AN906

AN906
Stepper Motor Control Using the PIC16F684
Author:
MICROSTEPPING
Reston Condit
Microchip Technology Inc.
Single stepping, or turning a stepping motor at its rated
step size, results in less than smooth movement.
Microstepping is a technique used to smooth the
motor’s movement between full steps and to improve
the step resolution of the motor. Microstepping also
improves the efficiency of the system, because the
current in the windings of the motor is manipulated in a
controlled manner rather than being turned on and off
abruptly.
INTRODUCTION
This application note describes how to drive a bipolar
stepping motor with the PIC16F684. The Enhanced
Capture Compare PWM (ECCP) module is used to
implement a microstepping technique known as hightorque microstepping. The microcontroller’s 8 MHz
internal oscillator allows the signals generated by the
ECCP module to achieve frequencies above the
audible range.
Note:
A microstepping technique known as high torque
microstepping alternately varies the current in the two
windings of a stepping motor. Figure 1 shows a graph
of the current in the windings vs. angular position using
this technique.
Please refer to AN907: Stepping Motor
Fundamentals for information on the types
of stepper motors, microstepping and
current limiting techniques.
HIGH TORQUE MICROSTEPPING
Current
FIGURE 1:
1st Winding
+3S
+6S
+9S
2nd Winding
Angular Position (S = rated step size)
A brief description of what is happening is that one
winding is powered while the current in the other winding is gradually dropped to zero, reversed, and then
ramped up again. This sequence is then repeated for
the other winding. Note that the transition between a
winding being energized in one direction and then
energized in the other direction has a sinusoidal shape
(refer to Figure 1). This shape gives the smoothest
transition between the motor’s rated step increments
(i.e., 7.5 degrees). The way this shape is achieved
using a microcontroller is through the use of pulsewidth modulation. Modulating the input to the drive
circuitry for a particular winding will result in a current
that is proportional to the duty cycle of the modulated
waveform.
 2004 Microchip Technology Inc.
For instance, if a 5V stepping motor is rated at 1 amp,
then modulating a 5V supply across the winding at 50%
will result in a current of 1/2 amp (assuming a low
inductance motor). Equation 1 shows this relationship:
EQUATION 1:
I = D× IMAX
where IMAX is the rated current of the motor and D is the
duty cycle.
DS00906B-page 1
AN906
In order to achieve the sinusoidal transition from a
positive to negative charge in a winding, numerous
microsteps are needed. The number of microsteps
typically ranges from 4 to 32 microsteps per rated step
size. Rather than calculating the duty cycle for a particular microstep on the fly, a duty cycle look-up table is
implemented in firmware. The number of table values
is equal to the number of steps desired for a particular
microstepping sequence. Equation 2 is used to obtain
the duty cycle values for the top half of the table. The
second half of the table is simply the top half in reverse
order.
EQUATION 2:
PWM Generation Using the ECCP Module
The ECCP module on the PIC16F684 is well suited for
generating the PWM signal required for microstepping.
The module is capable of generating a 10-bit resolution
PWM waveform at frequencies ranging up to 7.81 kHz
using the microcontroller’s 8 MHz internal oscillator.
Higher frequencies are more practical in motor control
applications because a motor will typically produce undesirable audible noise at frequencies less than 16 kHz.
Only 8-bit resolution is needed for this application, which
means frequencies up to 31.2 kHz can be achieved with
the ECCP module.
The ECCP module has four modes of operation:
D(step number) = cos((step number×π)/((number of
steps)+1)) × ((2^bits resolution)-1)
1) Single Output
2) Half-bridge Output
3) Full-bridge Forward Output
Using Equation 2, the following duty cycle values were
calculated for a 16 microsteps per full step sequence
using an 8-bit resolution PWM waveform:
TABLE 1:
DUTY CYCLE VALUES FOR
MICROSTEPPING
Step
Number
D
1
251
9
24
2
238
10
70
3
217
11
114
4
188
12
154
5
154
13
188
6
114
14
217
7
70
15
238
8
24
16
251
DS00906B-page 2
Step
Number
D
4) Full-bridge Reverse Output
In Half-bridge mode, the module modulates two pins
simultaneously, pins P1A and P1B. For this application,
these two outputs are used to drive the two windings of
a stepping motor. Only one pin is set active at a time.
Enabling and disabling one pin or the other is done by
modifying the TRISC register. The following circuit
diagram shows how these pins are connected to a
bipolar drive circuit.
 2004 Microchip Technology Inc.
AN906
FIGURE 2:
BIPOLAR DRIVE CIRCUIT
10K
R1
VSUPPLY
P1A
VSUPPLY
Winding 1
CTRLA2
CTRLA1
VSUPPLY
VSUPPLY
P1B
CTRLB1
 2004 Microchip Technology Inc.
Winding 2
CTRLB2
DS00906B-page 3
AN906
Note the pull-up resistor on pins P1A and P1B. These
resistors clamp the respective line high when the pin is
tristated. It is important that the non-modulated line be
clamped high so that the NAND gates on either end of
the winding can turn on the adjacent MOSFET when
the respective control line is enabled.
