Description
ECE 362 – Experiment 10 Rev 3/14
Bigger Bytes Lab Manual -1- © 2014 by D. G. Meyer
Experiment 10: D.C. Motor Speed Control and Digital Tachometer
Instructional Objectives:
· To illustrate a real-time embedded control application and the use of interrupts
· To provide experience using various 9S12C32 integrated peripherals, including the pulse width modulation (PWM) unit, the analog-to-digital (ATD) unit, the serial communications interface (SCI), the pulse accumulator (PA) unit, and the serial peripheral interface (SPI)
Parts Required:
· 4N28 optical isolator (included with DK-3 parts kit)
· TIP 120 or TIP 122 NPN transistor (included with DK-3 parts kit)
· 2 x 16 LCD and 16-pin single-row header (previous experiment)
· GAL22V10 programmed as an 8-bitshift register (previous experiment)
· Breadboard and wire
Preparation:
· Read this document in its entirety
· Review material on the ATD, PWM, TIM/PA, SCI, and SPI
· Create your solution in “C” using Code Warrior
1. Introduction
In lecture you have been introduced to a number of peripheral subsystems included on the 9S12C32 microcontroller. The best way to understand how these independent functional units operate, as well as to gain an appreciation for how they might be utilized in real-world applications, is to illustrate their use in a real-time control experiment. In this lab you will investigate the use of several key HC(S)12 peripherals: the pulse width modulation (PWM) unit,
the analog-to-digital converter (ATD), the serial communications interface (SCI), the pulse accumulator (PA) unit (itself part of an extensive timer subsystem), and the serial peripheral interface (SPI). In this real-time application, you will control the speed of a small D.C. motor using a pulse-width modulated signal derived from an analog input control voltage.
Your solution should utilize the two pushbuttons (“PAD7” and “PAD6”) on the docking module: the left pushbutton (“PAD7”) should stop the motor (if it is running), while the right pushbutton (“PAD6”) should start the motor (if it is stopped). The left LED (connected to port pin “PTT1”) should be on when the motor is stopped; the right LED (connected to port pin “PTT0”) should be turned on when the motor is running.
The 3-digit (geared down) drive shaft speed (in RPM) as well a percent-of-maximum bar graph will be displayed on an LCD (as shown below). An external shift register is used to interface the LCD to the microcontroller via the SPI module (MOSI, port pin PM4; and SCK, port pin PM5), as in Experiment 8.
NOTE: The “C” code for
“main” and the ISRs must
be written and debugged
during your scheduled lab
period (LCD device driver
should be written and
tested as part of your prelab
preparation.)
ECE 362 – Experiment 10 Rev 3/14
Bigger Bytes Lab Manual -2- © 2014 by D. G. Meyer
2. Software Overview
The objective of this experiment is to control the speed of a small D.C. motor using pulse width
modulation. The PWM duty cycle will be a function of an input analog D.C. voltage (in the
range of 0 to 5 volts). The motor speed will be determined by the number of pulses detected by
the pulse accumulator; its instantaneous value will be estimated based on the number of pulses
accumulated from the motor’s 64-slot “chopper” over a one-second integration period. An
updated calculation of the (geared-down) drive shaft RPM (in BCD format) will be displayed on
the terminal screen once every second. The motor, to which the chopper is attached, drives a
gear-head, which then drives the external shaft; the gear ratio is approximately 28:1. Therefore,
the three-digit value displayed for the drive shaft speed should be the motor speed divided by 28.
The timer module (TIM) will be used to establish the ATD sampling rate as well as the
tachometer integration period and display update rate. The real time interrupt (RTI) will be used
to establish the pushbutton sampling rate.
The SCI will be used to print the RPM of the motor to the terminal in a buffered, interrupt-driven
fashion. To print characters you should call bco which writes them into the buffer TBUF. Also
in bco, the transmit interrupt should be enabled so that the buffer can be emptied. The SCI
transmit ISR will send out characters when the buffer is not empty. When the transmit ISR
encounters an empty buffer, it should turn off the SCI transmit interrupt so that no unintended
characters are sent. This methodology allows the processor can do other things in-between
character transmissions.
The RTI will be used to sample the pushbuttons on the docking board every 2.048 milliseconds.
Data for the LCD display will be shifted out to an external shift register (GAL22V10) and will
be interfaced to the SPI module through Port M (PTM).
You should reuse the LCD driver you created for Lab 8 in this experiment for all interfacing. On
the first line of the LCD, you should display the RPM, updated every second. On the second line
of the LCD, one box character (ASCII $FF) should be displayed for each 10%. Additionally, the
percentage-of-max should be right aligned. Noting the fact that each line of the LCD is 16
characters will be useful in doing this.
NOTE: Before generating the bar graph, the maximum RPM must be determined.
ECE 362 – Experiment 10 Rev 3/14
Bigger Bytes Lab Manual -3- © 2014 by D. G. Meyer
Left LED
Right LED
PT0
PT1
PAD7
PAD6
5 V
Left PB
Right PB
PAD0 10K
5 V
1 6
2
5
4
4N28
PT2
1N4001
TIP122
220 1000
5 V
12 V
MOTOR
5 V
PT7
CHOPPER
3. Hardware Overview
The basic interface circuit you will need to construct is illustrated in Figure 1. PWM output
Channel 3 (routed to port pin PTT3 based on MODRR register setting) is used to drive an
(optically isolated) NPN switching transistor. The PWM sampling frequency should be
approximately 100 Hz. Attached to the motor shaft is a 64-slot “chopper” disk that passes
through an optical detector. Signal conditioning circuitry included with the motor assembly
produces a pulse each time a hole in the disk passes through the aperture of the optical detector.
