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I have an Avocent SwitchViewDVI (4SVDVI10) KVM that looks to be run by an ATmega64 microcontroller. Every now and then the KVM locks up and I need to reset the unit by pulling all USB power sources to it, a considerable hassle to access.

I know that the ATmega64's pin 20 is a reset and according to the datasheet "a low level on this pin for longer than the minimum pulse length will generate a reset". I suspect this can be used to provide a more convenient reset than pulling all power. I know a bit about electronics and am good with an iron, but microcontrollers are new territory for me. I'm a geek, but without proper EE cred.

There is a 21k Ohm resistor between VCC and the reset pin, I assume "pulling reset high" is possibly for power-on reset. My theory is that if I "pull low" (ground) the reset pin (easiest attach point being the reset side of the resistor) via a momentary switch I can generate that reset signal.

  • Is this correct?
  • Do I need to open the circuit between reset and VCC?
  • Should I include a resistor between the reset pin and ground to be a current limiter?

My suspicion is I need to keep VCC attached via the 21k Ohm resistor and use a resistor between reset and ground to form a divider, as the datasheet specifies a reset pin threshold voltage range of 0.2VCC-0.85VCC. Since VCC via USB should be somewhere around the 5V USB spec (I measured 4.1V in testing) I'm thinking a 3k Ohm resistor between reset and ground should get me there.

ATmega64 datasheet

I'm sure this is very EE 101 and I welcome responses/critiques of my described problem.

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1 Answer 1

Reset to default
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My theory is that if I "pull low" (ground) the reset pin (easiest attach point being the reset side of the resistor) via a momentary switch I can generate that reset signal. Is this correct?

Yes. No need to do extra stuff. So, ...

Do I need to open the circuit between reset and VCC?

... no, and...

Should I include a resistor between the reset pin and ground to be a current limiter?

... no.

But resetting the MCU may not be a 100% solution as the main cause of lockup could be something else (e.g. parasitics, fluctuations/spikes/unwanted noise along the supply line, heat, etc.). So maybe you should find the cause of lockup.

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  • \$\begingroup\$ Thanks! I really have no idea what the cause could be, it is just something that seems to happen after the device sits for a period. This is powered by USB from multiple computers all supported by a UPS so I would hope input would be stable. Even happens if I use a direct power supply. (has an optional 9V input) The manufacturer long since discontinued this KVM and it is already running the latest "bug free" firmware they released in 2009. \$\endgroup\$
    – cpow
    Commented Nov 20, 2020 at 3:47
  • \$\begingroup\$ Salvaged a NO momentary switch from the recycle pile and wired it in. With just power applied press of the momentary switch will trigger the boot beep of the KVM as well as switch it back to device 1 if a different device was selected. The PC didn't detect a disconnect of the USB hub so this is also promising that since the rest of the device remains powered I may also avoid resolution mismatches that would sometimes occur after power removal for reset. Will try to remember to follow up after the first lock-up occurs and I try it in a live scenario. \$\endgroup\$
    – cpow
    Commented Nov 20, 2020 at 4:45

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