Application Note

Cordless Drill Motor Control with

Battery Charging Using Z8 Encore!®
F0830 Reference Design

AN025504-0910

Abstract
Currently, most hand-held electric drilling machines operating on batteries need a separate external battery
charger to charge the batteries. This reference design describes the implementation of motor control for a 350-
W hand-held electric drilling machine along with Nickel Cadmium (NiCd) battery charging in a single unit.
This design is based on Zilog’s Z8 Encore!® F0830 microcontroller, which primarily controls the speed of the
motor, motor current monitoring, fault detection, and controlled dv/dt charging of NiCd battery. All
functionalities of the design are implemented with minimum hardware. The on-chip peripherals of Z8 Encore!
F0830 are used to drive the drill motor at Low—Medium—High speeds using the pulse width modulation
(PWM). The battery voltage and the charger input voltage are monitored by an analog-to-digital converter
(ADC), and the batteries are charged depending on the voltage read from the batteries and the charger. The
light emitting diodes (LEDs) are provided to indicate the motor running, motor fault, low battery, and battery
charging condition.

Note: The source code (AN0255-SC01) associated with this reference design has been tested with ZDS II
version 4.11.0.

Features
The key features of this reference design include:

 Motor control and battery charging in a single unit

 Smooth startup of motor, reducing the starting current of motor
 Three-step speed control of the motor using PWM
 Microcontroller based over current protection
 Monitoring of battery charger input voltage and battery voltage
 Controlled dv/dt charging of NiCd battery
 LED indication of motor running, overload, and fault condition
 LED indication of battery charging and low battery status
 Three-way switch for Low—Medium—High motor speed selection

Copyright ©2010 by Zilog®, Inc. All rights reserved.

www.zilog.com
 Cordless Drill Motor Control with Battery Charging
Using Z8 Encore® F0830 Reference Design

 Two-way switch for Forward and Reverse operation of the motor

Discussion
The drill motors used in most of the cordless handheld electric drilling machines are controlled by an
electronic circuit. This electronic circuit mainly comprises of a simple square wave generator to control the
speed. Usually batteries used in these machines are charged using a separate charging unit. By designing a
control circuit based on Z8 Encore!® F0830 microcontroller, it is easy to accomplish motor control at different
speeds and battery charging as a single unit. This is an added advantage because the battery used to drive the
motor is charged in the drilling machine without a separate battery charger.

The functions of the drilling machine like motoring, stop (break), and the steps of speed (High, Medium and
Low) can be effectively controlled by changing the duty cycle of the PWM generated by the microcontroller.
LEDs are provided for monitoring fault condition like overload, short circuit of motor, and the charging status
of the battery. Motor control operation resumes after the overload and short circuit faults are rectified.

The controller circuit based on the Z8 Encore!® F0830 can also be used to charge Nickel Metal Hydride
(NiMH), NiCd, or Lithium ion batteries. The battery status such as low battery, charging, and charge
completed are displayed using LEDs.

This reference design is implemented with very minimum hardware changes to accommodate interfacing
motors and batteries rated for different voltage and current ratings.

This reference design can be easily ported to a Z8 Encore! F083A microcontroller with a 20-MHz internal
precision oscillator (IPO) for better operation in terms of processor speed and ADC conversion. The required
changes are modifying the setting of the clock source frequency that is defined as a macro in the header files
and configuring the ADC Register used in the project.

Theory of Operation
The basic functions of the hand-held drill are classified as forward motoring, reverse motoring, speed control,
and torque adjustment. Motors used in the cordless hand-held drives are available at different voltage ratings.
The commonly used voltage ratings for the motor are 7.5 V, 12 V, 14.4 V, 18 V, 24 V, and 36 V DC. These
motors have the maximum constant current rating and so can be operated at the maximum specified current
rating, which in turn specifies torque. Motors used in this application are rated from 300 W to 500 W.
Generally, the no load current consumption of a 1/4-inch drill is in the range of 2 A to 2.5 A, and the stall
current of the motor is in the range of 80 A to 100 A. The speed of these drilling machines is adjustable from
150 rpm to 1200 rpm. Speed variation is necessary for different type of work from screw driving to drilling
metal sheet.

