There are two ways of transmitting a byte between two digital devices. We can either transmit the byte in parallel or we can transmit the byte in serial form. Figure 22 illustrates the difference between these two types of communication mechanisms. In parallel interfacing, each bit has a single wire devoted to it and all bits are transmitted at the same time. In serial interfacing, the bits are transmitted as a series of pulses.
The Motorola 68HC11 supports both parallel and serial communication. The MicroStamp11 uses the parallel ports PORTB and PORTC as interfaces to external memory chips. So the MicroStamp11 programmer does not have access to these parallel ports. The MicroStamp11 programmer has access to two types of serial communication subsystems through PORTD. These are the asynchronous serial (SCI) and the synchronous serial (SPI) communication interfaces.
This learning module presents basic methods associated with using the SCI and SPI serial communication subsystems in the MicroStamp11. The learning module shows you how to build a system that measures the voltage over a potentiometer and displays this voltage on a liquid crystal display (LCD). The potentiometer's voltage will be measured using an 8-bit analog-to-digital converter IC (ADC0831). The ADC0831 does an 8-bit analog-to-digital conversion and then transmits the converted measurement over a synchronous serial interface (SPI) to the micro-controller. The micro-controller then displays the converted measurement on a 2 line by 16 character LCD module (BPI-216). Communication between the MicroStamp11 and the BPI-216 is accomplished through an asynchronous serial interface (SCI).