MICRO PORT EXPANSION
The two methods of input-key matrixing described above represent one form of port expansion. In many cases the computing power required (small in relation to that of home computers and calculators) can easily be provided within the compass of one IC package. Difficulties arise, however, with the sheer number of pins required on the package, and particularly with the physical arrangement of printed conductors on the chip’s mounting panel. Between 80 and 160 pins is the norm for a microprocessor.
Expander systems are used to route information to and from a micro on a time-sequential (strobing) basis under the control of expander-address bus data generated by the micro. Fig. 16.6 gives an idea of how this system works. There are three expander address inputs to a port expander chip: E0, E1 and E2. Three binary input lines give eight possible combinations from 000 to 111, and these eight addresses in the expander chip are accessed on a continually rotat- ing basis. Each address corresponds to one input (usually a ‘latched’ port which holds one signal until it is replaced by another). The input is routed through a switch to one input port of the micro each time its address comes up. The pre-programmed micro thus polls each of the eight input points in turn, recognising them by the address-code being generated at that time. In the particular case of Fig. 16.6 three micro ports can thus monitor 24 inputs via six connecting leads – three in the address bus and three in the data bus. For the sort of
status, command and feedback information used in electromechanical control of videotape and videodisc decks and peripherals, the relative slowness and discontinuity of this form of data transfer is not important. The same port-expander principle can be and is applied to microprocessor outputs too, whereby the micro holds the output data intended for each specific output point until its destination address is generated, when it is released onto the data bus. It is also possible (following standard computer practice) to utilise a bi-directional data bus system with programmable I/O ports, but this degree of complexity is seldom necessary in domestic entertainment systems. An example of a bi-directional data bus is given in Chapter 22.
Other artifices to reduce the interconnection- and IC pin-count are used as alternatives to strobe port-expansion. Some processors – typically custom-designed types for miniature equipment – use ternary (tri-state) logic in and which three levels (high, medium, low) of volt- age are used and recognised. Another approach is the use of serial data, which generally requires the use of shift-registers as clocked parallel-to-serial and serial-to-parallel converters at sending and receiving ends respectively.