Rechargeable batteries are used to provide power to the drill motor. Most common rechargeable batteries are
NiCd, NiMH, or Lithium ion. The design uses NiCd cells of 1.2 V each connected in series to form 14.4-V
battery pack. NiCd battery can be charged by a constant current from an adapter plugged to the drive unit. The
theory of charging the NiCd batteries is described in Appendix C—Battery Technology on page 15. The
charge termination to the battery is done by observing the zero or negative dv/dt on the battery terminals or by
charging for a fixed time interval. In this design, the charge termination is done by zero/negative dv/dt or fixed
interval timeout, whichever occurs first.

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Motors
Brushed Universal Motors are commonly employed in the cordless electric hand-held drives.
These motors can be operated using a DC power supply. Brushed DC motors are classified as permanent
magnet and temporary magnet motors. Permanent magnet DC motors are employed where very low
power/torque is needed (for example, toys, tape players, and instrument cooling fans). Similarly, the temporary
magnet DC motors are further classified based on the type of magnetic field winding used for their
construction. Temporary magnet DC motors are classified as the following:

• Shunt motor
Shunt motors are employed where the constant speed is required.
• Series motor
Series motors are employed where high torque is required, but series motors rotate at very high speed
when they are not loaded.
• Compound motor
Compound motors combine the features of series and shunt motors.

Hand-held drilling machines require a high torque to drill objects, and the maintenance of speed is not a
criterion in drilling applications, so series motors are the most suitable for most of the drilling machines.

Rechargeable Batteries
Batteries are used to power cordless electric handheld drill motor. Drill motors consume high power during
their operation. The no load current consumed by a 350 W motor can be in the range of 2 A to 2.5 A, and
motor stall current can be in the range of 80 A to 100 A. The batteries required for this application should have
a high charge density to meet the power requirements of the motor. Rechargeable NiCd or NiMH batteries
have a moderate charge density, which can be considered suitable to the application. NiMH batteries exhibit
higher power density compared to their NiCd counterparts. The voltage per cell of the NiCd battery type is 1.2
V. NiCd batteries are charged using the constant current charging method.

Hardware Architecture
Block Diagram
Figure 1 on page 4 displays the functional block diagram of the Z8 Encore!® F0830 hand-held drill motor
control.

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Figure 1. Block Diagram of Z8 Encore! F0830 Hand-Held Drill Motor Control

The block diagram is divided into following functional blocks:

• Battery Charging Section (page 5)

• Controller Section (page 5)
• Power Electronic Drive (page 6)

All functional blocks are controlled by Z8 Encore! F0830 microcontroller operation using IPO at 5.5296 MHz.

The Z8 Encore! F0830 20-pin microcontroller pins are used for the functions listed in Table 1.

Table 1. Pin Function Descriptions

Pin
Pin Function Function Used Input/Output/PWR Function on Board
No.
1 PB1/ANA1 ANA1 Input Battery charger voltage sensing
2 PB2/ANA2 — Not Used —
3 PB3/CLKIN/ANA3 — Not Used —
4 VDD — PWR 3.3 V supply
PA0/T0IN/ T0OUT
5 /XIN — Not Used —

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Pin
Pin Function Function Used Input/Output/PWR Function on Board
No.
6 PA1/T0OUT/XOUT — Not Used —
7 GND — PWR GND
8, 9 PA2, PA3 PA2, PA3 Input Three-level speed setting
10 PA4 PA4 Input Run/Break switch
11 PA5 PA5 Output Charger ON/OFF control

12 PA6/T1IN/ T1OUT — Not Used —

Output PWM to drive MOSFET
13 PA7/T1OUT T1OUT Output connected to motor

14 RESET /PD0 RESET Input RESET

15 DBG DBG Input/Output DEBUG
Current sense input for
16 PC0/ANA4/CINP/LED CINP Input comparator
17 PC1/ANA5/CINN/LED — Not Used —
18 PC2/ANA6/LED/VREF PC2 Output LED
19 PC3/COUT/LED — Output LED
20 PB0/ANA0 ANA0 Input Battery voltage sensing

The detailed descriptions below are reflected in the schematics in Appendix A—Schematics on page 10.

