titus-larsen-titus-1981-apple-interfacing

cal AND
pointer.
subroutine has been loaded, along with the three-b
F.xampie -11·5. A l&imple fis Teoti1111 !'rogrnm
1050 POKE 768,32: POKE 769,PEEK(49321)
1060 Q = USR(O)
1070 IF PEEK(770) = 0 THEN 50
1080 D = PEEK(49320)
1090 . . . Continue here after data input
Typical devices that use flags in this way are keyboards, Hopp
disks, analog-to-digital converters, and other devices that may pro
vide data bytes at irregular periods.
In some cases, devices may not the necessary .flag circuit
within them for easy :flag control, or they may not generate logic
levels that are stable for relatively "long" periods so that they can be
properly detected by the computer. In these cases, the "flag"
a very short pulse. In fact, some pulses are too short to be
tected by the computer, if they are simply input by means of a
state input port
In cases such as this, it is necessary to design a circuit that
"capture" the flag pulse SO that it may be detected the vV,CH!_)UC,JL
sometime later. Even if the computer can test a flag bit every
milHsecods, it will frequently "miss" short pulses of a few micro"
seconds duration.
Flip-flop or latch circuits are generally used to remember the
ence of flag pulses. Typical flip-fl.op devices are the SN7474
Hip-fl.op, and the SN7476 J-K flip-flop. Most introductory digital
tronics books provide a good coverage of flip-flop devices if you
to review their operation.
A typical flip-Hop-based flag circuit is shown in Fig. 4-7. In
circuit, the input device generates a READY pulse that clocks
flip-flop, transferring the logic level from the D input to the Q
put. The Q output is detected by the computer through the use of
input pmt that is separate from the input port that is used for th{)
transfer of the eight data bits. The status of the flag bit
tested by the computer, as has been described. Once the ne,ces:sai,·y
action has taken place, in this case, the input of data
device, the flag flip-flop is cleared. A logic zero pulse,
plied to the clear input of the flip-flop serves this purpose. While
RD 49360 pulse used to control the 8-bit input port could be used to
dear the flip-flop, we have shown a separate clear signal, so that the
timing relationships can be shown, as in 4-8.