user guide ktgps1

7/28/2019 User Guide Ktgps1

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GPS Receiver

Build and program your own GPS receiver.

Contents

OverviewChallenge 1: Understanding GPS Data

Collecting Your ComponentsoConnecting the Receiver to Your ComputeroSetting up a Terminal Emulatoro

Understanding GPS DataoChallenge 2: Controlling the Receiver

Collecting Your Componentso

Connecting the Receiver to the ChipoDisplaying Raw GPS Data on the LCDoParsing the Raw GPS DataoSelecting GPS Output StreamsoDisplaying Specific GPS Data ElementsoFormatting GPS Data on Two Lineso

Overview

In this project, you will build an interface to the GlobalSat EM-406A GPS Receiver. TheEM-406A is a 20-channel GPS receiver based on the SiRF StarIII chipset, and it features

a built- in antenna. After building the interface circuit, you will learn how to customize it

to display more than 30 standard GPS data types, including latitude, longitude, altitude,

velocity, heading, time, and da te.

Figure 1. GlobalSat EM-406A GPS Receiver.

Like most GPS receivers, the EM-406A conforms to the standard NEMA SiRF interface

protocol, so the code you write in this project will work with most other GPS receivers.

Challenge 1: Understanding GPS Data

In order to better understand the GPS data format, you will first connect your GPSreceiver directly to your computer through the Machine Science

programming cable. GPS receivers are capable of transmitting a large

volume of data, which can be difficult to see on a small LCD screen. By

viewing the data on a computer monitor, you will be able to learn about thedifferent GPS data streams and data types.

Collecting Your Com ponents

In order to connect your GPS receiver to your computer, you will need the followingcomponents:

Part Quantity Description

A 1 GlobalSat EM-406A GPS Engine

B 1 Connector harness (6-pin)

C 1 Bent header (6-pin)

D 1 Bent header (4-pin)

Connecting the Receiver to Your Computer

Using Figure 2 as a guide, connect the GPS receiver to the programming board, which

will link it to your computer. To avoid intefering with the microcontroller, build this

circuit in an area of the board away from the chip. Figure 3 shows one way to lay out this

circuit. To orient the GPS unit properly, note the position of the grey wire in Figure 3.


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Figure 2. Connecting the GPS receiver to the computer (schematic).

Figure 3. Connecting the GPS receiver to the computer (photo).

Setting up a Terminal Em ulator

Connected as shown in Figure 2, the GPS receiver will send satellite data to your

computers COM port. To view the data, you will need to run a terminal emulator--a

program that emulates an older-style text-only computer terminal. Depending on youroperating system, your computer may already have a terminal emulator, or you may need

to download one from the Internet:

Windows (Vista): TeraTerm Pro Web is available here:http://www.ayera.com/teraterm/Windows (pre-Vista): HyperTerminal is available in the Start menu underPrograms > Accessories > Communication.


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Macintosh: ZTerm comes pre-installed with OSX and above.Linux: CuteCom can be found at: http://cutecom.sourceforge.net/

The instructions in this section are for installing TeraTerm Pro Web, but should give you

enough information to help you set up any of the other programs listed above.

1. Determine which COM port your Machine Science programming board is usingby viewing the Options menu in the Programming Window.

2. Download TeraTerm Pro Web from http://www.ayera.com/teraterm/

3. Unzip the .zip file to a convenient location on your hard drive.

4. Open the folder and double click on the file ttermpro.exe. You will see a dialogbox like the one shown in Figure 4.

Figure 4. New connection dialog box.

5. Select Serial and the COM port being used by your Machine Science

programming board. You should now see unintelligible characters scrolling acrossand down your screen as shown in Figure 5.


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Figure 5. TeraTerm Window.

6. The reason you see a jumble of characters is that you need to set the baud rate inbaud rate in your terminal program to match the baud rate of the GPS receiver.

Select Serial port... from the Setup menu. You should now see a dialog box like

the one shown in Figure 6.

