Receiver and PIC not working together
I am having a nightmare of a time with an fsk receiver module and a pic16f877a. I have built a project that receives data from an fsk receiver using 5v rs232 / ttl. When the receiver is powered up independantly from the PIC, everything is working great. When they are both connected to the same power supply I do not even get a kbhit(). The power is from a 12v switching adaptor. The receiver is powered directly from the adaptor and the pic is supplied by a 5v regulator, which is fed from the adaptor. The receiver is connected to the hardware uart and I have checked that all is working. I have tried to decouple the supply with various capacitors and I have managed to get some data but it is corrupted. I am thinking I might need to isolate the two devices using an opto-coupler? Any help provided would be very much appreciated.
Are you saying that it works fine when the receiver and controller are on different boards, but stop working when you put them on the same board?
Other than obvious stuff, like a mistake on the PCB or a missing ground plane, I would look at interference from the controller getting into the Receiver and screwing up the data.
I would make sure there are plenty of bypass capacitors right at the receiver chip. I would add a bandpass filter right at the input to the receiver chip. The bandpass filter will tend to eliminate any controller clock noise getting into the receiver front end.
Hi Biff,
Thanks for your reply. Yes everything works fine when the receiver and controller are on different boards, then when they are together nothing happens. I suspect interference is my problem but I am not sure what to do. I have placed numerous capacitors across the supply and the receiver will output corrupt data, as appose to nothing at all. I have checked the board and everything is ok, no shorts anywhere. I have built the thing on vero board, keeping all the tracks to the minimum size. As for the bandpass filter do you have any idea what frequency I should aim for. I have two 20mhz crystals and the receiver operates at 433.92MHZ and uses FSK modulation. I have tried to see if anything obvious came up by testing the supply with an oscilloscope, the lines seemed noisy but I have seen worse. Any info on this bandpass filter would be greatly appreciated.
Thanks for the help :D
Hi Biff,
I have solved the problem. From your suggestions, I decided to investigate any noise going in to the receiver. The problem was that the signal noise was getting onto the UART line from the receiver. So I used a comparator with a 0.001 uf cap connected across the input and output and everything is working great! The comparator is ignoring the small signals and so is reducing the noise, however it is performing better with the cap connected. Thanks for all the help :)