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Extremely high current requirement of RESET line.

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  • Extremely high current requirement of RESET line.

    Hello fellow forum members.

    I have been experiencing some reliability issues with the SOMO-14D that I'm using in a serial controlled application.

    I set up the attached microcontroller to issue a reset about 0.5 seconds after power on. I figured 0.5 seconds would be along enough to ensure the power was stable.

    For the reset I had the micro take the reset line low for 200mS.

    What I found is that following this reset the SOMO would not always play a file. As if the SOMO was not reset properly.

    Looking at the reset line with a DSO I found that the reset line was not going all the way to ground. It went down to only about 1V, which is not low enough to guarantee reset.

    The micro is an Atmel ATmega8L. It is capable of sinking around 20mA, which is more than adequate to pull a logic line low.

    With the micro removed from the circuit I connected a DMM in series with reset to ground and found the current to be 34mA!!!!

    What the heck! How can a micro be expected to pull the reset line close to ground? The reset current is not mentioned in the rev5 datasheet, so the assumption I as anyone else would make is that the current would be less than one milliamp.

    Why does the datasheet not give a specification for the reset current, given it is so extraordinarily high?

    Why does the datasheet not show that an external transistor is required to operate the reset from a micro?

    Hoping someone can shed some light on this, thanks, Neill

  • #2

    You should have at least a 330R resistor in series with the RESET line to deck, to limit current. This is not in the manual, from what I remember, but this is what I did, as I was connecting to a 5v PIC. Same idea as the resistors on CLK and DTA when used with 5v systems. Even with a 3v3 system, I would still like the series resistor.


    • #3

      Thanks for the response Graeme. It has been a while, but only just now am I again looking to use this module in a small run of about 20 units.

      Unfortunately limiting the current in the RESET line is not going to solve the problem. The RESET line needs to be pulled low enough to ensure a low logic level. Adding a resistor will only reduce the current and raise the Voltage at the reset pin.

      The solution is to use a buffer transistor to do the hard work that is beyond the capability of a typical micro controller output. But is this really a solution?

      So I'm still left wondering if this problem has been fixed in silicon. Is the RESET line actually the output of an internal regulator and in fact when the line is pulled low it effectively shorts out the internal power supply? Is there an internal pull-up resistor on the RESET line which was meant to be perhaps 100k Ohm but ended up been a 100Ohm?

      I wish I could have more confidence in this module as apart from the dodgy reset issue it's a great thing.