Figure 6. Serial port setup.

7. Select 4800 for the baud rate. You should now see lines of GPS data being

updated every second on your screen, as shown in Figure 7.


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Figure 7. Term inal emulator.

Understanding GPS Data

GPS receivers are capable of transmitting eight types of GPS data streams, shown below:

Option Description

GGA Time and position data

GLL Latitude, longitude, time and status

GSA GPS receiver operating mode, satellites used in the position solution,

and DOP values

GSV The number of GPS satellites in view, satellite ID numbers, elevation,

and azimuth

MSS Signal-to-noise ratio, signal strength, frequency, and bit rate from a

radio-beacon, receiver

RMC Time, date, position, course and speed data

VTG Course and speed information relative to the ground

ZDA Pulse Per Second (PPS) timing message

Referring to Figure 7 (or your terminal emulator, if it is still running on your computer),

you should see four types of GPS data streams: GGA, RMC, GSV, and GSA. These are

the four default data streams that are transmitted by the GPS receiver. Each stream is

transmitted on a single line. An example of a GGA data stream is shown below.

$GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M, , , ,0000*18


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Each GPS data stream can have up to sixteen GPS data fields. The data fields are

separated by commas, as shown in the sample above. Table 3 below details the data fields

contained in the GGA data stream.

Nam e Example Description

Message ID $GPGGA GGA protocol header

UTC Time 161229.487 hhmmss.sss (h=hours,m=minutes, s=seconds)

Latitude 3723.2474 ddm.mmmm (d=degrees,

m=minutes)

N/S Indicator N N=north or S=south

Longitude 12158.3416 dddmm.mmmm (d=degrees,

m=minutes)

E/W Indicator W E=east or W=west

Position Fix Indicator 1 0=fix not valid, 1,2, or 3= fixvalid

Satellites Used 07 Range 0 to 12

HDOP 1.0 Horizontal dilution of

precision

MSL Altitude 9.0 Meters above sea level

Units M Meters

Geoid Separation Meters

Units Meters

Age of Diff. Corr. Seconds

Diff. Ref. Station ID 0000

Checksum *18 Check for valid data

Carriage return and line feed

indicating end of message

Don't worry about understanding all of the GPS data types shown in this table. The ones

of greatest interest to you will probably be time, latitude, longitude, and altitude. In thenext challenge, you will learn how to display this information in a meaningful way on

your LCD.

Challenge 2: Controlling the Receiver

In this challenge, you will construct a circuit connecting the EM-406A to

the Atmega Board and program the chip to display readable GPS data onthe LCD.


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Collecting Your Com ponents

In order to build the circuit, you will need the following components:

Part Quantity Description

A 1 GlobalSat EM-406A

B 1 Connector harness (6-pin)

C 1 Bent header (6-pin)

D 1 Atmega Board

Connecting the Receiver to the Chip

Using the schematic in Figure 8 as a guide, add your components to the breadboard.

Figure 9 shows an example of a completed board for this circuit. Since the leads on the

GPS receiver are not labeled, you may need to examine the picture to make the correctconnections.

Figure 8. Connecting GPS receiver to the Atmega168 (schem atic).


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Figure 9. Connecting GPS receiver to the Atmega168 (photo).

Displaying Raw GPS Data on the LCD

The following code will allow you to begin receiving data from your GPS receiver and

displaying the information on your LCD. In this first step, you will not be parsing the

data from your receiver. As you learned in the previous challenge, GPS receivers can

produce a lot of data, so your tiny LCD will quickly fill up with a lot of numbers, letters,

and symbols. While you will not be able to read the data, it will indicate that your circuitis working properly. (It also looks kind of cool!)

IMPORTANT NOTE: The GPS receiver and the Machine Science programming board

both use the same pin for transmitting data to the Atmega168. You must disconnect thewire linking the GPS pin 4 to the Port D0 of the Atmega168 before downloading your

code. You can immediately reconnect this wire once the code has been downloaded.