TABLE 2:
The ECCP module is set up so that the waveforms on
pins P1A and P1B are identical. This is done by configuring the CCP1CON register so that P1A is active high
and P1B is active low. With no dead band delay, these
pins will behave identically. Configuring the module in
this way enables each winding control block to use the
same duty cycle look-up table values for it’s transition
sequence. The following table shows all eight winding
states.
WINDING STATES
STATE
0
1
2
3
4
5
6
7
Winding 1
Polarity
+ to 0
0 to -
-
-
- to 0
0 to +
+
+
Winding 2
Polarity
+
+
+ to 0
0 to -
-
-
- to 0
0 to +
P1A Duty Cycle
100% to 0
0 to 100%
100%*
100%*
100% to 0
0 to 100%
100%*
100%*
P1B Duty Cycle
100%*
100%*
100% to 0
0 to 100%
100%*
100%*
100% to 0
0 to 100%
TRISC, P1A
0
0
1
1
0
0
1
1
TRISC, P1B
1
1
0
0
1
1
0
0
CTRLA1
1
0
0
0
0
1
1
1
CTRLA2
0
1
1
1
1
0
0
0
CTRLB1
1
1
1
0
0
0
0
1
CTRLB2
0
0
0
1
1
1
1
0
* Pin is tristated and the pull-up resistor is clamping the line high.
In states 0, 2, 4 and 6, the first half of the duty cycle sine
look-up table (decreasing values) is referenced. In
states 1, 3, 5 and 7, the second half of the duty cycle
sine look-up table (increasing values) is referenced.
DS00906B-page 4
 2004 Microchip Technology Inc.
AN906
EXAMPLE APPLICATION
CONCLUSION
This example application demonstrates how to drive a
3.6 degree-per-step stepping motor. The motor used is
a bipolar stepping motor rated to draw 1/2 amp at 12V.
The PIC16F684 has an ideal set of features for lowcost stepper motor control. High torque microstepping can be implemented using its ECCP module and
very few external logic components. The
PIC16F684's 8 MHz internal oscillator will allow the
ECCP module to drive the transitioning phase of the
bipolar stepping motor at a frequency of 31.2 kHz and
still provide 8-bits of duty cycle resolution. This
frequency effectively eliminates unwanted audible
noise generated by the motor.
Hardware
Appendix A shows a schematic for the example
application included with this application note. The drive
circuit is composed of four Fairchild® Semiconductor
half-bridge MOSFET ICs (part number FDC6420C).
Two Microchip logic-input CMOS quad drivers are used
to drive the MOSFET ICs and to provide the logic
necessary for the implementation described in this
application note. The TC4467 has four on-chip NAND
gates and the TC4468 has four on-chip AND gates. The
inputs to each of the AND gates on the TC4468 are tied
together because this IC is used as a non-inverting
quad MOSFET driver for this implementation.
REFERENCES
AN907: “Stepper Motor Fundamentals”
Firmware
A flowchart illustrating the microstepping firmware
implementation of this example is in Appendix B. The
source code for this application note is included with
this application note on Microchip’s web site,
www.microchip.com.
Operation
There are five modes of operation in the example that
are sequenced through with single button presses. The
modes of operation are:
1.
2.
3.
4.
5.
Motor Off
Single-step mode
Half-step mode
Microstep mode
Position Control mode
In modes 2, 3 and 4, the speed of the stepping motor is
controlled by turning the potentiometer. Mode 5 uses
the potentiometer as a position dial. Turning the
potentiometer will cause the motor to microstep in one
direction or the other for a distance proportional to the
distance the potentiometer was turned.
 2004 Microchip Technology Inc.
DS00906B-page 5
R20
10K
100R
2
R9
CCW
10K
RC3
R18
1K
POT-3352E
RC3
P1B
P1A
RA4
RA5
CW
+5V
SW-B3F1000
S1
R10
0.1 uF
3
1
RA0 13
RA1 12
2 RA5
3 RA4
10K
RC2
R8
100R
R19
10K
RA5
RA4
10K
R7
AN2
RC2 8
7 RC3
PIC16F684
RC1
RC1 9
6 RC4
R6
P1B
RC2
RC0
RC0 10
RA1
RA0
5 RC5
4 RA3/MCLR RA2 11
GND 14
1 VDD
U1
P1A
+5V
AN2
R2
+5V
10K
C16
+5V
R3
DS00906B-page 6
10K
+5V
GND
12
GND
7
3Y 11
4Y 10
2Y
VDD 14
1Y 13
TC4468
6 3B
8 4A
9 4B
3 2A
4 2B
5 3A
2 1B
1 1A
U3
7
3Y 11
4Y 10
1Y 13
2Y 12
VSUPPLY
VSUPPLY
VDD 14
TC4467
9 4B
8 4A
6 3B
5 3A
4 2B
3 2A
2 1B
1 1A
U2
1 G1
S1
5
Q9:A 6
FDC6420C D1
FDC6420C D2
Q10:B 4
2
S2
3 G2
6
Q4:A
FDC6420C D1
1 G1
S1
5
FDC6420C D2
Q6:B 4
2
S2
3 G2
D5
3
BAT54S
3
1
1
Winding 1
BAT54S 2
VSUPPLY
2
D4
1 BAT54S
D6
3
1
3
2
Winding 1
D3
BAT54S 2
VSUPPLY
6
Q7:A
D1 FDC6420C
G1 1
S1
5
2
S2
G2 3
D2 FDC6420C
4 Q8:B
6
Q3:A
D1
FDC6420C
G1 1
S1
5
2
S2
G2 3
D2 FDC6420C
4 Q5:B
AN906
APPENDIX A:
