titus-larsen-titus-1981-apple-interfacing

49318
WR
TO SN7474 PIN 1
Fig. 6-22. Simple flag-clearing circuit schematic.
flop, be sure that you remove the wire that was used to connect·+
volts to pin 1 on the SN7474 HiJ flop.
The circuit shown in Fig. 6-22 will allow you to clear the flip-Ho
with a POKE 49318 command.
Modify your program so that line 65 is added:
65 POKE 49318,0
When this command is executed, the flag will be cleared. Since yo .
may not know the state of the flag when you start the program, you
might want to add a flag-clearing command at the start of the program,
too. Now run the program. When the flag is detected, the Ha
is immediately cleared. Then th count is incremented and dis
played. .
One of the benefits of using this type of flag, and using the assem
bly-language subroutine to check the flag, is that you do not "staff
the computer waiting for a flag, unless you want to. Thus, you ca
write a program to check for a flag. If the flag is not present, th
computer goes about some other task. If the flag is set, the device
associated with it is serviced, and the computer then goes on.
The BASIC interpreter in the Apple has a flag-checking command
called WAIT. This command may be used to test for a flag, but if
the flag is not found, the program continues to wai t for it, and it
cannot do anything else. If a program "hangs up" waiting for a flag
that never occurs, you must press the RESET key to re-establish
control of the Apple. We refer you to Basic Programming Reference
Manual for the Apple for more information about the WAIT com"
mand. This command does not incorporate any flag-clearing commands.
Purpose
EXPERIMENT NO. 16
A SIMPLE ANALOG-TO-DIGITAL CONVERTER
In this experiment, you will interface an 8-bit analog-to-digital
converter to the computer. Several different types of measurements
will be made.
Discussion
There are many applications for analog-to-digital converters, or
AID converters in computer systems. The A/D converters allow the
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