The signal produced by the optical detector is fed to the pulse accumulator input PTT7. Finally,
a potentiometer is used to provide a reference D.C. voltage (in the range of 0 to 5 volts) for ATD
input Channel 0 (PAD0). This value, when digitized, is used to control the PWM duty cycle.
Figure 1. Wiring diagram for potentiometer and motor interface.
Microcontroller docking board
Motor assembly
PT5
NOTE: Motor assemblies will only be available for checkout during times that a course
staff member is on duty. Debug your software using an oscilloscope (to view PWM output
generated on PT3) and a function generator (connected to PT7) to simulate the chopper.
PT3
ECE 362 – Experiment 10 Rev 3/14
Bigger Bytes Lab Manual -4- © 2014 by D. G. Meyer
You will interface to the LCD in the same way that you did in Experiment 8 (except for the Port
T connections) The description of the LCD interface is reproduced below for your convenience.
An external 8-bit shift register (GAL22V10) will be used to interface LCD to the microcontroller
via the SPI module (MOSI, port pin PTM4; and SCK, port pin PTM5). The LCD will be
interfaced as described to the microcontroller module as described in the table below:
LCD
Pin # LCD Pin Description Connected to
Microcontroller
1 Vss (ground) Vss (ground)
2 Vcc (+5V) Vcc (+5V)
3 VEE (contrast adjust) Vss (ground)
4 R/S (register select) PTT4
5 R/W’ (LCD read/write) PTT5
6 LCD Clock PTT6
7 DB[0] (LSb) Q[0]
8 DB[1] Q[1]
9 DB[2] Q[2]
10 DB[3] Q[3]
11 DB[4] Q[4]
12 DB[5] Q[5]
13 DB[6] Q[6]
14 DB[7] (MSb) Q[7]
15 Not connected
16 Not connected
Some of the LCD pins require a bit more explanation:
Mnemonic Name Description
RS Register
select
This pin is logic 0 when sending an instruction
command over the data bus and logic 1 when
sending a character.
R/W’ Read/write This pin is logic 0 when writing to the LCD,
logic 1 when reading from it. For this lab, we
will only write to the LCD.
LCDCLK LCD clock This pin latches in the data on the data[7:0]
bus on the falling edge. Therefore, this line
should idle as logic 1.
Step 1. Interfacing
Interface the LCD and LEDs to your microcontroller kit as described in the Hardware Overview
section (note that different Port T pins are used than in previous experiments – see table).
This will require programming your GAL22V10 to function as an 8-bit shift register, as
described in problem 5 of the Module 2 homework. Complete all the interface wiring on your
breadboard as part of your pre-lab preparation.
NOTE: Port T connections are different than those used
in previous experiments.
ECE 362 – Experiment 10 Rev 3/14
Bigger Bytes Lab Manual -5- © 2014 by D. G. Meyer
Step 2. Software
Prior to your scheduled lab period, copy the LCD device driver routines you wrote for Lab 8 and
test them with the interface circuitry completed for Step 1. Complete the remainder of the “C”
skeleton file provided on the course website during your scheduled lab period. Note that the
“finished product” should work in a “turn key” fashion, i.e., your application code should be
stored in flash memory and begin running upon power-on or reset. Demonstrate the completed
motor speed control system to your lab T.A.
Step 3. Submission
Submit your “.C” solution file on-line after demonstrating it to your lab T.A. (but before leaving
lab). Be sure identifying information (i.e., name, class number, and lab division) is included in
the files you submit – credit will not be awarded if identifying information is omitted.
Bonus Credit
Add a “musical” (frequency modulation) mode to the motor speed control function. Here, use the
left pushbutton to toggle motor run/stop, and the right button to toggle between potentiometer
(voltage) speed control (“stock” solution) and a frequency control mode. Indicate the mode of
operation in column 16 of the first line of the LCD display: “V” for voltage, “F” for frequency.
For the frequency control mode, use the function generator at your lab station as a “musical”
tone: connect the SYNC output of the function generator (CMOS compatible square wave) to
PTT2 (timer channel 2), configured for input capture mode. Write a function that converts input
capture timestamps to frequency, and convert the frequency to a proportional (8-bit) PWM duty
cycle: the lower the frequency, the slower the motor should run, and the higher the frequency,
the faster the motor will run. So, for example, if you wish to allow a frequency control range of
1000 Hz, you will want to divide the frequency detected by four to produce a PWM duty range
of 250 (assuming a PWM period of 255).
While beyond the scope of this experiment (but well within the realm of an exciting potential
Mini-Project), a microphone/preamp could be added along with some input conditioning
circuitry to create a “ hummer” – a device that converts musical tones to different motor speeds.
The speed of a color wheel (or disco ball?) could be modulated by music.
As they tend to screech on most infomercials, “Amazing!”…
ECE 362 – Experiment 10 Rev 3/14
Bigger Bytes Lab Manual -6- © 2014 by D. G. Meyer
Thought Questions
Answer the following thought questions in the space provided below:
(a) Why is an optical isolator used to interface the 9S12C32 PWM output pin with the NPN
switching transistor that controls the motor? Could an NPN switching transistor like a TIP
120 be interfaced to a 9S12C32 port pin without using an optical isolator?
(b) What are the advantages of using PWM to control the speed of a D.C. motor? (Compare
PWM to any alternatives you think might be feasible.)
(c) Why not just connect the motor directly to the potentiometer to control its speed? What
would happen? (DO NOT TRY THIS!)
(d) What would happen if a higher PWM sampling frequency were utilized? Is there a practical
limit to the sampling frequency that can/should be utilized? (This one you CAN safely try!!)
(e) Given a 64-slot chopper and a 1.0 second integration period, what is the RESOLUTION of
the tachometer (i.e., the difference in RPM estimate caused by pulse count difference of one
over the integration period)?
