Upload
priya-suresh
View
212
Download
0
Embed Size (px)
Citation preview
8/3/2019 College_Automation
1/47
INTRODUCTION
1.1 OBJECTIVE
1.2 SCOPE OF THE PROJECT
1
8/3/2019 College_Automation
2/47
1.1 OBJECTIVE
Embedded systems have already improved our lives in numerous ways.
A good example is an automatic washing machine or a microwave oven. Due to the
breathtaking developments in microelectronics, processor speeds are increasing tremendously
and memory costs are dropping rapidly. As a result, we are witnessing revolutionary changes
in embedded systems.
This project is indented to completely automate certain functions of a
class room. Such as recording attendance of students and teacher, automate the working of thecollege bell and displaying notifications by principal. This system is connected with the
computer in principals or HODs room so that he can download the attendance of students at
any time. Also from this computer he can change the college bell ringing time. This project has
the following features,
Stores the attendance of students.
A message can be passed to any class directly by the principal.
The attendance and strength of the class can be downloaded at any time.
It automates the functioning of the college bell.
This project is implemented using Philips LPC2148 ARM controller. In
this system attendance of students are recorded with the help of a RFID tag. A student entering
the class has to swipe his/her tag into the system. This information is stored into the internal
memory of the microcontroller. Using the connection provided with the computer in
principals room, the memory of the system can be downloaded at any time. Also the details
will be automatically sent to the principal once the teacher enters the class. This helps the
college authority to keep track of the number of students present in a class room at any time.
The system completely automates the working of the college bell. From the center computer
the bell time can be modified easily.
8/3/2019 College_Automation
3/47
1.2 SCOPE OF THE PROJECT
The project COLLEGE AUTOMATION is intended to automate the certain
functions of a classroom. The main duties of the system is to store the attendance of students,
keep the current strength of the classroom, automate the working of college bell, displaying
notifications by principal, detect the presence of teacher inside classroom and send the
attendance to the PC in the office room once the teacher enters and on demand too.
It helps teachers to easily handle the attendance details of students. There is no
need for take attendance manually. This reduces the wastage of class time for taking
attendance. Also no need to keep the hardcopy of attendance and enter it into the computer to
calculate the percentage of each student at the end of each semester. By installing suitablesoftware all attendance information can be handles easily. The device helps the working of
college bell; it reduces the human efforts for ringing the bell.
The device is much helpful in handling the attendance of students. It reduces the
wastage of valuable time for entering the attendance details of each student, which consumes a
lot of time.
2
8/3/2019 College_Automation
4/47
BLOCK DIAGRAM
2.1BLOCK DIAGRAM DESCRIPTION
2
8/3/2019 College_Automation
5/47
8/3/2019 College_Automation
6/47
8/3/2019 College_Automation
7/47
3.1BLOCK DIAGRAM DESCRIPTION
The major parts of the system are ARM Microcontroller, IR Sensor,
ZIGBEE, RFID, LCD, Buzzer, and Magnetic Lock.
1. MICROCONTROLLER
Microcontroller is the heart of the project. It is responsible for all the activities done inside
the system. Here microcontroller used is Philips LPC2148 ARM. Microcontroller scans the
RFID for any new entry. According to the RFID tag swiped, it stores the attendance. Theattendance information is stored in the internal EEPROM of the microcontroller. It also
communicates with the Personal Computer and sends the attendance information on
demand.
2. IR SENSOR
8/3/2019 College_Automation
8/47
This section is used to count the students passing in and out of the class. For this infrared
transmitter/receiver pairs are used. This pairs are connected in parallel at the door. The
direction of passing can be detected by the priority in which the IR is blocked.
3. ZIGBEE
ZigBee is designed to provide highly efficient connectivity between small
packet devices. As a result of its simplified operations, which are one to
two full orders of magnitude less complex than a comparable Bluetooth
device, pricing for ZigBee devices is extremely competitive, with full
nodes available for a fraction of the cost of a Bluetooth node. ZigBee has
been developed to meet the growing demand for capable wireless
networking between numerous low-power devices. In industry ZigBee is
being used for next generation automated manufacturing , with small
transmitters in every device on the floor, allowing for communication
between devices to a central computer . Due to its low power output,
ZigBee devices can sustain themselves on a small battery for many
months, or even years, making them ideal for install-and-forget
purposes, such as most small household systems.
