cd00270970

AN3208
Application note
Microstepping motor drive
with STM8A and STM8S microcontrollers
Introduction
Stepper motors are electrically powered motors that create rotation from electrical current
driven into the motor.
They are used in a wide variety of applications such as printers, automated machine tools,
disk drives, automotive dashboard instrument clusters, and other applications requiring
precise motion control. They are well-suited for positioning applications since they can
achieve very good positional accuracy without complicated feedback loops associated with
servomechanism (servo) systems. However, their resolution, when driven in the
conventional full or half step modes of operation, is limited by the configuration of the motor.
Many designers today seek methods to increase the resolution of stepper motor drives.
Dedicated stepper motor controllers/ICs are available on the market. These controllers
contain the special logic and high-current drive circuits necessary to operate the stepper
motors. In some applications, for example in automotive dashboards, stepper motors with a
lower current rating (20 mA) are used to power the needles or pointers that display
parameters such as vehicle speed or the engine RPM.
Stepper motors need to be driven in microstepping mode (see Section 4: Driving stepper
motors using STM8A and STM8S microcontrollers). However, in this case, the use of
dedicated stepper motor controllers may increase the system cost and complexity. As an
alternative, the motors can be driven easily using the resources located within a
microcontroller (example, pulse-width modulation timers and I/O pins), thus reducing the
hardware cost and complexity. CPU load is very low when using internal resources and the
microcontroller is not precluded from performing other control activities or driving other
external peripherals. For example, the STM8 is able to drive two stepper motors together
with an LCD glass containing a high number of segments such as a motorcycle dashboard
application.
The application described in this document is a software/low cost solution to drive stepper
motors in microstepping mode using the STM8 microcontroller. The main focus of this
application note is to explain how to drive the microstepping motor with STM8A and STM8S
devices. An overview of the various stepper motor types is given in Section 2: Types of
stepper motor. Stepper motor basics are explained inSection 4: Driving stepper motors
using STM8A and STM8S microcontrollers. Section 5 summarizes stepper motor software.
Reference documents
●
STM8A reference manual (RM0009)
●
STM8S reference manual (RM0016)
●
STM8A/S datasheets
Reference firmware
●
July 2010
STM8A/S firmware library
Doc ID 17411 Rev 1
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www.st.com
Contents
AN3208
Contents
1
Winding arrangement in two-phase stepper motors . . . . . . . . . . . . . . . 5
2
Types of stepper motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1
Variable-reluctance (VR) motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2
Permanent magnet (PM) motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3
Hybrid synchronous motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
Microstepping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4
Driving stepper motors using STM8A and
STM8S microcontrollers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5
Software
6
2/19
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1
Preliminary information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2
Software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.1
Main program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.2
TIM1 interrupt routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Doc ID 17411 Rev 1
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List of tables
List of tables
Table 1.
Table 2.
Table 3.
PWM duty cycles for an M-S motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Specific duty cycles of the TIM1 registers and I/O values . . . . . . . . . . . . . . . . . . . . . . . . . 15
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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List of figures
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List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
4/19
Unipolar winding arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Bipolar winding arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Variable-reluctance motor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Permanent magnet motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Hybrid synchronous motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Current waveforms with 90 ° phase difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Functional block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Stepper motor schematic layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Stepper motor pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Current waveforms (Switec M-S motor) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Stepper motor configuration pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Main program flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
TIM1 interrupt (microstep output) flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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1
Winding arrangement in two-phase stepper motors
Winding arrangement in two-phase stepper motors
There are two basic winding arrangements for the electromagnetic coils in a two-phase
stepper motor: bipolar and unipolar.
The unipolar stepper motor has two identical coils which are not connected electrically and
each coil has a centre tap.
Figure 1.
Unipolar winding arrangement
The bipolar stepper motor is the same as the unipolar stepper except the motor coils do not
have the center taps. Bipolar motor can produce higher torque in comparison to the unipolar
motor as the entire coil is energized and not just half-coils.
Figure 2.
Bipolar winding arrangement
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Types of stepper motor
2
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Types of stepper motor
There are three main types of stepper motors:
2.1
1.
