from the Nuts & Volts website at www.nutsvolts.com to experiment with and use as a template for applications. The library assumes that the Experimenter I/O expansion bus is configured for SPI and digital control outputs. This I/O programming is accomplished at the beginning of the MAIN code in GRAPHICDEMO.

The VMAP, Bitmaps, Font, and Graphics Library Operations

There are several aspects in using the library to be aware of. The library requires — and sets up — a dedicated RAM area within the Experimenter PIC24F designated as VMAP. It is here where all the drawings occur prior to display. The VMAP RAM area is a C array that is configured as a linear memory map 128 bytes x 8 bytes, representing all pixels on the display. VMAP contents are manipulated using the graphic primitives and then dumped onto the display for a drawing or screen presentation. The VMAP is used to do the drawing and then the low level driver “dumpVmap ()” is used to transfer the contents to the display.