4. RFID
http://www.wisegeek.com/what-is-automated-manufacturing.htmhttp://www.wisegeek.com/what-is-a-computer.htmhttp://www.wisegeek.com/what-is-automated-manufacturing.htmhttp://www.wisegeek.com/what-is-a-computer.htm8/3/2019 College_Automation
9/47
RFID is a more recent term referring to a family of sensing technologies that has been in
use for more than 50 years. The technology of radio identification was devised for military
applications during the Second World War. RFID is now a generic term for a variety of
technologies that use radio waves to automatically identify individual items.
5. LCD
Here for the display an LCD module is used. The advantage of using LCD display over 7-
segment or Led is that LCD can display alphanumeric characters (i.e.-; both alphabets and
numbers). Another advantage is that LCD does not require constant refreshing, which
reduces the complexity of the software.
6. BUZZER
8/3/2019 College_Automation
10/47
This is for alarm and belling purpose.
7. MAGNETIC LOCK
Electro-magnetic locks (maglocks) are increasingly being used in the access control and
security industries. Application areas include airports, hospitals, office buildings, and mass
transit stations. They are also being used on doors where access is restricted for security
reasons or where doors double as an emergency exit.
8/3/2019 College_Automation
11/47
CIRCUIT DIAGRAM
3
8/3/2019 College_Automation
12/47
8/3/2019 College_Automation
13/47
CIRCUIT DESCRIPTION
4
8/3/2019 College_Automation
14/47
MICROCONTROLLER
ARM7 LPC2148 is ARM7TDMI-S Core Board Microcontroller that uses 16/32-Bit 64 Pin
(LQFP) Microcontroller No.LPC2148 from Philips (NXP). All resources inside LPC2148 isquite perfect, so it is the most suitable to learn and study because if user can learn and
understand the applications of all resources inside MCU well, it makes user can modify, apply
and develop many excellent applications in the future. Because Hardware system of LPC2148
includes the necessary devices within only one MCU such as USB, ADC, DAC,
Timer/Counter, PWM, Capture, I2C, SPI, UART, and etc.
LPC2148 BOARD FEATURES
Processor: LPC2148
2xSerial ports(One for ISP and other for Serial Communication)
12.00 MHz crystal
On board Reset Circuit with a switch.
Dual Power supply (either through USB or using external power adapter).
Power on LED supply.
Three on-board voltage regulators 1.8V, 3.3V and 5V with up to 800mA current
Extension headers for C ports.
Graphic LDC display interfacing port.
USB Ports.
CAN controller interfacing.
MMC/SD card interfacing.
8 Bit LED interfacing.
EEPROM Interfacing.
On board UART
8/3/2019 College_Automation
15/47
APPLICATIONS
Industrial control
Medical systems
Access control
Point-of-sale
Communication gateway
Embedded soft modem
General purpose applications
8/3/2019 College_Automation
16/47
PINOUT DIAGRAM OF LPC2148
8/3/2019 College_Automation
17/47
LPC2148 PINOUT DESCRIPTION
8/3/2019 College_Automation
18/47
8/3/2019 College_Automation
19/47
8/3/2019 College_Automation
20/47
8/3/2019 College_Automation
21/47
8/3/2019 College_Automation
22/47
GENERAL PURPOSE INPUT/OUTPUT PORTS (GPIO)
Features
Every physical GPIO port is accessible via either the group of registers providing an
enhanced feature and accelerated port access or the legacy group of registers
Accelerated GPIO functions:
GPIO registers are relocated to the ARM local bus so that the fastest possible
I/O timing can be achieved
Mask registers allow treating sets of port bits as a group, leaving other bits
unchanged
All registers are byte and half-word addressable
Entire port value can be written in one instruction
Bit-level set and clear registers allow a single instruction set or clear of any number of
bits in one port
Direction control of individual bits
All I/O default to inputs after reset
Backward compatibility with other earlier devices is maintained with legacy registers
appearing at the original addresses on the VPB bus
Applications
General purpose I/O
Driving LEDs, or other indicators
Controlling off-chip devices
Sensing digital inputs
Register description
LPC2141/2/4/6/8 has two 32-bit General Purpose I/O ports. Total of 30 input/output and a
single output only pin out of 32 pins are available on PORT0. PORT1 has up to 16 pins
8/3/2019 College_Automation
23/47
available for GPIO functions. PORT0 and PORT1 are controlled via two groups of 4 registers
as shown in table.