 2004 Microchip Technology Inc.
AN906
APPENDIX A: (CONTINUED)
J7
DIN5P_RECEPTICAL
VSUPPLY
1
1
RA5
1
RA5
2
2
RA4
2
RA4
3
3
VPP
3
RA3
4
4
RC5
4
5
5
RC4
5
6
6
RC3
6
7
7
ICSPDAT
7
8
8
ICSPCLK
8
9
9
10
RA2
9
10
RC0
10
11
11
RC1
11
12
12
RC2
12
13
13
13
14
14
14
HDR1X14
C13
0.1 uF
C12
LM78L05ACM
OUT
IN
GND GND GND GND
2
3
6
7
1
+5V
P3
P4
P5
1 uF
7
C11
6
100 uF
4
2
5
8
0.1 uF
1
3
U10
C14
VSUPPLY
HDR1X14
 2004 Microchip Technology Inc.
P1A
P1B
RC3
RA0
RA1
AN2
RC0
RC1
RC2
+5V
HDR1X14
DS00906B-page 7
AN906
APPENDIX B:
Initialize
Internal Oscillator
Frequency = 8 MHz
Assign I/O Pins
Setup ADC Pin: AN2
Setup ECCP Module:
Half-bridge mode, 31.25 kHz
Waveform
Set TMR0 Parameters
Turn on TMR2
goto MotorState
DS00906B-page 8
 2004 Microchip Technology Inc.
AN906
APPENDIX B: (CONTINUED)
MotorState
Clear TMR0 Interrupt
Flag
Yes
TMR0
Interrupt Flag
Set?
Delay = Delay - 1
No
No
Is Delay equal
to zero?
Yes
Read Potentiometer
R3
Move high 4-bits of ADC Value into
Delay and Increment
Duty Cycle
Look-up Table
Initiate next ADC read
Load CCP1CON an CCPR1L with
Next Duty Cycle Value
No
State?
0
0
1
goto State0
2
3
goto State2
goto State1
 2004 Microchip Technology Inc.
Yes
End/Middle of
Look-up
Table?
Increment State
4
5
goto State4
goto State3
6
7
goto State6
goto State5
goto State7
DS00906B-page 9
AN906
APPENDIX B: (CONTINUED)
State0
State1
State2
State3
Enable P1A
Disable P1B
CTRLA1 = 0
CTRLA2 = 0
CTRLB1 = 1
CTRLB2 = 1
Enable P1A
Disable P1B
CTRLA1 = 0
CTRLA2 = 1
CTRLB1 = 1
CTRLB2 = 0
Disable P1A
Enable P1B
CTRLA1 = 0
CTRLA2 = 0
CTRLB1 = 1
CTRLB2 = 1
Disable P1A
Enable P1B
CTRLA1 = 0
CTRLA2 = 1
CTRLB1 = 0
CTRLB2 = 1
goto MotorState
goto MotorState
goto MotorState
goto MotorState
State4
State5
State6
State7
Enable P1A
Disable P1B
CTRLA1 = 0
CTRLA2 = 1
CTRLB1 = 0
CTRLB2 = 1
Enable P1A
Disable P1B
CTRLA1 = 1
CTRLA2 = 0
CTRLB1 = 0
CTRLB2 = 1
Disable P1A
Enable P1B
CTRLA1 = 1
CTRLA2 = 0
CTRLB1 = 0
CTRLB2 = 1
Disable P1A
Enable P1B
CTRLA1 = 0
CTRLA2 = 0
CTRLB1 = 1
CTRLB2 = 1
goto MotorState
goto MotorState
goto MotorState
goto MotorState
DS00906B-page 10
 2004 Microchip Technology Inc.
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•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
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•
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Company are registered trademarks of Microchip Technology
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© 2004, Microchip Technology Incorporated, Printed in the
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Printed on recycled paper.
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 2004 Microchip Technology Inc.
DS00906B-page 11
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DS00906B-page 12
 2004 Microchip Technology Inc.
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