Caps and Inductors next to power supply pins of AVR

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I'm reading the best practices and harsh environment application notes with the AVR and it's clear I need to add an inductor and capacitor right next to the supply pins of the AVR.

Trouble is, I'm using the mega644 which has three Vcc pins as well as an Aref and AVcc pin. I'm not doing any analog to digital stuff. I'm told the AVcc pin should be connected to 5V even if you're not using it and the Aref doesn't need to be connected to anything.

That makes four pins of incoming 5V. Should I have the inductor/capacitor combo for each of the four? What about the three common pins? Should I throw anything extra on them?

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The series inductor is for avcc only. Using this for the digital pins would not gain you much. This forms a lc filter to knock off some of the digital noise for the adc. Nevertheless, you should place the bypass caps as close as possible to the pins on the avr. Having a ground plane is beneficial. For a robust design, layout is critical. Understand where currents are flowing or want to flow - we know they flow in a loop. External wires are antennas and secondary windings in a transformer. Seek to minimise these effects.

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Not sure where digital pins came into this. I'm talking about the three Vcc pins and the Avcc on. Should each of those have a cap and inductor combo? What about the common pins?

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That's what i'm talking about! You would seek to minimise inductance on the common pins.

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I'm sorry kartman. I'm just not understanding you. Should I put a cap/inductor set on each of the four 5v pins or share one cap/inductor set across all four?

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A cap for each 5V/0V pair. The inductor is not required if you don't use the adc.

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Thanks Kartman. Appreciate it.

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AVR042 shows a ferrite bead in the digital power supply pins. I like to isolate chips that cause noise to power supply pins, and isolate chips that are sensitive to noise coming from power supply pins.

Basically, all AVR power should go through one ferrite bead filter to whatever amount of VCC pins, and each VCC pin should have 100nF capacitor near it. And as you don't use ADC, just connect the AVCC to same node as VCC, and also 100nF on AVCC.

I recall some threads here were discussing what is best way to filter AVCC if you do use ADC. Some say ferrite, some say resistor, some say inductor. Inductor appeared to cause some peaking. Resistor might not be so effective and causes more resistive losses. Ferrite bead might not filter enough. Anyway, in case of RC filter and obscene amount of capacitance on AVCC pin, at powerup and powerdown the voltage difference between AVCC and VCC nodes may be too large and smoke comes out.

Anyway, if you put an inductor on the AVR supply, make sure it does not make things worse. Put enough bulk capacitance on AVR side. This again might make power supply rise slower than before, and therefore the AVR might not boot properly, if it comes out of reset and starts running code, before voltage is high enough to actually run it at that high speed. Sometimes there is a damping resistor over the inductor to reduce the LC filter Q factor so that it does not peak. Also make sure the resonant frequency of the LC filter is much much lower than the frequency the AVR is running at.