GPIO register map (legacy VPB accessible registers)
UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER 0(UART0)
FEATURES
16 byte Receive and Transmit FIFOs
Register locations conform to 550 industry standard.
Receiver FIFO trigger points at 1, 4, 8, and 14 bytes.
Built-in fractional baud rate generator.
Mechanism that enables software and hardware flow control implementation .
PIN DESCRIPTION
8/3/2019 College_Automation
24/47
UART0 BLOCK DIAGRAM
UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER 1 (UART1)
FEATURES
8/3/2019 College_Automation
25/47
UART1 is identical to UART0, with the addition of a modem interface.
16 byte Receive and Transmit FIFOs.
Register locations conform to 550 industry standard.
Receiver FIFO triggers points at 1, 4, 8, and 14 bytes.Built-in fractional baud rate generator with autobauding capabilities.
Mechanism that enables software and hardware flow control implementation.
Standard modem interface signals included with flow control (auto-CTS/RTS) fully supported in
hardware (LPC2144/6/8 only).
PIN DESCRIPTION
8/3/2019 College_Automation
26/47
UART1 BLOCK DIAGRAM
TIMER/COUNTER TIMER0 AND TIMER1
8/3/2019 College_Automation
27/47
FEATURES
A 32-bit Timer/Counter with a programmable 32-bit Prescaler.
Counter or Timer operationUp to four 32-bit capture channels per timer, that can take a snapshot of the timer value
when an input signal transitions. A capture event may also optionally generate an
interrupt.
Four 32-bit match registers that allow:
Continuous operation with optional interrupt generation on match.
Stop timer on match with optional interrupt generation.
Reset timer on match with optional interrupt generation.Up to four external outputs corresponding to match registers, with the following
capabilities:
Set low on match.
Set high on match.
Toggle on match.
Do nothing on match.
APPLICATIONS
Interval Timer for counting internal events.
Pulse Width Demodulator via Capture inputs.
Free running timer .
DESCRIPTION
The Timer/Counter is designed to count cycles of the peripheral clock (PCLK) or anexternally-supplied clock, and can optionally generate interrupts or perform other actions at
specified timer values, based on four match registers. It also includes four capture inputs to
trap the timer value when an input signals transitions, optionally generating an interrupt.
LCD DISPLAY
8/3/2019 College_Automation
28/47
Liquid crystal displays are generally more flexible than LED displays
because they allow for a variety of text and/or graphics. LCDs require less power LEDs
making them suitable for lose power requirements.) LCDs are more readable in sunlight and
an use backlights for night viewing. However, LCDs are more expensive than LED displays.
LCDs come in a variety of sizes. Text displays are specified by their character size, the number
of lines, and the number of characters per line.
LCD MODULE
The LCD module, made by Crystallonics, is 16x2 line interactive displays. It
needs a power supply of +5v. The module has inbuilt controller chip, such as an HD44780,which acts as an interface between CPU and the row and column drivers. The controller takes
care of generating characters, refreshing the display, and so on. The module has a back light
driven by a pair of pads separate from the interface pads .The LCD module works in two modes
for communicating with the micro controller - 8 bit (byte) mode & 4 bit (nibble) mode. In the
later case only the higher nibble i.e. pins DB4-DB7 is used for communication. For controlling
the LCD module we have used only the port D.
LCD module
PINOUT DIAGRAM
8/3/2019 College_Automation
29/47
PIN NO. SYMBOL FUNCTION
1 Vss Ground
2 Vdd +5v
3 Vo Contrast Adjust
4 RS(H/L)Register Select
H=Data/L=Instruction
5 R/W (H/L)Read/Write
H=Read/L=Write
6 E Enable
7 DB0 Data Pin 1
8 DB1 Data Pin 2
9 DB2 Data Pin 3
10 DB3 Data Pin4
11 DB4 Data Pin 5
12 DB5 Data Pin 6
13 DB6 Data Pin 7
14 DB7 Data Pin8
15 BL- Back Light
16 BL+ Back Light
PINOUT DESCRIPTION
8/3/2019 College_Automation
30/47
Contrast: A variable voltage applied to this pin controls the contrast. Use a potentiometer
and adjust until you see the background.
