PCPIC

The PCPIC can perform many functions, although not necessarily all at once. Many of the functions are concerned with counting - either counting pulses in a fixed time interval (frequency measurement),counting a fixed clock for a variable time (time measurement) or simply counting and totalising input pulses. The three 8254 chips on the PCPIC contain a total of nine 16-bit counters to perform the counting. Of these nine, eight can be connected to the input pins on the connector in some way, and the remaining one is used as a timebase. The eight counters are in two sets of four; each set is independent, although they do share the same timebase.

The clock for the timebase can be jumper-selected as either 1MHz or 25kHz. This can then be divided down by up to 65536 in the timebase counter to give a very wide range of possible timebase outputs. Generally, the higher frequencies are used for time measurement and the lower ones for frequency measurement.

It is not just the timebase frequency that is programmable; what the timebase output does can also be controlled by software. For example, it can be used to open the counter gates for a precisely determined time period - this is the basis of frequency measurement.Alternatively, an external signal can open the counter gates while a known frequency from the timebase is counted, so times can be measured.

The counter gates determine whether the counters can count or not.In some cases - for example free-running counters - they can be permanently open. In other cases - for example measuring the time between two signals - the gates must be opened by one signal and closed by another. To cope with this case, the PCPIC has eight latches which can be set and reset by pairs of signals.

As can be seen, a fair amount of 'rewiring' is necessary to configure the PCPIC to perform these different functions. This is all achieved by setting up bits in a register on board and changing the mode of operation of the counters.

The PCPIC can also be used for digital I/O. One group of eight lines on the connector can be read directly as inputs. Another group can also be read as inputs but is connected to the counter outputs; since these are open collector the lines can be used as general-purpose inputs if the counter outputs are disabled. In addition, these eight lines can be used as interrupt inputs, so interrupts from either the counter outputs or the external inputs can be generated. A third group of lines acts as outputs - they are also used to reset the input latches if necessary.

Interrupts may be generated on three different conditions and may be fed to the PC's IRQ2, 3, 4, 5 and 7 interrupt lines. These conditions are that either any one or more inputs are high, or that any one or more inputs are low, or that all inputs are low. Interrupts can also be disabled. Any of the interrupt generating conditions or the interrupt disable condition, may be selected by software.

A 50-way D-type connector is used to connect to the PCPIC. This allows a ribbon-cable to connect to individual cable connectors or to other boards which either modify the signal in some way or contain other types of connectors. These are called signal-conditioning boards.