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I am currently connecting an AVR ATXmega 16A4U in my circuit. According to AVR (http://www.atmel.com/Images/doc8278.pdf, p6) the external reset circuit should look like

schematic

simulate this circuit – Schematic created using CircuitLab
But I want to use the RESET/PDI_CLK-Pin also for programming. For this case AVR suggests to remove all capacitance from the reset line.
What should I omit now: The capacitor, or should I risk getting a smeared CLK-signal while programming the µC?

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  • \$\begingroup\$ Related post with discussion about the ATMega reset circuit. (Not a duplicate.) \$\endgroup\$ Feb 17, 2015 at 2:52

2 Answers 2

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You would omit the 100nF capacitor when you want to support the programming mode. In this case it is recommended that you add an additional external reset supervisor chip to your design. The reset supervisor that you select should be one with an open drain output. Depending upon which supervisor chip that you select the capacitor and reset button would be connected into the input side of the supervisor.

This provides the isolation needed to allow the programming function to operate reliably but still allows your board to get a clean reset signal over to the MCU in case of power up or switch press.

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  • \$\begingroup\$ Another idea I just had: Using a jumper for disconnecting the Cap, would that work, too? \$\endgroup\$
    – arc_lupus
    Feb 12, 2015 at 14:17
  • \$\begingroup\$ You can use a jumper which will work just fine for a single prototype and you remember to set the jumper correct every time. However if you go toward building some product you will want a solution that is less prone to an operator leaving the jumper in the wrong state. \$\endgroup\$ Feb 12, 2015 at 14:21
  • \$\begingroup\$ I just need one version for our lab (which is as I presume programmed only once), so for the moment I think that a jumper would do it then, but I keep your idea in mind. \$\endgroup\$
    – arc_lupus
    Feb 12, 2015 at 14:22
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The reset input is very likely to be high input impedance for normal use so you could, on the face of it, place another 10k resistor in series with it. This would not likely affect the RC reset circuit - it would connect to the pin via this "new" 10k resistor. When it comes to programming the device, connect directly to the reset pin i.e. not via the 10k resistor.

You will need to check the data sheet for normal reset operation with the added resistor. Check to see what the range of R1 (your circuit) can be - I suspect that it can be higher than 20k so this idea should work and no links or components to remove when programming.

As far as programming this will be fine because the data sheet says the impedance should be 10k or greater.

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