Variable-reluctance stepper motor
2.
Permanent magnet stepper motor
3.
Hybrid synchronous stepper motor
Variable-reluctance (VR) motor
This type of motor contains a soft iron multi-toothed rotor and a wound stator. When the
stator windings are energized with DC current, it magnetizes the stator poles. Rotor teeth
are attracted towards the energized stator poles and the rotation occurs. The variablereluctance motor does not use permanent magnets, so the field strength can be varied. The
VR motor generates less torque so it is generally used for small positioning loads. Figure 3
shows a cross section of a typical VR stepper motor.
Figure 3.
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Variable-reluctance motor
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2.2
Types of stepper motor
Permanent magnet (PM) motor
This motor is known as a tin-can or can-stock motor. The permanent magnet stepper motor
is a low cost and low resolution type motor with a typical step angle of 7.5 ° to 15 °. PM
motors use permanent magnets and the rotor does not have the teeth of the VR motor. The
PM motor has improved torque characteristics compared with the VR motor.
Figure 4.
2.3
Permanent magnet motor
Hybrid synchronous motor
The hybrid stepper motor contains features of both the PM and VR motors. Hybrid motors
are more expensive than PM stepper motors but provide better performance regarding step
resolution, torque and speed. Typical step angles for the hybrid motor range from 3.6 ° to 0.9
°.The rotor is multi-toothed like the VR motor and contains an axially magnetized concentric
magnet around its shaft. The teeth on the rotor provide an even better path which helps
guide the magnetic flux to preferred locations in the air gap. This further increases the
detent, holding, and dynamic torque characteristics of the motor compared with both VR
and PM motors.
Figure 5.
Hybrid synchronous motor
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Microstepping
3
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Microstepping
Physically, stepper motors can be large but, often they are small enough to be driven by
current in the mA range. Current pulses are applied to the motor which generates discrete
rotations of the motor shaft. Although it is possible to drive a stepper motor in a manner
where it has near continuous rotation, doing so requires more finesse of the input waveform
that drives the stepper motor.
Microstepping is a way of moving the motor shaft more smoothly than in full- or half-step
drive modes, allowing the stepper motor to stop and hold a position between the full- or halfstep positions. The jerky character of low stepping motor operation is reduced. There are
fewer vibrations and less problems with resonance which makes noiseless stepping
possible down to 0 Hz. With microstepping, smaller step angles and better positioning is
possible.
The ideal current waveform for driving a stepper motor is a sinewave. Two sinewaves, 90 °
out of phase (or another angle depending on the motor construction), form the ideal drive
current. If the stepper coils follow these current waveforms, the motor runs quietly and
smoothly, which is the ideal condition. In fact, the steps associated with stepper motors will
disappear.
Figure 6.
Current waveforms with 90 ° phase difference
In microstepping mode, the current magnitude in the motor coil has to be controlled in a
proper sequence. The current can be controlled using a H-bridge circuit or PWM technique.
Section 4: Driving stepper motors using STM8A and STM8S microcontrollers describes the
software/low-cost solution to drive the stepper motors in microstepping mode using STM8A
and STM8S microcontrollers.
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4
Driving stepper motors using STM8A and STM8S microcontrollers
Driving stepper motors using STM8A and
STM8S microcontrollers
This application solution makes use of a PWM current control technique. The stepper motor
is driven directly by PWMs, I/O pins and a 74HC/HCT244 buffer/driver (so the position of the
motor can be controlled with precision without any feedback mechanism) of an an STM8A or
STM8S microcontroller. Two ends of each motor winding are connected to one PWM and
one I/O line. TIM1 (16-bit advanced control timer) peripheral on the STM8A or STM8S
allows two independent PWM signals to be generated. The PWM signals have the same
frequency and are controlled by the counter clock period and the capture/compare register
values (TIM1_CCRxH and TIM1_CCRxL). For detailed information about TIM1, refer to the
STM8A and STM8S device datasheets. Figure 7 shows the functional block diagram of a
stepper motor, STM8A PWM and I/O signals, and an 74HC244 buffer.
Figure 7.