Register Select : This pin selects whether you are sending the module a command or data.
Read/Write : This pin allows for bi-directional communications. For the discussions here, uni-
directional communications will be used. Ground this pin.
Enable : This is the latch pin. A high-to-low transition causes the value on the data lines to be
latched by the module.
DB0-DB7 : Apply the data or commands to these pins.
RFID
Radio-frequency identification (RFID) is the use of an object (typically referred to
as an RFID tag) applied to or incorporated into a product, animal, or person for the purpose of
identification and tracking using radio waves. Some tags can be read from several maters away
and beyond the line of sight of the reader.
Most RFID tags contain at least two parts. One is integrated circuits for storing
and information, modulating and demodulating a radio frequency (RF) signal, and other
specialized functions. The second is an antenna for receiving and transmitting the signal. There
are generally three types of RFID tags:-
1. Active tags:-
Which contain a battery and can transmit signals autonomously.
2. Passive tags:-
Which have no battery and require an external source to provoke signal
transmission
8/3/2019 College_Automation
31/47
3. Battery assisted passive (BRP):-
Which require an external source to wake up but have significant higher forward
link capability providing read rang
MAX 232
It is a voltage level converter. It converts RS232 voltage levels to TTL .The serial
port of PC uses RS232 voltage levels, and microcontroller uses TTL levels. To match these
voltage levels MAX232 IC is used. This IC includes a pair of transmitter and receiver. One
advantage of using MAX232 is that, no negative voltage is required for its working. So need of
dual supply is eliminated.
The MAX232 is an integrated circuit that converts signals from an RS-232 serial port
to signals suitable for use in TTL compatible digital logic circuits. The MAX232 is a dual
driver/receiver and typically converts the RX, TX, CTS and RTS signals.
The drivers provide RS-232 voltage level outputs (approx. 7.5 V) from a single + 5 V
supply via on-chip charge pumps and external capacitors. This makes it useful for implementing RS-232 in devices that otherwise do not need any voltages outside the 0 V to
+ 5 V range, as power supply design does not need to be made more complicated just for
driving the RS-232 in this case.
The receivers reduce RS-232 inputs (which may be as high as 25 V), to standard 5 V
TTL levels. These receivers have a typical threshold of 1.3 V, and a typical hysteresis of 0.5 V.
The later MAX232A is backwards compatible with the original MAX232 but may
operate at higher baud rates and can use smaller external capacitors 0.1 F in place of the
1.0 F capacitors used with the original device.
The newer MAX3232 is also backwards compatible, but operates at a broader voltage
range, from 3 to 5.5V.
http://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/wiki/RS-232http://en.wikipedia.org/wiki/Transistor-transistor_logichttp://en.wikipedia.org/wiki/Charge_pumphttp://en.wikipedia.org/wiki/Power_supplyhttp://en.wikipedia.org/wiki/Transistor-transistor_logichttp://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Baudhttp://en.wikipedia.org/wiki/Faradhttp://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/wiki/RS-232http://en.wikipedia.org/wiki/Transistor-transistor_logichttp://en.wikipedia.org/wiki/Charge_pumphttp://en.wikipedia.org/wiki/Power_supplyhttp://en.wikipedia.org/wiki/Transistor-transistor_logichttp://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Baudhttp://en.wikipedia.org/wiki/Farad8/3/2019 College_Automation
32/47
When a MAX232 IC receives a TTL level to convert, it changes a TTL Logic 0 to
between +3 and +15V, and changes TTL Logic 1 to between -3 to -15V, and vice versa for
converting from RS232 to TTL. This can be confusing when you realize that the RS232 Data
Transmission voltages at a certain logic state are opposite from the RS232 Control Line
voltages at the same logic state. To clarify the matter, see the table below.
