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EENG 383
Microcomputer Architecture and Interfacing
Spring 2015
Lab 2 – Stepper Motor
January 22, 2015
In this lab you will use the HCS12 microcontroller on the SSMI board to control a stepper motor.
You will implement simple delay loops in order to control the timing and speed of the stepper
motor. You will also gain experience with a voltage regulator and a Darlington driver.
1
Prelab Questions (answer prior to coming to lab)
Read this handout before coming to lab and answer the following questions.
1. Voltage regulator:
a. What is the minimum input voltage needed in order to generate an output voltage
of 5V? Hint: see “recommended operating conditions” on the device datasheet.
b. Estimate the value of R2 in Equation 1, so that the output voltage is 5V.
2. ULN283 Darlington driver:
a. How much current can the ULN2803 sink on each output (refer to the data sheet
on the course website)?
b. What should pin 10 of the ULN2803 be connected to?
c. What is the typical collector-emitter saturation voltage of the ULN2803?
3. Looking at the schematic for the SSMI board (on the course website), you can see that
pin 1 of jack J5 (this is what you connect your power supply to) comes out to a signal
labeled Vpwr. This signal is also brought out to header H1 on the SSMI board. What pin
is Vpwr on H1?
4. Port T is another input/output port, just like Port M. Where are Port T, pins 0..3 located
on H1?
5. Write a little assembly language code snippet that outputs a logic high (+5V) on PT31.
2
Background
The stepper motor in lab is unipolar, permanent magnet type
with six pins (Figure 1). The “VM” pins will be connected to a
+5V source.
By pulling the other four pins low in sequence, we energize
the coils in sequence and make the rotor turn. For example,
the sequence A→B→A’→B’ will cause the rotor to turn to
positions: 8→6→4→2.
1
“PT3” means Port T, pin 3.
1
Figure 1. Unipolar stepper motor (from
http://www.solarbotics.net/library/pdflib/
pdf/motorbas.pdf)
EENG 383
Microcomputer Architecture and Interfacing
Spring 2015
Pulling a pin low means that a lot of current passes through the coil and must be sunk by the
driving device. The microcontroller has far too little current output sinking capability to drive
this stepper motor (or any motor, for that matter).
Therefore, we will use the ULN2803 driver chip to interface to the motor. Study the data sheet
of this chip. This device contains 8 bipolar power transistors, configured as “Darlington drivers”
(Figure 2). Each of the outputs is independently controllable. The output of each driver
transistor is open collector. Our strategy will be to use four of these outputs to drive the four
windings of the stepper motor. Each output, when asserted, must be able to sink the current
flowing through its respective winding of the motor.
Figure 2. (Left) ULN2803 pinout. (Right) Each output is configured as a “Darlington driver”. Note the flyback
diodes.
One final issue has to do with the power supply for the stepper motor. The power supply that
you have in the kits is a +9V AC-DC wall adapter2. However, the stepper motor is rated for only
+5V DC. If you were to connect the motor to the +9V supply, the motor will overheat.
Therefore, we need to lower the voltage that is supplied to the motor.
We can do this with a voltage regulator. A voltage regulator is a circuit that can take a variable,
unregulated input voltage (such as from a battery or your AC-DC wall adapter) and output a
stable, constant voltage that doesn’t depend on the load. We will use the LM317 adjustable
voltage regulator, which is a chip with 3 terminals (“input”, “output”, and “adj”). See the data
sheet on the course website for the pinouts of this chip.
The LM317 voltage regulator adjusts its output voltage so as to make the voltage drop between
its “output” pin and its “adjust” pin equal to Vref = 1.25 V (see Figure 3). The input voltage must
2
Actually, if you measure the voltage with no load, you will probably get a higher voltage than +9V. This is a nonregulated power supply, so the output will fluctuate and will depend on the load. The +9V is just a nominal value.
2
EENG 383
Microcomputer Architecture and Interfacing
Spring 2015
be higher than the desired output voltage. From Figure 3, you can calculate the output voltage as
a function of R1 and R2:
 R 
VO  Vref 1  2   I Adj  R2 
R1 

Here, IAdj is a small current (typically 50 A) and is negligible in most applications.
3
(1)
Voltage Regulator
With the power supply unplugged,
assemble the circuit shown in
Figure 3 on the protoboard area of
the SSMI board. Vi should be
connected to Vpwr on H1. For R2,
use the little “trimpot” that comes
in the kit. Adjust the trimpot to the
value that you calculated in the
pre-lab exercise. Show your
circuit to the instructor and get his
or her approval before proceeding.
Input
Vi
Output
LM317
Adjust
Vref =
1.25V
VO
R1
1K
Iadj
R2
Figure 3. LM317 adjustable voltage regulator. Capacitors can be
added to improve ripple rejection (see the device datasheet).
