The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Control Circuits
Listing 11-2. Demonstrates gain control of op amp.
10 REM A=address of output port
20 A=0E400H
30 DO
40 FOR I=0 TO 7
50 XBY(A)=I
60 PRINT “Gain = ”,I
70 PRINT “Press any key to continue...”
80 G=GET
90 DO : G=GET : UNTIL G0
100 NEXT I
110 WHILE 1=1
120 END
to output adds stability, as recommended by the LF13006/7’s data sheet. I found that the
capacitor did keep the op amp’s output from oscillating at certain gain settings.
The LF13006/7 also has two matched, uncommitted resistors of about 15K each, which you
can use as you wish.
Listing 11-2 steps through the available gains. You can verify circuit operation by connecting
a signal such as a sine-wave output of a signal generator to VIN, and monitoring VOUT with
an oscilloscope. To use the full range of gains, the signal at pin 3 must be quite small. For
example, if your input is 100 millivolts peak-to-peak, at a gain of 128 the output is 12.8
volts.
Controlling a Stepper Motor
Figure 11-4 shows an 8-bit output port controlling a four-phase unipolar stepper motor.
Applying power to the motor’s four phases, or coils, in sequence causes the motor to turn.
The port uses a 74HCT374 latch addressed at 0E400h. If you use a 74LS374, add 10K
pullups to the outputs, to ensure that logic-high inputs to U3 and U5 meet the specified
minimum of 3.5V.
You can choose from several modes of operation for the motor, with each using a different
sequence of pulses. The frequency and sequence of the pulses determine the speed of the
motor.
In the circuit shown, after you write values to the port to set the speed and operating mode,
the motor continues to run automatically, using the selected parameters. This frees the
The Microcontroller Idea Book 191