#include "mxapi.h"1.#include "usart.h"2.

#include "lcd.h"3.4.

int main ( void )5.{6.

char gps_characters; //Declare a variable for7.storing GPS characters


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usart_init ( 4800 ) ; //Initialize the USART to8.4800 baud

lcd_init () ; //Initialize your LCD9.while ( 1 == 1 ) //Start an infinite loop10.{11.

gps_characters=usart_read () ; //Read each12. character from the GPS

lcd_character ( gps_characters ) ; //Display13.each character on the LCD

}14.}15.

Parsing the Raw GPS Data

In order to better view the data on your LCD, you will need to parse the raw data stream

from the GPS receiver. In Figure 7, you can see that the terminal emulator on yourcomputer displays each data stream on a separate line. It does this by checking for a

carriage return and line feed, the ASCII characters 13 and 10, at the end of each data

stream (as shown in the last rows of Table 3). When the terminal emulator finds these

two characters, it starts the next data set on a new line, making the data much easier to

read than if it were simply a continuous stream.

The microcontroller has no built-in ability to parse data. As you may have seen in the

previous exercise, the carriage return and line feeds at the end of each data string are

displayed on the LCD as more data. As a result, using the previous code, the GPS data

was displayed on the LCD in long stream without easily discernible breaks.

The next code sample introduces breaks after each line of data, just like the terminal

emulator did. It checks continuously for a line feed character (ASCII 10), and when it

gets one, it repositions LCD's cursor at the start of the first line before displaying the next

character. The data on the display remains somewhat difficult to read, since all of thedifferent GPS data streams are displayed, but it is easier to pick out patterns in the data.

#include "mxapi.h"1.#include "usart.h"2.#include "lcd.h"3.

4.

int main ( void )5.

6. {char gps_characters; //Declare a variable to7.

store GPS characters


lcd_init () ; //Initialize the LCD9.while ( 1 == 1 ) //Start an infinite loop10.


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{11.gps_characters = usart_read () ; //Read12.each character from the GPS


if ( gps_characters == 10 ) //Check for the14. line feed (ASCII 10) character

{15.lcd_instruction ( FIRST_LINE ) ; //Move16.the cursor to the first line

}17.}18.

}19.

Programming Challenge

Modify your code so that each line new line scrolls up in the same way that it displays onyour computer monitor. This is difficult!

Selecting GPS Output Streams

The following code turns on and off specific GPS data streams. Remember that the

receiver is capable of transmitting any or all of the eight different GPS streams listed in

Table 2 at a rate of up to 100 Hertz each. In order to display the data on the LCD, it isbest to turn on only one stream and turn off the rest, using a function called gps_setdata.

This function takes two arguments: the first argument selects the GPS stream that you

want to turn on or off; the second argument determines how frequently (in Hertz) the

GPS receiver transmits the stream. A value of 0 turns off the stream entirely, while avalue of 1 to 99 specifies a transmission frequency of 1 to 99 times a second.


#include "lcd.h"3.#include "gps.h"4.

5.

int main ( void )6.7. {

char gps_characters; //Declare a variable to8.

store GPS characterusart_init ( 4800 ) ; //Initialize the USART to9.4800 baud

lcd_init () ; //Initialize the LCD10.delay_ms ( 2000 ) ; //Delay while GPS receiver11.warms up

12.


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/* Turn off all NMEA formats except the one13.you want to see. */

gps_setdata (NMEA_GGA, 1 ) ; //Turn on GGA14.format at 1 Hertz

gps_setdata (NMEA_GSA, 0 ) ; //Turn off GSA15.

format (0 Hertz)gps_setdata (NMEA_GSV, 0 ) ; //Turn off GSV16.format (0 Hertz)

gps_setdata (NMEA_RMC, 0 ) ; //Turn off RMC17.format (0 Hertz)

while ( 1 == 1 ) //Run the following code in an18.infinite loop

{19.gps_characters = usart_read () ; //Read20.each character from the GPS


if ( gps_characters== 10 ) //Check for the22.line feed (ASCII 10) character

{23.lcd_instruction ( FIRST_LINE ) ; //Move the24.cursor to the first line

}25.}26.