Plug in the 9V power supply and
connect it to the SSMI board at connector J5. Measure the output voltage Vo. It may be
necessary to adjust the potentiometer a little so that Vo equals 5V.
4
Stepper Motor
1. Estimate the step size (in degrees) of the stepper motor, by manually turning the shaft. You
will feel a detent at each step. Hint - it is one of these possibilities: 5.0 degrees, 7.5 degrees,
10.0 degrees.
2. Measure the resistance across the coils of the stepper motor with the multimeter, and identify
the pins in Figure 1. Note that the resistance from the center tap (e.g., VM in the figure) to
one of the ends (e.g., Phase A) will be half the resistance from one end of the coil to the other
(e.g., Phase A to Phase A’).
3. Estimate the current draw through the coils and make sure that the ULN2803 can handle this.
To compute this, divide the voltage drop across the coil (5V minus the collector-emitter
saturation voltage of the ULN2803) by the resistance of the coil. Include your calculations in
your lab report.
3
EENG 383
5
Microcomputer Architecture and Interfacing
Spring 2015
Driver Circuit
With the power off, place the ULN2803 onto the protoboard area of the SSMI board. We will
control the ULN2803 with four output pins of the HCS12 microcontroller chip. These are pins 0
through 3 of Port T. Identify where these come out to header H1.
Draw a schematic diagram of the connections from the microcontroller to the ULN2803, and
from the ULN2803 to the motor. Don’t forget the ground and Vcc (common) pins on the
ULN2803. The power to the motor should come from the voltage regulator output.
In this section you will design the circuit and make the connections. However, don’t plug in the
power adapter until you show your schematic and the circuit to the instructor and get his or her
approval before proceeding.
Connect the components.
1. Sign-Off 1: Show your schematic and the circuit to the instructor and get his approval before
proceeding.
6
Program
Create the following short program to configure PT0..PT3 as output pins, and then go into an
infinite loop:
ldaa
staa
bra
#%00001111
DDRT
*
; configure PortT, bits 0..3
; .. for output
; loop here forever
Compile and load this program and start the debugger. Manually write to Port T to turn on and
off the PT0..PT3 pins. You can do this by simply double clicking on address $240 in the
memory window and typing in a new value there. For example, to turn on PT0 you would write
a $01 to Port T; to turn PT1 you would write a $02, and so on. Find out what the sequence of
values should be to make the motor rotate.
Write a short program that continuously outputs the above sequence of values to Port T in order
to drive the motor continuously. Include a short (~1 sec) delay after each value (you can use the
module from class). The pseudo code for this program is:
Configure PT0..PT3 as output pins
while (true) do
store first value to Port T
delay
store second value to Port T
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EENG 383
Microcomputer Architecture and Interfacing
Spring 2015
delay
store third value to Port T
delay
store fourth value to Port T
delay
end while
1. Sign-Off 2: By adjusting the delay times, you can drive the motor at different speeds.
Calculate the delay necessary to achieve a rotation speed of exactly 1 revolution per second.
You may find it helpful to put a little mark on the shaft of the motor so you can see its
position. Demonstrate your program to the instructor. Include your calculations and the
assembly program in your report.
2. Sign-Off 3: Modify your program above so that instead of rotating continuously, the motor
will rotate through a one full revolution and then stop. To do this, replace the infinite loop
with a loop that tests a counter and stops when the count limit is reached. Demonstrate your
program to the instructor. Include the assembly program in your report.
3. Now increase the pulse rate of the motor until it starts losing steps (this will be evident
because the motor will not rotate smoothly through a complete turn). What is the maximum
pulse rate in terms of cycles per second, for which the motor does not lose steps?
7
Extra Credit
Modify the program so that it uses “half steps” instead of “full steps” (see Lecture 9, slide 5).
Have the motor rotate through a one full revolution and then stop. How many steps do you need
to have a full rotation?
5
EENG 383
Microcomputer Architecture and Interfacing
Spring 2015
Lab 2: Stepper Motor
Name: ________________________________Name: ________________________________
Task
Description
Initials
Sign-Off 1 Schematic and Implementation
Sign-Off 2 One RPS
Sign-Off 3 360 Degrees
8 Rubric
Deliverables
20 pts Pre-Lab
Code with flowchart/psuedocode
/7
Schematic
/ 5 Demonstrations
Circuit and program Explanation
/3
Graph, Scope traces
/5
Composition
Questions
Sign-Off 1:
Schematic/Implementation
5 pts
Total
/ 50 pts
6
5 pts
/5
20 pts
/5
Sign-Off 2: One RPM
/7
Sign-Off 3: 360 Degrees
/8