Battery Charging Section

The output of a 110/230 V AC to 20 V DC, 1-A power adapter is connected to the input of the battery charger
section. The battery charging section comprises the charging current limiting resistor, transistor to turn on/off
the charging current, trickle charging resistor, and 14.4-V NiCd battery pack. The resistor across the transistor
provides trickle charging current of C/40 to the battery, where C is the rated battery capacity in Ampere Hours
(AH). The transistor switching is controlled by the Z8 Encore!® F0830 microcontroller. The charging input
voltage and the battery terminal voltage are attenuated to a voltage level acceptable by the ADC peripheral
within the Z8 Encore! F0830 microcontroller. The attenuated voltage is connected to the respective pins of the
microcontroller. The microcontroller monitors the attenuated charging voltage and battery voltage for charging
the battery. The microcontroller measures the voltage slope of the battery every 32 s. When the batteries show
a negative voltage slope (-dv/dt), the microcontroller turns off the charging transistor by making the GPIO pin
low.

This design accomplishes either battery charging or the drill motor control function at a time. It is not
possible to run the motor when the batteries are charged and vice versa.

Controller Section
The controller section comprises of Z8 Encore!® F0830 microcontroller operating at 5.5296 MHz using IPO.
The power supply for the controller is derived from the battery or the charger input voltage. Battery voltage of
14.4 V and charger input voltage of 20 V are logically ORed using the diode and stepped down to 3.3 V. The

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stepped down voltage is achieved using a transistor and zener diode combination. The microcontroller is
connected to a three-position switch for speed control of the DC motor. Based on the switch position, the
PWM duty cycle is varied to achieve Low—Medium—High speed operation of the motor. A trigger switch
(ON/OFF) to turn on/turn off the drill motor is connected to PA4. The LEDs to indicate battery and motor
status are connected to PC2 and PC3 pins of the microcontroller respectively. The voltage developed across the
current sensing resistor, when the current flows through it, is fed to the positive input of the on-chip
comparator. When the voltage on the positive input of the comparator exceeds the on-chip reference voltage
connected to the non-inverting input of the comparator, PWM stops. Every 10 ms, the PWM is initialized to
check fault. If PWM is not started the motor status LED blinks to indicate motor fault/overload.

Power Electronic Drive

The power electronic drive unit consists of transistors to drive an IXYS highly efficient Trench Gate metal
oxide semiconductor field effect transistor (MOSFET), with ultralow Rds, connected to the low side of the
supply voltage. The transistor drive stage forms a voltage level converter stage to drive the gate of the
MOSFET with appropriate voltage. A switching frequency of 100 Hz gives a smooth variation of the motor
speed. The MOSFET is switched at a frequency of 100 Hz. The source pin of the MOSFET is connected to the
ground through a current sense resistor. The voltage drop across the current sense resistor is the input to the
CINP pin of the microcontroller. The CINP pin is the input connected to the positive input of the comparator
within the Z8 Encore! F0830 microcontroller. The negative input of the comparator is connected to the
programmable internal reference voltage generated within Z8 Encore! F0830 microcontroller.

The user interface consists of switch inputs for forwarding, stopping or breaking, and reversing the motor,
setting the speed of the motor to Low—Medium—High. Two LEDs are provided for status indication of motor
and battery.

Software Implementation
The motor control and battery charging software implementation procedures include the following sequences
of events:
1. Initialize the comparator, timer 0, timer 1 PWM, ADC, WDT, and GPIO upon power up or external
pin reset.
2. Read battery voltage and update the status flags reflecting the battery condition.
3. If the battery voltage is above or below the threshold limit, turn on the battery damage Flag and charge
the battery for 60 seconds.
4. If the trigger switch is not pressed, continue to step 8.
5. If the trigger switch is pressed and the battery has sufficient charge, set the speed of the motor to the
value set by the settings switch.
6. Continuously monitor for changes in the speed setting switch and trigger the switch release.
7. If the trigger switch is released or the battery is completely discharged, turn off the PWM.
8. If the battery is not completely charged and the charger voltage is present, turn on the charger.
9. Continuously monitor the battery status and trigger the switch press.
10. If the trigger switch is pressed, repeat step 5 through step 7.
11. If the charger voltage is not present or the battery is completely charged, turn OFF charger and enter
the stop mode.
12. The stop mode is recovered when the WDT times out or the trigger switch is pressed.