}27.

You do not need to select a GPS data stream every time you program the device. Once a

stream is selected, the receiver displays only that stream until the gps_setdata function iscalled again.

Programming Challenge

Try changing the GPS stream displayed by your GPS receiver. Try changing how often

the GPS stream is updated.

Displaying Specific GPS Data Elem ents

With the following code, you can display specific GPS data elements, such as time,

latitude, longitude, and altitude, from the selected data stream. To simplify matters, every

GPS data stream is parsed in the background by a function within the gps.h header file.The parsed data is stored in an array called gps_data[ ], with each value in the array

representing a different GPS data element. You can reference any element in the

gps_data[ ] array in your code.


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For example, with the GGA data stream, selected a line of data might look like this:

$GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M, , , ,0000*18

In this instance, gps_data[0] would equal $GPGGA, gps_data[1] would equal

161229.487, gps_data[2] would equal N, and so on.

The following table shows the gps_data[] values for some of the more useful data

elements in the GGA, RMC, and VTC data streams.

GPS Data Example Units Format GGA RBC VTG

Message ID $GPGGA 0 0 0

Latitude 3723.2475 Degrees ddmm.mmmm 2 3and

minutes

North/South N 3 4Longitude 12158.3416 Degrees dddmm.mmmm 4 5

and

minutes

East/West W 5 6

Altitude 9.0 Meters 9

Speed 0.2 Kilometers 7

/ hour

Course 309.62 Degrees 8 1

Time 161229.487 hhmmss.ss 1 1Date 120598 ddmmyy 9

Satellites 07 0 to 12 7

This program uses the chip's internal interrupts, which allow the microcontroller toexecute multiple tasks at the same time. A few new statements are added to enable and

initialize the interrupts.


#include "lcd.h"3. #include "gps.h"4.5.

int main ( void )6.7. {



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usart_interrupt_rx ( ENABLE) ; //Enable9.interrupts for the USART

sei () ; //Turn on interrupts10.lcd_init () ; //Initialize the LCD11.

12.

while ( 1 == 1 ) //Start an infinite loop13. {14.lcd_instruction ( FIRST_LINE ) ; //Move the15.cursor to the first line

lcd_instruction ( CLEAR) ; //Clear the LCD16.lcd_text ( gps_data [ 2 ]) ; //Display the 3rd17.value in the selected data stream

}18.}19.

Formatting GPS Data on Two Lines

The last code example in this project will display and format multiple GPS data fields on

both lines of the LCD. The added functions in this example should be familiar to youfrom other Machine Science projects.

#include "mxapi.h"1.#include "usart.h"2.#include "lcd.h"3.

#include "gps.h"4.5.

int main ( void )6.

7. {usart_init ( 4800 ) ; //Initialize the8.Atmega168s USART to 4800 baud

usart_interrupt_rx ( ENABLE) ; //Enable9.interrupts for the USART

sei () ; //Turn on interrupts10.lcd_init () ; //Initialize your LCD11.

12.

while ( 1 ) //Run the following code in an13.infinite loop

{14.lcd_instruction ( FIRST_LINE ) ; //Move the15.cursor to the first line

lcd_text ( gps_data [ 2 ]) ; //Display value16.of third data field

lcd_character ( ':' ) ; //Display ':'17.character


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lcd_text ( gps_data [ 3 ]) ; //Display value of18.fourth data field

19.

lcd_instruction ( SECOND_LINE ) ; //Move the20.curse to the first line

lcd_text ( gps_data [ 4 ]) ; //Display value21. of fifth data field

lcd_character ( ':' ) ; //Display ':'22.character

lcd_text ( gps_data [ 5 ]) ; //Display value of23.sixth data field

lcdBlankLine () ; //Send blanks to24.clear the lines

}25.}26.

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