problem with connection between gps evalution board and sk40

[chuaweekok]

i baught a pic start-up kit, SK40C, GPS evaluation board, and 32 Channel GPS Receiver, EB-85A recently.

i want to send the the GPS data to the microcontroller using usart and display to LCD.

i connect the Tx and Gnd pin from gps evaluation board directly to Rx and Gnd pin of SK40C using jumper wire
but, LCD only display null characters.

i switch to connect using RS232 to SK40C
LCD display unknown character.

i have check the functionality of the LCD
because it display normally when display character that i have store in the memory
connection of GPS evaluation board, and 32 Channel GPS Receiver, EB-85A with computer have been successful
the data seen to can perform localizatioin using GPS trace software.

i am using PIC16F877A and 20MHz crystal.
and i set the baud rate to 38400.

so i think i am having problem to establish the connection between gps evaluation board and the PIC in the SK40C

please help me for this connection problem

[ober]

You can also try to verify the UART on SK40C by connecting it to computer. From HyperTerminal, try to type something. The character should be displayed on LCD. Of course the baud rate and setting should be the same.

[chuaweekok]

thank Mr. Ober for your suggestion.
i Have verified the UART connection between computer and SK40C using UC00A
LCD display the character i type in the hyperterminal
i also verified the UART connection between computer and GPS evaluation board with 32 channel receiver using UC00A
Hyperterminal show the NMEA code from the GPS

but when i connect GPS's Tx, Rx. and Gnd pin directly to SK40C using same configuration without UC00A
i didnt connected the VDD pin of SK40C
The LCD only display character 'll' repeatly

then, i think the problem is about the connection between the SK40C and GPS evaluation board.But, i'm not really sure the connection that i have make because the GPS evaluation board dont have any user manual.

please help me with this communication problem. thanks.

[ober]

i Have verified the UART connection between computer and SK40C using UC00A
LCD display the character i type in the hyperterminal
i also verified the UART connection between computer and GPS evaluation board with 32 channel receiver using UC00A
Hyperterminal show the NMEA code from the GPS

Good troubleshooting skill. So you are connecting the GPS Evaluation board to PC using USB? I assume is it because this is the most simplest way.

but when i connect GPS's Tx, Rx. and Gnd pin directly to SK40C using same configuration without UC00A
i didnt connected the VDD pin of SK40C
The LCD only display character 'll' repeatly

You are connecting directly from GPS to SK40C? Em..... so how are you power the GPS? From Evaluation board? Did you power with positive and negative too? Or you connect the Gnd to SK40C only? Another concern is 3.3V and 5V signal. If the GPS is 3.3V power, the Tx pin of SK40C will pump 5V to Rx pin of GPS, though it will not cause major problem, just be careful on that.

then, i think the problem is about the connection between the SK40C and GPS evaluation board.But, i'm not really sure the connection that i have make because the GPS evaluation board dont have any user manual.

Yup, the evaluation board is just help user to interface, referring schematic given is sufficient. Are you sure you have cross connect the communication line? Tx to Rx, Rx to Tx?

[chuaweekok]

thank for the reply.

You are connecting directly from GPS to SK40C? Em..... so how are you power the GPS? From Evaluation board? Did you power with positive and negative too? Or you connect the Gnd to SK40C only? Another concern is 3.3V and 5V signal. If the GPS is 3.3V power, the Tx pin of SK40C will pump 5V to Rx pin of GPS, though it will not cause major problem, just be careful on that.

i connect the gps to SK40C through gps evaluation board. i power up both of them by 9v adapter separately. i select the external disconnect switch on the GPS evaluation board to isolate the Rx pin of GPS evaluation board to GPS. So, are this problem cause by 3.3v signal from the Tx of the GPS evalaution board which is lower than the 5V from the Sk40C.

Are you sure you have cross connect the communication line? Tx to Rx, Rx to Tx?

i already cross connect the communication line as Tx to Rx, Rx to Tx.

I'm wandering whether is the 38400 baud rate is too fast for the communication of PIC to GPS evaluation board.
i have try to change the baud rate of the gps, but not success.

[waiweng83]

So, are this problem cause by 3.3v signal from the Tx of the GPS evalaution board which is lower than the 5V from the Sk40C.

Yes, you are right. In fact, the UART signal from the GPS module is just 2.8V and this may be a problem. To solve this, you can use the following circuit:

[chuaweekok]

thanks for your circuit diagram.

i have constructed and test the circuit with the Vcc from Sk40C which is around 4.8V.
now the LCD display different character like "G", "$", "e", and other. but it is still different from the NMEA code shown in the PC using Hyperterminal.
i have check the voltage of the Rx pin of SK40C. It have rise from about 2.5v to about 3.5v.

so, is it cause by the baud rate problem?

[waiweng83]

You have tested the following and they are working rite?

- GPS connect to PC
- SK40C connect to PC

What is the working baud rate?

[chuaweekok]

i have tested the both connection.
the working baud rate is 38400.

when connect gps to pc
the hyperterminal show the NMEA code

when connect Sk40c to PC
the LCD display the character i type in the Hyperterminal

[waiweng83]

what happen when the length of the character you type is over the length of the LCD? Cause the GPS is sending the NMEA character continuously.