Connect LVTTL to PIC

[shahrul]

Hi,
I want connect GPS SKM53 to PIC16F887. The GPS output is 2.85V LVTTL.
I have done connect GPS to USB-to-UART and display on VB. This is succesfull.

I connect GPS TX directly to PIC RX, PIC can't read signal.
Then I make transistor level like below, also PIC can't read signal.

Schematic-GPS-Receiver[1].jpg (9.42 KiB) Viewed 6522 times

This is weird, I use this method for FV-M8 (formerly named as EB-85A) before.

[robosang]

Seem to be correct, but cannot just verify the signal with USB to UART converter? Which is after the transistors.

BTW, what is the baudrate?

[shahrul]

Seem to be correct, but cannot just verify the signal with USB to UART converter? Which is after the transistors.

BTW, what is the baudrate?

The baudrate 9600. If GPS leave open, (measure with multimeter) the voltage goes 2.XX V and 1.XX V, so I know there is UART signal high and low. But, after connect the transistor, GPS TX fixed at 2.XX V and PIC RX fixed 5V.
I don't know why this connection 'stuck' the signal.

[robosang]

OK, would you mind posting the hardware setup? A clear photo if possible

[shahrul]

OK, would you mind posting the hardware setup? A clear photo if possible

Ok, let me take photo tomorrow.

[shahrul]

Here the photo
Method 1, connect GPS >> USB-to-UART

Method 2, connect GPS >> XBee ----- XBee >> USB-to-UART

The result on VB

GPS Data.jpg (55.31 KiB) Viewed 6462 times

The transistor circuit is below the XBee

The bottom of this, I simply use jumper to connect what I want

GPS TX >> PIC RX, PIC TX >> XBee or GPS TX >> Transistor >> PIC RX, PIC TX >> XBee
These 2 can't function.

I program sending in 2 place, like

CODE: SELECT_ALL_CODE

while(1){
if(receive_gps(){
uart_string("1");}

uart_string("2");
__delay_ms(100);
}


But, only no 2 appear on Serial Communication. That I know, PIC not read GPS UART signal.

[robosang]

Hey, shahrul, very nice photo! Super clear!

Seem to me the hardware is correct and tidy!

CODE: SELECT_ALL_CODE

while(1){
if(receive_gps(){
uart_string("1");}

uart_string("2");
__delay_ms(100);
}


I think you misses at ")" after the receive_gps() for the if statement. But it should be your typo because this is not escape the compiler

What is the specification of GPS? Can you give the link of the datasheet?
Another thing you can verify is to use UC00A, connect to transistor and GPS.

UC00A's RX ---- Transistors ---- GPS's TX. To verify that the transistor logic converter is working or the problem at PIC.

[ABSF]

Wow !! Your arrangement of components and soldering on the back of PCB is really FIRST CLASS. It must have taken a lot of patience and time to do it. I normally use stripboard and a 1/8" drill bit with hand drill to separate the trace. Plus some wire wrapping pins.

Allen

[ABSF]

Seem to be correct, but cannot just verify the signal with USB to UART converter? Which is after the transistors.

BTW, what is the baudrate?

The baudrate 9600. If GPS leave open, (measure with multimeter) the voltage goes 2.XX V and 1.XX V, so I know there is UART signal high and low. But, after connect the transistor, GPS TX fixed at 2.XX V and PIC RX fixed 5V.
I don't know why this connection 'stuck' the signal.

Try put another 330 ohms between the B and E of first 2N2222. The voltage at logic 0 there might be too high and made it saturate.

You can also replace 2N2222 with DTC114ES or DTC144ES if you still can get them. They are transistors with resistors built in.

Allen

[shahrul]

@Robosang, the code is typo error, because that is not actual code. The code I use is, I'm using UART Interrupt. If receive UART, I set a flag byte. At main, I detect this flag and do the next.
Then, I will try your suggestion GPS >> Transistor >> UC00A

@Allen, that soldering not so hard. I always do that and take fast time (1 day while surfing internet, forum and facebook )
I will try your suggestion to add higher resistor value.

Other method I will try opto isolator 4N25.