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This chapter considered with two main parts of the project which are the global positioning system known as GPS and the programmable intelligent computer PIC.
1.2 Programmable Intelligent Computers PIC
PIC stands for programmable Intelligent Computer, Circuit or Control.
1.2.1 The PIC used in the projectknown as 18F452 PIC
Each number stands for something:
18: stands for the family of the PIC controller.
This controller has 18 bits: two bits for something we call chip select or it means that the controller has four banks and these two bits are for selecting of these banks, the other sixteen bits and these are the bits for the data bus in this controller.
Note: The selecting of the banks is performed by the first two bits by logic method for example 00, 01, 10, 11, and these is always four banks in the PIC controller.
The letter F stands for FLASH MEMORY and that is the type of the controller, some types have the letter C, and that's stands for COMMERCIAL.
The difference between these two types is that we can reprogram the flash memory type 100000 times and we can add or delete information from it, but in the commercial type it cannot be reprogrammed, there is programmed one time only and that's why they are cheaper too, the reason that we didn't use the commercial type in this project is that our project needs some future developments, and it doesn't have reached its final version, so we needed the flash memory type to edit the code.
18F452 PIC use CMOS technology which stands for Complementary Metal Oxide Semiconductor, this type is voltage controlled, on the contrary of the TTL which is current controlled.
1. it has less power consumption.
2. It has high speed.
The CMOS is faster because it doesn't work on P-N junction, because it has a propagation time delay.
The numbers 452: they stand for the series number.
This PIC has analog to digital converter (ADC) and bandwidth modulation built in it.
1.2.2 The pins of the PIC
To operate the PIC, there are some pins has to activated or connected, they are called (MUST). The other pins are not important and the PIC can operate without them, they called (OPTIONAL).
MUST: consists of:
(1.) Supply: its ranges are (4.5 - 5 - 5.5) volts.
Less than 4.5 volts its functions would be shorted and more that 5.5 volt it can be destroyed or burned, It uses four pins; two for Vdd = 5 volt and two for Vss = 0 volt.
(2.) Oscillator: crystal oscillator.
Crystal oscillator made of quartz and 8000 MHz written on the oscillator represents the oscillation or in other words like a clock.
The oscillator make square waves at a frequency equals to 8 MHz and that is represents the speed too, There are many types of oscillators (RC, LC, etc), But in this project usage prototype board, the crystal oscillator is used because it saves time and effort more than the oscillator that works on the principle of the piezoelectric.
The crystal oscillator consists of the quartz crystal and those are ordered in a way that can provide 8 MHz as a frequency, and if high impedance occurs it can decrease it at a high speed. This oscillator needs 2 pins of the PIC.
Note: If we said that for each 4 cycles of the frequency, only one instruction can be performed by the PIC controller, how many instructions can the PIC perform in one second?
Four cycles equal one instruction which mean two million instructions per second.
(3.) M_CLR: Master Clear.
It's like the restart button in the computer; it works on the Active Low principle, so that must be connected on the board on 5 volts. It needs one pin.
OPTINAL: the rest of the pins are all optional and they are 33 pins.
They were divided onto ports for easier way of handling or managing with the PIC controller. There is something called TRIS for each port, each port has 1 byte = 8 bits.
1.2.3 PIC sourcing and sinking current
It is important to study the PIC sourcing and sinking current to protect the PIC and LED is taken as example
Active high case
(5 - 1.3) / 0.3K = 12mA the current that will pass through the led.
If (0 volts) provided in this case, the led will not light, and no current will pass through.
When (5 volts) passes, the led operates, and the current will pass through it.
This case we call it SOURCING. (Because the PIC gives current).
If we provide (5volt), the led will not operate, because it has the same voltage on its both sides, so no current will pass through it. But when we provide (0 volts), the led will light, and the current will pass through it and to the PIC, in the opposite of the first case. This we call SINKING. (Because the PIC absorbs current).
The current in the LED must be more than ten ampere I led >= 10 mA, the resistance is used for limiting (decreasing) the current (for protection and safety).
So in conclusion:
And it's important to know that the limits of the current in the case of sourcing are 20 mA, and in the case of sinking the limit of the current equal 25 mA. In general and for safety we use it (the limits) as 20mA maximum.
In both cases there is no current passing in the circuit, because its voltage controlled.
1.3 The Global Positioning System GPS
1.3.1 GPS distribution
There are at least four up to twelve satellites cover any target or region on the earth as shown in.
Each satellite sends a messages continuously and the message includes the current time, the climate conditions( which has an obvious effect on special calculations such as atmosphere layers which consists of multi materials controls the message delay), satellite position , general information about the satellite and so on. And in this project parallax GPS receiver is used (figure 2.7) .
1.3.2 GPS Theory
There's three points determined and each point faraway from another at specific distance and intersects in one point, the simple theory in two dimensions. In three dimensions there's a third satellite determines the altitude in reference to sea level, this process occurs via a device called (GPS receiver). The function of this receiver which is one of the main parts in our project is collecting the messages transmitted from those satellites regardless the position.
1.3.3 Determination method
In the unknown co ordinations point, the messages are sent continuously as we mentioned and the time identified and via some calculation the reaching time of the message is taken into consideration and the time difference is determined, the distance is already known and the velocity is the light velocity which is 3*102 so the co ordinations of this point can be detected as shown in (figure 2.9).
Note: the satellite sends messages all the time 24 hours every Nano seconds.
One disadvantage of GPS receiver that it is taking a direct line between the farthest points in the path formed in regardless of the distance transferred, but it has a self solution for this disadvantage by differentiation formulas where it determines the velocity each ten to fifteen meters transferred.
Parallax GPS receiver also has important advantage which is Existence of a buffer (memory system) where the recent data depends on previous data (velocity data).The velocity is determined by the memory and the distance was moved also the time.
Note: All the satellites have the same timing (one clock) in nano seconds, it is synchronized.
In many positions there are more than three satellites sharing the GPS calculations, four or five, sex, up to twelve. If the position is already determined by the three satellites then a fourth satellite show that the position is at another point ,which means the fourth point doesn't intersects with the other three points or co ordinations, then the GPS receiver takes the position decided by the majority of satellites.
1.3.4 GPS receiver components
Parallax GPS receiver made by PARALLAX co. and contest of the following:
1) Antenna: the outer part of the receiver which has many shapes such as (square shape, circular shape, spiral shape) The receiver used in the circuit has the square shape antenna because it's better in this project due to the limit free space and the advance connection Properties.
2) Four pins (from right to left order as shown in figure 2.8):
RAW: its function is easy of handling with serial, smart mode or standard mode.
There are two modes for the serial:
a) Standard mode: which we use already in the system, this mode sends back all the data we need or we don't need so it takes more time.
b) Smart mode: which sends back one data we had already asked for so it doesn't too much time and has much rate, this mode is used with some types of PICs that requires high rates to operate.
SIO: serial input (data in), output (data out)
PIN RC7 in the PIC is the only pin which interfaces with serial and this is one of the reasons of using 18F452 PIC which use USART 8n1 (synchronous asynchronous receiver transmitter),Eight stand for data bits, n stand for no parity check, and one for stop bit between each two pockets. Band rate of GPS data is (2400 complications), so the rate of RC7 in the PIC made to be 4800 bps
In the GPS USART sentences all sentences is ignored except the RMC which has the data needed in digital retarder project (longitude, latitude, altitude, velocity, etc...)
VCC: source of 5 volts voltage.
GND: Ground (0 volts).