Bit-mapped graphics on a budget using the Freescale S10 microcontroller

Graphics displays are handy output devices in microcontroller systems. This paper describes a simple and efficient graphics display using a bit-mapped approach, generated by the Freescale S12 microcontroller, implemented on Wytec's Dragon development board. The display uses an inexpensive CRT monitor or standard television monitor for output, and requires only a few passive components added to the Dragon development board. This graphics display uses hardware that has been developed earlier, and that has been disclosed and discussed in earlier ASEE papers¹'². This paper describes software that implements the bitmapped graphics output, and presents some lab experiments that use this graphics display, appropriate for assignments in introductory microcontroller classes. The critical task in generating a bit-mapped graphics display is getting the bits out of memory to the display's video signal fast enough to produce adequate resolution on the display. The Serial Peripheral Interface (SPI) unit on the S12 microcontroller is especially suited to high-speed bit stream output. With the bus clock on the Dragon development board at 24 MHz, the SPI can emit streams of bits from memory at up to 12 megabits per second, resulting in a resolution along a scanline of the display of approximately 600 pixels. The resolution along the axis of the display perpendicular to the scanlines is limited to the number of scanlines in the image, which is 239 scanlines in this design using an inexpensive television-type CRT display. In order to reduce the memory demands on the limited resources of the Dragon board, the bit rate emitted by the SPI is reduced to a quarter of the maximum rate, resulting in a display resolution of 239 x 128 pixels. Although this does not compare well with commercial high-resolution graphics displays, it is certainly useful in an educational environment, and is entirely adequate for teaching students how to use bit-mapped graphics. This paper describes required software and its interaction with the S12 microcontroller SPI unit to produce a bit-mapped graphics display on a standard television or inexpensive CRT display. The paper also investigates several experiments using this graphics display that are possible in an introductory microcontroller course lab setting.