Video Graphics Using The Spi On The Mc68 Hc11 Microcontroller

The Serial Peripheral Interface (SPI) input/output capability of the MC68HC11 microcontroller is a feature of the MC68HC11 architecture that is often overlooked by casual experimenters. It is designed to interface to input/output devices that include special hardware specifically meant to connect to the SPI. However, the SPI provides a handy way to output a generic high-speed stream of bits from the MC68HC11 without requiring additional external hardware. That capability has been employed to generate the video signal for an alphanumeric text display on a standard video monitor, as described in an earlier ASEE paper 1 . This paper details a technique for producing a simple graphics display on a standard video monitor, using the SPI unit to generate the high-speed bit stream necessary for the video signal driving the monitor. The display produced is adequate for simple line graphs or other comparable displays. The heart of the technique described in this paper is controlling the timing of data emerging from the SPI very carefully, at the clock cycle level, and thus establishing the position of various graphical elements along the scanlines of the standard video display. The technique relies heavily on creative programming techniques to achieve this clock-cycle-level control of the signal timing, clearly demonstrating the operation of the SPI unit while at the same time serving as a useful graphics output utility that can be used by other software. The software routines that control the MC68HC11’s SPI unit to produce the graphics output are revealed in this paper, as are the few discrete components necessary to produce a composite video signal to drive a standard video monitor. Equipping an MC68HC11 microcontroller with this feature adds a handy output function that can be used in any MC68HC11 system.