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Topic: TTL Serial noise affecting entire board (Read 701 times) previous topic - next topic

dpmattox

I have asked quite a few questions to get my ShowStopper LED lighting controller going, and it's finished. However, through all 5 PCB revisions so far, When the bluetooth module is connected to an Android phone (http://dx.com/p/jy-mcu-arduino-bluetooth-wireless-serial-port-module-104299), everything is beautiful, as soon as the bluetooth link is disconnected, the LED strips flicker with pulsing of the status LED on the bluetooth module. I have tried moving the bluetooth module away from and next to the ATMEGA, placing it under the board or on top, connecting the module with a cable even. The flickering also feeds back into the MSGEQ7 readings from User audio, even into the vehicles audio system as a popping sound. But again, simply opening the Android app to establish a connection, gets rid of the popping, the flickering, and the bad readings... I am adding a decoupling capacitor to the power lines of the module in the next revision (.1uf), but is there something I can do to the serial lines?

pylon

Can you post the schematics of your circuit?

dpmattox

As soon as I finish this revision, give me 30 minutes!

Grumpy_Mike

Quote
I am adding a decoupling capacitor to the power lines of the module in the next revision

Why not add them now? A surface mount capacitor can easily be tacked on the track side of any board. I am sure there are places where the power rails are close enough for this.

dpmattox

I don't have any boards on hand, all 3 from this batch (I use OshPark) have been sold. Otherwise I would just add one to the board. Here is the schematic for the next revision, I added a cap to the BT power lines.

http://imgur.com/NgONhVS

Grumpy_Mike

Quote
I don't have any boards on hand, all 3 from this batch (I use OshPark) have been sold.

Well there is no way you are going to know if this fixes the problem until you get it made. The normal development process is that you hack about with the PCB until you get it to work and then you have another revision made.

dpmattox

Please allow me to rephrase the question, is it safe to add a capacitor in parallel with RX and TX?

pylon

In parallel? You should add it to the power supply pins (Vcc and GND) and not the serial interface.

dpmattox


davew

Hmm,

maybe I'm not thinking well yet, just woke up, but if the BT is disconnected, there is NO serial data..
So what's affecting the board?
1) the BT LED. Check the widths and routing of the VCC and GND to the BT header, make sure they are as direct as possible to the VCC supply/ regulator.
OR,
2) if the BT is ramping up it's RF power level to find the Android phone, the BT module is drawing more current. Can the supply handle it?
OR,
3) the RF is affecting the drivers, try RF bypass on the gates.

several things that are not TTL serial problems.

Dave

Grumpy_Mike


Please allow me to rephrase the question, is it safe to add a capacitor in parallel with RX and TX?

As a general rule adding capacitors to signal lines is a bad idea. Like the shift out tutorial in the learning section. It is often done by people who do not know what they are doing.

If a line is floating you might like to try adding a pull up resistor to the signal line.

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