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The two AVRs I'm using right now (ATmega1284P and ATtiny4313) are both powered by 4.96V. I'm measuring a voltage on their /RESET pins of 6.33 volts. Is that normal? Where is it coming from?

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The /RESET pins are inputs, hence measuring the voltage at them is meaningless.

Also, like any high impedance input pin, the RESET pin will float to an arbitrary value when not provided with a pull-up or pull-down resistor.

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  • \$\begingroup\$ I thought they might be internally pulled-up by default (when used as reset pins, at least), so I expected them to be no more than Vcc. So if they're not, how high can they float to? What sets the limit? \$\endgroup\$ – Isvara Jul 21 '13 at 6:14
  • \$\begingroup\$ @Derecho There is no limit up to the point the protection diodes on the input pin kick in. That voltage is quite possibly injected due to some vagary of the multimeter used, check if that is powered by a 9 Volt battery. \$\endgroup\$ – Anindo Ghosh Jul 21 '13 at 6:39
  • \$\begingroup\$ The reset pin on an AVR is actually different from all other input pins. It has its own entry in the datasheet and allows for a voltage up to 13V (to enter high voltage programming mode). The higher voltage is not generated by the AVR though. \$\endgroup\$ – jippie Jul 21 '13 at 8:15
  • \$\begingroup\$ @jippie So there wouldn't be clamp diodes at all on that pin, I assume. \$\endgroup\$ – Anindo Ghosh Jul 21 '13 at 8:16
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    \$\begingroup\$ It is a good practice to have a pull up resistor to Vcc on \$\overline{\text{RESET}}\$, say 10kΩ. This will prevent spurious resets and may solve the measured high voltage if it is picking it up from the environment. \$\endgroup\$ – jippie Jul 21 '13 at 8:26
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I can think of two reasons:

  1. AC voltage between measuring person body and psu gnd may get rectified by input protection diodes (or non-linear input impedance). Touch the gnd with your finger while measuring to see if the 6V drops to 5V.

  2. (very unlikely) There might be an internal Vpp generator, for flash programming. It can have a little leak to the high-impedance reset/vpp pin.

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Pretty much and old post but still here are my findings:-

I did try to measure the voltage between RESET and GND which came out to be around 4.2V on AVR running on 5V rail.

Also someone said reset is an input, well I am not sure, at least not in atmega328p. or infact even in the AVR architecture. most people do get confused because of the reset switch commonly found or connected as a best practice which would send an interrupt to the reset handler, basically its nothing but grounding the reset pin.

Usually at run state the reset is high, and at programming or flashing state it is momentarily set to ground or a low state. This principle is leveraged for instance it is applied in designing a board where you can program while the circuit is already active with different peripherals connected to miso mosi pins and modules which are continuously running. (incase you are using spi programmer)

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