Functional block diagram
The stepper motor used in the current solution is the M-S motor X25.689 from Switec (see
Figure 8 and Figure 9). The main features are:
●
1/3 ° resolution per step
●
Low current consumption
●
High speed (greater than 600 °/s)
●
Can be driven directly by a microcontroller
The M-S motor has two types of movement:
●
Rotor movement
●
Pointer shaft movement
This motor is therefore a bipolar motor. It has four different wires which operate the motor in
both clockwise and anti-clockwise directions.
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Driving stepper motors using STM8A and STM8S microcontrollers
Figure 8.
Stepper motor schematic layout
Figure 9.
Stepper motor pin configuration
AN3208
The M-S motor has a gear reduction ratio of 1/180 meaning that 180 ° of rotor movement
(defined as a full-step) is converted to a one degree rotation of the M-S shaft. One full-step
is divided into three partial steps which in turn are further divided into four microsteps. So,
one complete rotation (360 °) of the rotor is equivalent to 24 microsteps of the M-S shaft.
One microstep is equivalent to the 1/12 ° movement of the M-S shaft.
In the M-S motor there is a 60 ° phase difference between the two motor winding currents.
Figure 10 shows the current waveforms.
Figure 10. Current waveforms (Switec M-S motor)
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Driving stepper motors using STM8A and STM8S microcontrollers
The current in the motor windings is controlled by varying the PWM duty cycle values. The
PWM duty cycle for each microstep depends on the phase angle of currents in the two
windings. The duty cycles of Table 1 are calculated using the current waveforms of
Figure 10. The same table can be generated for other motors using the same method. For
more details about duty cycle calculation see Section 5.2: Software description.
Table 1.
Note:
PWM duty cycles for an M-S motor
Step no.
Step
angle
(°)
Phase
angle (°)
coil 1
Phase
angle (°)
coil 2
Sine value Sine value PWM duty PWM duty
(°)
(°)
cycle (%) cycle (%)
coil 1
coil 2
coil 1
coil 2
1
0
60
120
0.87
0.87
87
87
2
15
75
135
0.97
0.71
97
71
3
30
90
150
1.00
0.50
100
50
4
45
105
165
0.97
0.26
97
26
5
60
120
180
0.87
0.00
87
0
6
75
135
195
0.71
-0.26
71
26
7
90
150
210
0.50
-0.50
50
50
8
105
165
225
0.26
-0.71
26
71
9
120
180
240
0.00
-0.87
0
87
10
135
195
255
-0.26
-0.97
26
97
11
150
210
270
-0.50
-1.00
50
100
12
165
225
285
-0.71
-0.97
71
97
13
180
240
300
-0.87
-0.87
87
87
14
195
255
315
-0.97
-0.71
97
71
15
210
270
330
-1.00
-0.50
100
50
16
225
285
345
-0.97
-0.26
97
26
17
240
300
360
-0.87
0.00
87
0
18
255
315
15
-0.71
0.26
71
26
19
270
330
30
-0.50
0.50
50
50
20
285
345
45
-0.26
0.71
26
71
21
300
360
60
0.00
0.87
0
87
22
315
15
75
0.26
0.97
26
97
23
330
30
90
0.50
1.00
50
100
24
345
45
105
0.71
0.97
71
97
25
360
60
120
0.87
0.87
87
87
Table 1 shows theoretical duty cycle values. Contact the stepper motor supplier for
exact/customized values.
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Driving stepper motors using STM8A and STM8S microcontrollers
AN3208
A negative sign shows the current flow in the reverse direction. The current flowing through
each motor coil is controlled using one PWM output (PWMx) and one I/O port (configured in
output push-pull mode) as shown in Figure 11. PWM channels are configured to provide the
required waveform in synchronization with the GPIOs.
Figure 11. Stepper motor configuration pin
4)-07- )/ 4)-07-
)/
AI
When the I/O output is 0, the current through a coil flows in one direction and when the I/O
output is 1, the current flows in the reverse direction.
Step-by-step explanation of Table 1:
●
Step 1: Coil 1 and coil 2 have the same magnitude (0.87, duty cycle 87 %) and in both
coils the current flows in the same direction, I/O value for both coils is 0.
