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Author Topic: uart serial current? What should be expected?  (Read 797 times)
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The other day I was playing with some serial communications from a GPS module and had resistors between the uart conections of the atmega328 and a gtpa010 GPS module and I noticed that the arduino would not receive serial inputs unless the resistor value was somewhat less than 1k ohm (i used 470ohm). I connected a 3v3 wire to the serial input Rx of the arduino and it read 5.5ma for a current! The FTDI chip on the arduino board worked fine with resistors at 2.2k ... (as high as i tested)

I suspect this number should be much lower than this! Is there some some setting for high impedance serial connections? Can someone explain why the current would be this high? How would i got about reducing the current?
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and had resistors between the uart conections of the atmega328 and a gtpa010 GPS module
Why did you have these resistors in place?

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I noticed that the arduino would not receive serial inputs unless the resistor value was somewhat less than 1k ohm
Likely because the GPS isn't supplying enough current to overcome the resistors.

There are already resistors on the TX and RX pins. Adding more is not needed.

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I connected a 3v3 wire to the serial input Rx of the arduino and it read 5.5ma for a current!
For your next experiment, will you be pounding the Arduino with a 25 pound sledge hammer?


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1) To limit the current in case a wire became shorted etc ... ( breadboard )

2) After noting your comment I looked up your schematic and verified there are indeed 1K ohm resistors, however they are not pull down resistors, they go to the RX TX portion of the ft232rl chip. For this scenario to be plausible the ft232rl chip would have to ground the TX and RX portions of the chip which would put me close to explaining the current draw I found. This scenario however seems increasingly likely as putting the resistance below 1k would then put the data signal slightly about vcc/2 ...

3) As funny as you think your comment is, connecting a data line from a chip is no different ... other than the 3v3 is pulsed... and i need to verify that the current draw was what I suspected ... 5 ma is well within specs.
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So it turns out that my ftdi chip packed it in ... I suspect that it has been failing for a bit as I've had problems with that board! Too bad it took out a gtpa010 gps chip before it died!
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