Hardware Reference
In-Depth Information
Figure 9.10
Simplified internal arrangement of the 74C922 keyboard encoder
where software overheads have to be minimized, the use of a fully encoded
keyboard is obviously much to be preferred.
Encoded keypads employ dedicated encoder chips such as the 74C922. This
device contains all the necessary logic to interface a 4
4 keypad matrix to four
lines of a data bus or digital input port. The output is presented in binary coded
decimal (BCD) form and an additional signal is provided to indicate that data is
available from the keyboard. This active-high Data Available (DA) output can
be used to drive an interrupt line when the keyboard is used in conjunction with
a bus processor or may be connected via an open-collector inverter to one of
the interrupt request (IRQ) lines of the PC expansion bus.
The simplified internal arrangement of the 74C922 is shown in Figure 9.10.
The keypad scan may be implemented by the internal clock using an external
timing capacitor (C
T
) or may be over-driven by an external clock. On-chip
pull-up resistors permit keypad switches with contact resistance of up to 50 k
.
Internal debouncing is provided, the time constant of which is determined by
an external capacitor (C
D
)
.
The Data Available output goes high when a key is depressed and returns to
low when a key is released even if another key is depressed. The Data Available
