RF tranceiver problem when working with microcontroller

[Munib]

RF tranceiver problem

Iam using RF transmitter TXM-418-LR and RF receiver RXM-418-LR from LINX TECHNOLOGIES both modules are connected with the mcu PIC18F452
Modules have seperate antennas.I am using them as half duplex tranceiver
transmitters datain pin is connected to TX of USART and recivers dataout pin is cnnnected to the RX of USART in mcu.supply votage of both modules is also controlled by mcu such that at a given time only one of them is working


I have tested the RF modules seperately.I gave a square wave at the input of the transmiiters datain pin and then checked the output on receivers dataout pin by an oscilloscope.Igot the same signal back which means that my RFmodules are working fine.

Now i connected the transmitter to mcu#! and reciver to mcu#2.
mcu#1 is transmitting a counter serially and mcu#2 is supposed to receive that counter.


Now the problem is that i am getting data in mcu#2 but its not correct some values are correct some are false.

can anybody tell me what is the problem when my transmitter and reciever and also microcontrollers are working fine individually the what is the problem when they are integrated

I think that the problem is in the synchroni9zation aof serial data when it is transmitted wirelessly

aNY SOLUTION TO THIS PROBLEM

 

[uk]

Re: RF tranceiver problem

....supply votage of both modules is also controlled by mcu such that at a given time only one of them is working
.....

Can you explain the sentence above more spesificly.

 

[BigBoss]

RF tranceiver problem

Do you only use serial data?? No syncronization bits ?? Or preamble ??
Maybe slicing level is not well adjusted ??
We will also need more specific informations..
I think you should observe the data that you sent with a communication analyzer or something else..

 

[Munib]

Re: RF tranceiver problem

....supply votage of both modules is also controlled by mcu such that at a given time only one of them is working
.....

Both RF modules have power down pins when a logic 0 is given to that power down pin that module goes into very low power mode and cant transmit or receive and by controlling the supply i meant that i am controlling the power down pins

Do you only use serial data?? No syncronization bits ?? Or preamble ??

yes i am using only serial data with one start bit , one stop bit and 8 data bits
baudrate is 9600
Can u plz explain what are synchronization bits and preamble

Maybe slicing level is not well adjusted ??

I couldnt get it please explain

I think you should observe the data that you sent with a communication analyzer or something else..

I have only a DMM and a oscilloscope available so please suggest some simple solution

 

[Borber]

Re: RF tranceiver problem

Check the time delay between power on and start of transmitting. Next you should try lower bitrate.

 

[gecky]

Re: RF tranceiver problem

Munib,

I have the same problem when doing a project using PIC16f628 and a 433MHz Tx-Rx pair. Sadly, I still can't solve the problem.

I sent hex numerals but receive some other stuff at the rx end... some data is correct, some isn't... those that are "corrupted" are not unique.... same in your case?

Since i only needed to send a few instructional codes, what i did is to do a look up table on the rx end... guess this is a trivial soln and will not work if you are doing serial comms and need all 256 char.

Sync shd not be a prob since the USART model in the PIC handles it. What could be wrong is the circuit path to the tx module and data coding - sending too many consequtive 1s/0s.

Also, jus to ensure that all the PIC coding is ok, connect the tx-rx pins by wires to check that the USART is working. Then work on the wireless comms part....

Hope you can solve the problem. Then you can tell me how you do it

Gd Luck!

 

[BigBoss]

Re: RF tranceiver problem

To establish a well communication between two idantical digital sources, some proven techniques should be used I believe.

Especially, during demodulation from FSK or similar waveform, due to assymmetric mixer and slicer behaviour, some pre-data should be sent to syncronize and to adjust the slicing level of the demodulator. We call these datas as "preamble" bits. These bits are used to syncronize and to pull down or pull up the slicing level of the demodulator. And these are 10101010101 ( any long). After sending certain preamble data bits, the dc level of the receiver slicer will be constant ( because of a charging capacitor in it ) and then main data will arrive and be sliced. If you don't use a preamble bits, the DC level will be changed accoording to data bits that is sent. So some bits will be missed and some of them achieved

In fact the methods that we use in digital communication have many techniques and should be carefully investigated.

 

[banh]

Re: RF tranceiver problem

Iam using RF transmitter TXM-418-LR and RF receiver RXM-418-LR from LINX TECHNOLOGIES both modules are connected with the mcu PIC18F452
Modules have seperate antennas.I am using them as half duplex tranceiver
transmitters datain pin is connected to TX of USART and recivers dataout pin is cnnnected to the RX of USART in mcu.supply votage of both modules is also controlled by mcu such that at a given time only one of them is working


I have tested the RF modules seperately.I gave a square wave at the input of the transmiiters datain pin and then checked the output on receivers dataout pin by an oscilloscope.Igot the same signal back which means that my RFmodules are working fine.

Now i connected the transmitter to mcu#! and reciver to mcu#2.
mcu#1 is transmitting a counter serially and mcu#2 is supposed to receive that counter.


Now the problem is that i am getting data in mcu#2 but its not correct some values are correct some are false.

can anybody tell me what is the problem when my transmitter and reciever and also microcontrollers are working fine individually the what is the problem when they are integrated

I think that the problem is in the synchroni9zation aof serial data when it is transmitted wirelessly

aNY SOLUTION TO THIS PROBLEM

the following problems are often encountered
+ power (Vcc to the transmitter is enough)
+ oscilloscope probe may affect your transmitter/receiver
+ if you have 2 transmitters both on at the same time, they may interfere with each other
+ antenna length

 

[FANT]

Re: RF tranceiver problem

When you send some NRZ data through air this type of problems are normal.
The average level depends on the data transmitted and the data slicer ( comparator that extracts the data ) is not working well.
also , usually the noise associated with no signal input changes the DC level at the input of the comparator.
As said above a good improuvment is possible to have with the use of the preamble bytes ( use fron 2 to 10 byte ), but the problem can be solved using biphase mopdulated data, like FM0 , FM1 or Manchester coding.
The bad news is that you need a 8530 or a lot of sotware to generate ( and much more to demodulate ) such type of data.

Mandi