●
Step 2: The magnitude of coil 1 is 0.71 (duty cycle 71 %) and the magnitude of coil 2 is
0.97 (duty cycle 97%). The currents in both coils flow in the same direction, I/O value
for both coils is 0.
.
.
●
Step 6: The magnitude of coil 1 is 0.71 (duty cycle 71 %) and the magnitude of coil 2 is
-0.26 (duty cycle 26 %). The currents in the two coils flow in the opposite direction. I/O
value for coil 1 is 0, I/O value for coil 2 is 1.
For more details about duty cycle calculation see Section 5.2: Software description.
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AN3208
Software
5
Software
5.1
Preliminary information
The software described in this section has been implemented using a 32-Kbyte
STM8AF6266 device with 32 pins. It performs the complete needle rotation of one stepper
motor (in a forward and back forward direction).
Two PWM channels and two I/O lines are necessary for driving the stepper motor.
Thus, four stepper motors can be driven by STM8AF6266 devices as it is able to provide up
to eight PWM signals.
Note:
Up to four PWM signals are provided by TIM1, two PWM signals are provided by TIMER2
(leaving one channel free ), and another two PWM signal are provided by TIMER3.
5.2
Software description
The software code is based on the STM8A/S standard firmware library available from
http://www.st.com.
The firmware consists of:
●
A main program
●
An interrupt routine.
TIM1 interrupt routine, (TIM1_UPD_OVF_TRG_BRK_IRQHandler()), is used for providing
PWMs signals with correct duty cycles.
5.2.1
Main program
The main program initializes the peripherals (GPIOs, timers, clock CNTRL), the variables,
and the interrupt routines. After initialization, it stays in an endless loop (see Figure 12).
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Software
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Figure 12. Main program flow
-AIN
#,+?).)4
)/?).)4
4)-?).)4
%NABLEINTERRUPTS
7HILE
AI
CLK_Init(): Is the clock initialization routine. For this application solution, an HSI clock has
been used. The CPU frequency is 16 Mhz.
I/O Init(): Is the GPIO initialization routine. I/O pins dedicated for stepper motor driving are
configured in output push-pull mode.
TIM1_Init(): Is the initialization routine of TIM1. TIM1 is configured in “PWM edge alignment
mode” providing four PWM signals at 60 Khz.
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5.2.2
Software
TIM1 interrupt routine
The microsteps are output from the TIM1 interrupt routine (see Figure 13: TIM1 interrupt
(microstep output) flow chart. Each microstep output is obtained by changing the PWM duty
cycle according to Table 1. Table 2 provides the TIM1CCRx register values required to
obtain the PWMx duty cycle and I/O output values for each microstep position.
Table 2.
Specific duty cycles of the TIM1 registers and I/O values
Step
no.
Step
angle
(°)
PWM duty PWM duty
cycle
cycle
coil 1 (%) coil 2 (%)
TIM1_CCR1
register values
(PWM1)
TIM1_CCR2
register values
(PWM2)
I/O
values
coil 1
I/O
values
coil 2
1
0
87
87
0x74
0x74
0
0
2
15
97
71
0x81
0x5F
0
0
3
30
100
50
0x86
0x43
0
0
4
45
97
26
0x81
0x22
0
0
5
60
87
0
0x74
0x00
0
0
6
75
71
26
0x5F
0x63
0
1
7
90
50
50
0x43
0x43
0
1
8
105
26
71
0x22
0x26
0
1
9
120
0
87
0x00
0x11
0
1
10
135
26
97
0x63
0x04
1
1
11
150
50
100
0x43
0x00
1
1
12
165
71
97
0x26
0x04
1
1
13
180
87
87
0x11
0x11
1
1
14
195
97
71
0x04
0x26
1
1
15
210
100
50
0x00
0x43
1
1
16
225
97
26
0x04
0x63
1
1
17
240
87
0
0x11
0x86
1
1
18
255
71
26
0x26
0x22
1
0
19
270
50
50
0x43
0x43
1
0
20
285
26
71
0x63
0x5F
1
0
21
300
0
87
0x86
0x74
1
0
22
315
26
97
0x22
0x81
0
0
23
330
50
100
0x43
0x86
0
0
24
345
71
97
0x5F
0x81
0
0
25
360
87
87
0x74
0x74
0
0
The TIMCCRx register values in Table 2 have been calculated under the STM8
configurations given below and using Equation 1.
