Servicing Personal Computers, 3rd Edition

Microcomputer systems 49

Φ2

RES

System clock phase 2. Data transfers only take place when Φ2 is

high.

This signal also

provides the timebase for the VIA timers and shift registers.

Clears all internal registers (to logic 0) except for those associated with the shift register and

timers. Also disables all interrupts emanating from the VIA.

Keyboards

As an example of the use of PIA devices, we shall now consider the operation of a typical

keyboard decoding arrangement. On most microcomputers, the keyboard consists of a matrix

of 60 or more switches, with the possible addition of further switches reserved for specific

functions. The main keyboar ...