GPS receiver does not locate

[andy40185]

Hi all,

I have a GPS module, EM411, its Tx pin connect to AT89S52(RxD) while the Rx connect to TxD. I waited for a long time but it does not locate my position, no data output from the AT89S52(RxD) pin. In the EM411 datasheet, it says the Rx pin always pull it high and connect a resistor to 3.5v. Besides, I have not do it yet.

Is it failed if do not follow this steps?

Regards,
Andy

 

[E Kafeman]

You will within a few seconds from start-up read NMEA data with information for each satellite. No need to wait for a fixation.
AT89S52 and EM411, are logical levels compatible? Guess EM411 is a 3.3 volt device and AT is 5 volt?
Verify if you get data from EM411, independent of actual voltage swing. A pull-up resistor can be enough for interfacing different logical voltage levels.
Guess it is covered in data sheet.

 

[lucho1045]

-Ensure you are Powering with 4.5V ~ 6.5V DC input.
-The maximun oput level for GPS Module TX is 2.85 so you need to use a 2N2222 to make the level transition so when is 2.85 the output will be 5V. Like this:
**broken link removed**

Best Regards.

 

[andy40185]

So, EM411's output voltage cannot fulfil the MCU's port operating voltage, is that right?
I have confused that AT89S52 and EM411 are TTL device, why cannot connect them directly?
In addition, which way will be better either pull-up resistor or 2N2222?

Regards,
Andy

 

[E Kafeman]

TTL or not, voltage swing and type of port must fit. If it does so, can you read in the data sheet.
If a pull-up resistor is mentioned for EM411 TxD is that port most likely an open collector output and then is a resistor a requirement. It is a common solution allowing same circuit to handle a wide range of voltages without modification, or share same wire for both TxD and RxD.
Either read data sheet to find out what kind of port EM411 have and/or measure with an oscilloscope.
A 500 Ohm resistor in serial with a LED can be used in case of lack of oscilloscope.
Pull-up resistor or any transistor design are both worthless solutions without knowing actual port requirements.

 

[andy40185]

To E Kafeman,

Thank you so much! In the AT series datasheet, Vcc(0.75/0.9) output high voltage for P1-P3 is required. I guess that it may be work when adding the pull-up resistor on
both usart pin. I will test it tomorrow and share the result;-)

 

[andy40185]

Oh, I have added the pull-up resistor(20k) on both sides(Tx and Rx) and tested it today but it's not work.
It's LED is lighting (searching location)
EM411 - twinkle means it located.

Have any ideas?

 

[E Kafeman]

With not working, do you mean a fixed low or high voltage at TxD? With a multimeter, what voltage can you read?
If you need to increase voltage, replace 20 kOhm with something lower. As low as down to 1 kOhm can be tested.

 

[andy40185]

To E Kafeman,

If it unable to locate the location, the Tx pin still have a voltage?

 

[E Kafeman]

It should normally be some data at TxD pin within seconds after start-up, sending information about hard and software version and such.
After that will it send data even if only one satellite is found. This data contains NMEA information about satellite ID, channel, signal level and sat. location.
If you are indoor, can it maybe not find any satellites at all, but most likely will it still send some kind of start-up information.

 

[andy40185]

now I am indoor and the result is Tx=2.89v, Rx=3.8v

Is it normal?

 

[E Kafeman]

Do not know if it is normal, but from my view seems it to be realistic upper values. Average TxD voltage should at least periodically go down a bit as indication that data is transferred. Try to check outdoor.

 

[andy40185]

It still lighting that means the GPS cannot locate my position but the voltage in Tx pin have a change between 1.x~2.8v while the Rx maintain to 3.8v.
Ah...what's happen??

I have tested before around 5 months with max232 receiving in PC and stored the data in txt. Besides, I remember that it has to wait around 10mins(it's so long).
I only success at that time:sad:

 

[E Kafeman]

Guess there is some data traffic, that is why you see that voltage drop.
A MAX232-ic with corresponding 2.8-Volt logic level is also an alternative to adapt to a common serial interface at a computer.

 

[andy40185]

I have no idea so I guess that its internal antenna is so weak and may be some interference from different way.

Regards,
Andy

 

[zia]

andy you should use translator circuit from 3 to 5 volt so that controller gets dat accuratly
and also chck it hyperterminal whether ur gps working or not

 

[E Kafeman]

Start with reading existing information from the GPS module. If it is correct that you can see voltage drops from 2.8 Volt, this module tries to say something to you. A voltage translator seems to be best way to decode this information?

 

[andy40185]

I find a link about how to build a voltage translator but I am not sure which is being suitable.

http://www.interfacebus.com/Design_Translation.html

 

[andy40185]

As I read the AT89s52 datasheet again, I find the minimum input high voltage is 0.2Vcc+0.9 not 0.75Vcc which is maximum output high voltage.
In addition, the low logic from the Tx also pull high when I add the pull-up resistor. For ATxx, the maximum input low voltage is 0.2Vcc-0.1.
If the pull-up resistor is 330 ohms, the low logic voltage will have 2.6-8v which cannot fulfil the ATxx maximum input low voltage and fail to define the logic(1 or 0)
Therefore, there is no need to add the pull-up resistor and my problem may be solved.
Please note that the GPS module should need around 20-30mins to receive the information from different satellite(at least 3 satellite)

 

[nandhu015]

Please note that the GPS module should need around 20-30mins to receive the information from different satellite(at least 3 satellite)

Where did you get this?