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Software
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STM8 configurations
●
f_Master = 16 MHz (HSIDIV[1:0] = 000)
●
TIM1_counter_clock = 8 MHz (TIM1_PSCR = 1)
●
PWMx frequency = 60 kHz (TIM1_ARR = 0x85)
Equation 1
TIM1_CCRx = ( PWMx_duty_cycle_coil_x ) × ( TIM1_ARR+1 )
Example
To obtain PWM_duty_cycle_coil1 = 87 %:
TIM1_CCR1 = (0.87) x (0x85 + 1) = 0x74
The maximum step rate/speed possible for the M-S motor is 600 full steps/s or 7200
microsteps/s or 7200 Hz. So, the duration between two consecutive microstep output
interrupts should be ≥ 139 µs. The 74x244 buffer/driver current limitation should also be
considered to determine the maximum speed with which the stepper motor can be driven.
In this application the step rate is set to 830 µs. This means that the next microstep is output
after 50 PWM interrupts because the PWM frequency is 60 kHz (16.6 µs). This choice of
step rate is guaranteed without incurring any problems over the minimum limit of 300 µs.
When an overflow interrupt on TIM1 occurs (@ 60 kHz or 16.6 µs), the TIM1 interrupt
routine is executed.
The flow chart of the TIM1 interrupt is shown in Figure 13.
After resetting the overflow flag, the PWM_Count is incremented by 1. When this variable
reaches a value of 50, the PWM_Count is reinitialized to 0, and according to Motor_dir
information, the needle of the stepper motor moves forwards or backwards (Motor_dir = 0 or
Motor_dir = 1 respectively).
If the stepper needle moves in forwards, the microstepping number (Microstep_No) is
incremented by 1 (up to 24).
In this application the pointer shaft of the stepper motors moves the stepper needle from 0
to 320 °. Taking into account that after 24 microsteps the pointer shaft has moved by 2°, a
complete movement of 320 ° (the Rotation_No variable which represents the number of
rotations) has to reach 160 (160 rotations multiplied by 2 °).
After one complete movement, No_rotation is set to 0, while Motor_dir is set to 1. From this
point on, the needle moves backwards and, the Microstep_No decreased from 23 to 0.
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Software
Figure 13. TIM1 interrupt (microstep output) flow chart
4IMERINTERRUPT
-ICROSTEP?NO-ICROSTEP?NO
#LEARTHEOVERFLOWFLAG
07-?COUNT07-?COUNT
-ICROSTEP?NO 07-?COUNT -ICROSTEP?NO
2OTATION?NO2OTATION?NO
07-?COUNT
2OTATION?NO
-OTOR?DIR
2OTATION?NO
-OTOR?DIR
-ICROSTEP?NO
-ICROSTEP?NO-ICROSTEPS?NO
-ICROSTEP?NO
4AKENEXTMICROSTEP
5PDATE4)-?##2X,REGISTERTOTAKENEXTMICROSTEP
CLOCKWISEORANTICLOCKWISE
07-OUTPUTGETSUPDATEDATNEXTOVERFLOWINTERRUPT
)/OUTPUTSALSOUPDATEDATSAMEOVERFLOWINTERRUPT
-ICROSTEP?NO
2OTATION?NO2OTATION?NO
2OTATION?NO
2OTATION?NO
-OTOR?DIR
-ICROSTEP?NO
4AKENEXTMICROSTEP
5PDATE4)-?##2X,REGISTERTOTAKENEXTMICROSTEP
CLOCKWISEORANTICLOCKWISE
07-OUTPUTGETSUPDATEDATNEXTOVERFLOWINTERRUPT
)/OUTPUTSALSOUPDATEDATSAMEOVERFLOWINTERRUPT
AI
%NDOFINTERRUPTROUTINE
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Revision history
6
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Revision history
Table 3.
18/19
Document revision history
Date
Revision
06-Jul-2010
1
Changes
Initial release
Doc ID 17411 Rev 1
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Doc ID 17411 Rev 1
19/19
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