RS232 Line Type & Logic Level RS232 Voltage TTL Voltage to/fromMAX232
Data Transmission (Rx/Tx) Logic 0 +3V to +15V 0V
Data Transmission (Rx/Tx) Logic 1 -3V to -15V 5V
Control Signals (RTS/CTS/DTR/DSR) Logic0 -3V to -15V 5V
Control Signals (RTS/CTS/DTR/DSR) Logic
1+3V to +15V 0V
POWER SUPPLY
The system requires a regulated +3.3v supply for the semiconductors and a +12V
unregulated supply for the relay. These can be delivered from the 230V domestic supply.
Before applying this to the system we must step down this high voltage to an appropriate
value. After that it should be rectified. To achieve +3.3 V DC we should regulate this. All this
are run in the power supply circuitry.
8/3/2019 College_Automation
33/47
Power supply is used to give sufficient power to the microcontroller. A step down
transformer and a bridge rectifier is used here to convert AC to DC. A regulator IC is also used
here to give constant supply.
A 12-0-12V step down transformer is connected to provide the necessary low
voltage. The transformer also works as an Isolator between the hot and cold end. The hot end
refers to the 230v supply, which is hazardous one, and the cold one refers to the low, safe
voltage .Now the hot portion appears only at the primary of the transformer. The secondary of
the transformer deliver 12v ac pulses along with a ground.
O /.
.
.1 5
4 8
A C I / P
.
R E C T I F I E R
R E G U L A T O R I C
This ac supply goes to a center tap rectifier, which converts the ac into a unidirectional
voltage. The ripples in the resulting supply is filtered and smoothed by a 2200micro FD/25V
capacitor. The 0.1 microfarad capacitor bypasses any high frequency noises. The resulting
supply has magnitude above 17V.
This voltage is given to the regulated IC S111733.This IC provides a regulated 3.3V
positive supply at its 3 rd pin.
DESCRIPTION
The SA1117 is a positive low voltage dropout regulator; voltage dropout is only 1.2V at 1A.
SA1117 has two versions: the fixed version and the adjustable version. VOUT has a tolerance
of less than 1% for fixed versions 1.5V, 1.8V, 2.5V, 3.3V, 5.0V and adjustable version or 2%
output accuracy for fixed version 1.2V. The SA1117 integrates overheating protection and
current limit circuits. It is suitable for all electronic products.
FEATURES
8/3/2019 College_Automation
34/47
1% accuracy for fixed output VOUT while the voltage is 1.5V, 1.8V, 2.5V, 3.3V,
5.0V and the adjustable output VOUT
2% output accuracy for fixed version 1.2V
Low Dropout Voltage: 1.2V at 1A output current
Current Limiting
Thermal Shutdown
Temperature Range: -40C to 125C
APPLICATIONS
Laptop, Palmtop, and Notebook Computers
Battery Charger SCSI-|| Active Terminator
Cellular Phone
Cordless Telephones
Battery Powered Systems
Portable Devices
SMPS Post-Regulator
IR DETECTOR
This section is built around IR transmitters and receivers. The IR
transmitters and receivers are placed at both sides of the entrance in line of sight. In normal
condition the IR receivers receive IR rays from TXr when a student enters the class room the
path will be blocked and output of the receiver will become high. This is detected by the micro
controller. A pair is used because we have to find the direction, is student passing in to or outof the class room.
8/3/2019 College_Automation
35/47
PCB DESCRIPTION
5
8/3/2019 College_Automation
36/47
PCB DESCRIPTION
The first step of assembling is to procure a printing circuit
board .The fabrication of the program counter plays a crucial role in the electronic field. The
success of a circuit is also depends on the PCB. As far as the cost is concerned the more than
25% of the total cost is gone for the PCB design and fabrication.
We are using a micro controller-based system that handles
high frequencies. In the high frequency circuit the data may easily be violated in the PCB due
to the physical parameters .That is the track capacitance and inductance can cause the cross
talk in the buses. Also unwanted noise can be induced to supply rails and from there it canaffect the total response. Hence the PCB design has a major role in system performance.
Design of a PCB is consider as the last step in electronics
circuit design as well as the first step in production of the PCBs. It forms a distant factor in
electronics circuits performance and reliability. The productivity of the PCB and its assembly
and service ability also depends on the design .The designing of the PCB consist of the
designing of the layout followed by the generation of the artwork. Orcad is low cost feature
rich software package for designing electronics circuit diagrams. The various tools in orcad
and their implementation and designing the PCB is discussed below.
