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 Posted: Aug 06, 2012 - 05:18 PM |
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Joined: Jan 10, 2012
Posts: 8
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I'm using the STK600 to run a circuit I've built which requires a somewhat heavy power load. When powering off the USB, I'd occasionally get a flashing red light, indicating that the power supply to the board was too low, so I decided to switch over to a separate DC power supply to power the STK600.
I've noticed strange behavior on the ADC conversions, i.e. readings jumping all over the place between 0 and 255, except when I have the ADC pin grounded or connected directly to 5V (VTG). In my circuit the ADC pin is normally connected to a voltage divider which is dividing VTG.
However, the ADC works as consistently as expected when I power off the USB connected to my computer.
Has anyone seen this behavior before? Any thoughts on why/potential fixes? |
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Posted: Aug 07, 2012 - 06:16 PM |
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Joined: Jan 10, 2012
Posts: 8
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I think this has something to do with a missing decoupling capacitor for the AVRISP-mkii which I am using for programming my chip-Mega164.
The datasheet for the mkii says
Quote:
Any de-coupling capacitor should not be larger than 10uF
which I suppose means that the mkii must have a decoupling capacitor?
I'm no EE, so this isn't totally obvious to me. I've been using the mkii without a decoupling capacitor for months now and this is the first time I've had any issues with it.
Edit-I discovered the issue with the mkii in my circuit when I programmed the chip, then pulled the mkii from the circuit to let it run overnight and the chip reset itself. |
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Posted: Aug 07, 2012 - 06:57 PM |
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Joined: Jan 07, 2012
Posts: 1201
Location: North of France
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Well USB powered cannot have more than 10 uF between +-5v (USB limits current : if a capacitor was too high, leading to current surges at the plugging in, I suppose the peripheral could not be enumerated, as the PC would "think" it is short circuited).
But typical decoupling condensators are 0.1uF (100nF)unless you have low frequency current spikes (then, higher decoupling condensators can be paralleled) |
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