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Testing

Test Setup
See the schematics in Appendix A—Schematics on page 10 and the Test Procedure to connect the test circuit.

Equipment Used
The equipments used for testing consist of the following:

• 14.4 V DC operated Cordless hand-held drill/screwdriver

• 20 V, 1 A DC power supply
• Digital multi-meter
• Oscilloscope
• Serial/USB Smart Cable
• ZDS II installed PC with a USB/serial port to compile code and download the code to the target

Test Procedure
Follow these steps to test the Z8 Encore!® F0830 microcontroller-based design:
1. Connect the circuit as displayed in the schematics in Appendix A—Schematics on page 10.
2. Connect the 14.4-V battery to the circuit.
3. Connect a Serial/USB Smart Cable to the debug connector in the circuit and to the PC.
4. Open the Project file Motor_Control.zdsproj in the source folder of this application
installation using the ZDS II Compiler, build the project, and download the code to the target device.
5. Disconnect the Smart Cable from the target device and recycle the power to the application.
6. Connect a multi-meter in series with the battery and the circuit to measure the motor current/battery
charging current.
7. Connect an oscilloscope across the terminals of the motor.
8. Press and hold the RUN/STOP switch.
9. Observe the motor speed as it gradually increases up to the maximum speed set by the speed position
switch.
10. Observe the waveforms on the oscilloscope.
11. Change the speed settings of the motor by changing the position of the speed setting slide switch.
12. Measure the speed and observe the waveforms for all of the speed settings.

Test Results
The results in the following table are obtained for various speed settings of the motor.

Trigger Switch Position Speed Switch Position No Load Speed (RPM) Current (A)
Released — 0 0
Pressed Low 400 2.00
Pressed Medium 800 2.90
Pressed High 1150 3.20

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Figure 2. Starting and Operating Current of Motor with Speed Set to Minimum

A load current of 5.6 A is utilized when drilling an aluminum sheet of 5-mm thickness.

LED D6 lights up to indicate low battery when the battery voltage is at 13.8 V or lower. The system shuts
down when the battery voltage reaches 12 V.

Battery Charging Test Results

The test results for battery charging are listed in Table 2.

Table 2. Battery Charging Test Results

Parameters Value
Battery type Nickel Cadmium
Battery voltage 14.4 V
Ampere Hour rating 1500 mAH

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Parameters Value
Charging type Constant voltage
Charging current 800 mA (initial, decreases with charging time)
Charging time 2 hours (approximately for completely discharged
battery pack)
Charge termination Negative voltage on battery terminals/constant time
interval
Maximum battery voltage when charging completely 18.2 V
discharged battery
Trickle charging current 40 mA
Voltage of the battery when completely discharged 12 V
using motor load

Summary
This reference design describes smooth speed control of a battery-operated drilling machine motor along with
an in-built battery charger using low-cost Z8 Encore!® F0830/F083A. This design has two LEDs that indicate
various conditions like motor operation, motor overcurrent, battery charging, and low battery.

This design also includes features like motor protection for overcurrent and short circuit, controlled NiCd
battery charging. The advantages of this design over the existing cordless hand-held drives are that there is no
need to plug the battery pack into a separate charger unit, and the smooth startup of the motor reduces high
starting current of the motor.

References
The following documents associated with Z8 Encore! F0830 MCU or battery charger are available on
www.zilog.com:

• Z8 Encore!® F0830 Series Product Specification (PS0251)

• Z8 Encore! ® Based AA Type NiMH and NiCd Battery Charger Reference Design (AN0229)
• Z8 Encore! XP® Based NiCd Battery Charger Application Note (AN0221)

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Appendix A—Schematics
Figure 3 displays the implementation of the cordless drill motor control with battery charging using the Z8 Encore! F0830 Series MCU.