Electronics Design Automation (EDA) tools:
With the advent of powerful computing system and
interactive software, several stages in the design and development of an electronic circuit has
undergone automation .The software and this hardware tool, which enables this automation, is
called EDA tools. This tool helps us in such a way that we can draw that circuit ; list the
functioning of the circuit in response to the best input in assimilation software after successful
simulating the circuit. The placing and routing software does the PCB artwork in the project
the design automation tool used in orcad, which includes.
8/3/2019 College_Automation
37/47
Or cad Capture:
For circuiting the diagram, create schematic and net list.
Or cad Layout:
For creating the PCB artwork the design process is of the following steps:
1. Drawing the circuit schematic:
This is done in orcad schematic capture. It includes many libraries withthousands of component symbol. We can select the required symbol from library and place it
on the schematic page. After placing the component symbol, the inter connection is completing
using bus tool. After drawing the schematic, the following operations are performed.
2. Routing:
Routing is the interconnection of component using upper tracks of required
width .Before starting routing the following thinks are done.
1. Enabling/disabling required layers:
The number of layers used and enabling the artwork depends upon the complexity
of the circuit, and fabrication technology available. If the board is single sided , enable only
bottom or solder side layer, so that track will come only on one side of the PCB If the circuit is
much more complex the enable the required number of inner layer consider the fabrication
technique and cost.
Manual routing:
In this, the PCB design has to manually connect each track. This is time
consuming process, but is required some cases. On this also the software checks for errors and
reports.
8/3/2019 College_Automation
38/47
8/3/2019 College_Automation
39/47
WORKING
When a student enters the class the path of IR breaks and microcontroller
senses the direction and increments the count which represents strength. Then the student has
to mark the attendance. All these information are stored in to the internal memory with the roll
number of the student. If a student goes out of class microcontroller detects this from IR
detectors and decrements the class strength. Through the PC interface the computer to the
principal can download the attendance of the students at any time. The computer can send the
bell time to the system using the PC interface. Principal can sent an urgent message to the
LCD display which is placed in the class room through serial transmission. And the principal
can also clear the display in the class room and data stored.
8/3/2019 College_Automation
40/47
SOFTWARE DESIGN
6
8/3/2019 College_Automation
41/47
FLOWCHART
8/3/2019 College_Automation
42/47
8/3/2019 College_Automation
43/47
SOFTWARE
8/3/2019 College_Automation
44/47
SOFTWARE TOOLS
The program was written and simulated using keil, which is a complete IDE. It consist a text
editor, C compiler and a simulator to simulate our code. The keil for ARM is a powerful,
feature-rich development tool. It is designed to provide the programmer with the easiest
possible solution to developing applications for embedded systems, without compromising
performance or control.
Features
Keil allows you to quickly develop and deploy complex applications:
Write your C source code using the built-in Code Editor (Code and Parameter
Assistants, Code Folding, Syntax Highlighting, Auto Correct, Code Templates, and
more.)
Monitor your program structure, variables, and functions in the Code Explorer.
Generate commented, human-readable assembly, and standard HEX compatible with
all programmers.
Get detailed reports and graphs: RAM and ROM map, code statistics, assembly listing,
calling tree, and more. Keil provides plenty of examples to expand, develop, and use as building bricks in your
projects. Copy them entirely if you deem fit thats why we included them with the
compiler.
8/3/2019 College_Automation
45/47
CONCLUSION
6.1 RESULT
6.2 FUTURE EXPANSION
6
8/3/2019 College_Automation
46/47
6.1RESULTThe COLLEGE AUTOMATION system was developed and tested successfully.
6.2 FUTURE EXPANSION
The system presented here has a lack of security features; we can provide high
security to this system by replacing keyboard with a FINGERPRINT IDENTIFICATION
module. The fingerprint module identifies each student by scanning their fingerprint so the
level of security is very high. Also we can expand the memory of the system so that the
attendance details for a month or greater than that can be stored in the system.
8/3/2019 College_Automation
47/47
BIBILIOGRAPHY
1. www.datashhet4u.com
2. www.dallas.com
3. www.maxim.com