Figure 3. Cordless Drill Motor Control with Battery Charging Schematics

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Appendix B—Flowcharts
This appendix contains the flowcharts of the main function and interrupts in the application of cordless drill
motor control with battery charging using Z8 Encore!® F0830. See Figure 4 through Figure 8 (page 14) for
details.

Figure 4. Flowchart of Main Function

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Figure 5. Motor Control Algorithm

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Figure 6. Battery Charging Algorithm

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Figure 7. Comparators Interrupt

Figure 8. Timer0 Interrupt

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Appendix C—Battery Technology

The four popular battery types (NiCd, NiMH, SLA, and Li-Ion) displays different charging and discharging
characteristics. The battery life and performance mainly depends upon the battery charging mechanism.
Therefore, batteries must be charged in a proper mechanism. Charging must be terminated when the battery is
completely charged as overcharging of the battery invariably results in poor performance and can also damage
the battery. Different batteries require different charge termination techniques as they behave differently when
approaching the full charge state. While charging, batteries exhibit marked rise in voltage above the rated
battery voltage. The NiCd and NiMH rechargeable battery types used in this reference design are briefly
discussed below.

• Nickel Cadmium (NiCd)

NiCd batteries are used in portable consumer equipments. The single-cell voltage for NiCd batteries is 1.2
V. These batteries are charged using the constant current charging method. While charging, as the voltage
crosses the full charge point, the voltage gradually drops. This voltage drop is approximately 15 mV per
cell in the battery. This voltage drop is recognized as full charge condition resulting in the termination of
the charge. This termination mechanism is known as -dv/dt termination. The battery voltage rises to 1.65 V
per cell during charging. The main disadvantage of the NiCd battery is that it must be discharged
periodically to protect the performance. This phenomenon is known as memory effect.

• Nickel Metal Hydride (NiMH)

NiMH batteries exhibit high power density compared to the NiCd batteries. The per cell voltage of the
NiMH battery type is 1.2 V which is similar to NiCd batteries. NiMH batteries are charged with constant
current charging method. While charging, the voltage drop is not as low compared to NiCd batteries.

Therefore, -dv/dt charge termination is not recommended. Instead of the drop in cell voltage, the battery tends
to stabilize after a small drop. This flat region is the indication for full battery charging. This termination
mechanism is known as zero dv/dt termination. NiMH batteries do not suffer with memory effect as compared
to NiCd batteries. As a result, they replace NiCd batteries in devices such as cell phones. The increase in price
is justified by the reduction in weight and absence of memory effect.

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Warning: DO NOT USE IN LIFE SUPPORT

LIFE SUPPORT POLICY

ZILOG'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE
SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE
PRESIDENT AND GENERAL COUNSEL OF ZILOG CORPORATION.

As used herein
Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b)
support or sustain life and whose failure to perform when properly used in accordance with instructions for use
provided in the labeling can be reasonably expected to result in a significant injury to the user. A critical
component is any component in a life support device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or system or to affect its safety or effectiveness.

Document Disclaimer
©2010 by Zilog, Inc. All rights reserved. Information in this publication concerning the devices, applications,
or technology described is intended to suggest possible uses and may be superseded. ZILOG, INC. DOES
NOT ASSUME LIABILITY FOR OR PROVIDE A REPRESENTATION OF ACCURACY OF THE
INFORMATION, DEVICES, OR TECHNOLOGY DESCRIBED IN THIS DOCUMENT. ZILOG ALSO
DOES NOT ASSUME LIABILITY FOR INTELLECTUAL PROPERTY INFRINGEMENT RELATED IN
ANY MANNER TO USE OF INFORMATION, DEVICES, OR TECHNOLOGY DESCRIBED HEREIN OR
OTHERWISE. The information contained within this document has been verified according to the general
principles of electrical and mechanical engineering.

Z8, Z8 Encore!, and Z8 Encore! XP are registered trademarks of Zilog, Inc. All other product or service names
are the property of their respective owners.

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