ATMega32u4 board layout questions...5v USB and 3.3v IO pins.

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I'm laying out a board with a AtMega32u2 chip.

 

I wish to run the USB portion of the chip off of the 5V USB VBUS.
At the same time, I wish to run the rest of the IO pins at 3.3 volts.
I plan to do this by taking the 5V USB VBUS power and running it through a 5.0V-3.3V LDO regulator, and using the 3.3Volts to connect to VCC and AVCC.

 

Here are some pictures out of the ATMega32u2 Datasheet.

 

This picture shows the USB portion of the chip. The UVCC Pin takes 5v coming in on the USB VBUS line and sends it to an internal regulator, changing it to 3.3V to power the D- and D+ pins.

 

 

 

 

This picture shows a wiring diagram to attain 3.3 volts on all the IO pins.

The picture above assumes UCAP, VCC, and AVCC are attached to a 3.3 volt line.
Not shown is the line I will run from the 5volt VBUS...into a 5v to 3.3v LDO...then running the 3.3 volts into VCC and AVCC.

 

What troubles me in the diagram is the 3.3v going into UCAP.
I don't want to do that.
I want to run the USB portion of the chip using the 5-volts coming into UVCC which then goes to the internal regulator.

 

That means erasing the line in this picture showing a 3.3v connection into the UCAP pin.
Can I do that?
Will everything still work?

 

Also, I understand VCC and AVCC should have some bypass capacitors in close vicinity to the pins.
I've seen values of 1.0uF and 0.1uF suggested.
Which value should I use?

 

Should I also put a bypass capacitor on UVCC.
If so, what value?

 

I will have a 10uF capacitor on the 5volt VBUS line very close to where it enters the board off the Mini USB receptacle.

 

 

Last Edited: Mon. Jul 27, 2015 - 04:07 PM
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Also, I understand VCC and AVCC should have some bypass capacitors in close vicinity to the pins.
I've seen values of 1.0uF and 0.1uF suggested.
Which value should I use?

I'll let others answer the split voltage USB-Micro question.

 

When the digital circuits inside the uC change state, (think lots of gates switching at millions of times / second), lots of current is drawn.

That means there are high current demand intervals taking place, frequently.

The By-Pass caps, mounted very close to the uC's physical pins, with very short, wide traces, can act like a power source for the uC's current demands.

Drawing the current through long, relatively narrow, PCB traces from the power supply doesn't work so well.

The traces have inductance, and the sudden current in-rush needed for the chip can't be instantly provided.

One might also view them as a bit of a filter for the noise exiting the uC, onto the PCB traces.

 

The different size (and type) of caps all have different frequency responses and abilities to source the current spikes.

A 0.1 uF cap is generally a good general purpose By-pass cap.

You might see on some data sheets for highly sensitive (analog) circuitry, (16-Bit ADC's), or some analog chips, (op-amps, VCO's, etc), a recommended By-Pass configuration, such as three caps, 0.1 uF, 1 uF, and another at either end of the series, (10 uF, or 0.01 uF).

Each one is there to squelch the frequencies it can best cover, and work together to By-Pass the pin.

 

As a general rule, every Vcc (and AVcc) pin on the uC should have its own By-Pass cap.

This is a PITA on some of the 100 pin Xmegas which can have 8 Vcc pins...(IIRC).

 

You also, ideally, want them as close to the chip's physical pins as possible, using as wide a PCB trace as possible.

That minimizes the inductance of the trace between he cap and the chip.

Of course, in the real world, sometimes compromises are made.

Certainly the By-Pass cap should be the physically closest device or bus trace to the chip.

i.e. Don't run a bus line to the chip, and then tack on the By-Pass cap at a distance.

 

West Coast Jim had a nice discussion on By-Pass caps in another Thread, and the topic comes up and and then.

Unfortunately I can't find the Thread I'm thinking of.

 

Atmel App Note AVR042 discusses the HW layout of the uC's, and touches on this topic, I think, (but not in detail).

 

AVcc is the Analog Vcc pin, and typically powers Port A and the Analog circuitry inside the uC, (ADC, DAC, AC, etc.).

It is usually spec'd to be equal to Vcc +/- a small range.

For improved analog performance one would often feed the AVcc pin with an LC filter, (Not an RC filter, in other to avoid the "extra" voltage drop across the resistor, small though it may be).

For many applications, just tying AVcc to Vcc, and adding a 0.1 uF By-Pass cap is all that is necessary.

 

Atmel has a number of demo Xplain PCBs for various uC's.

You could look at a few of their schematics to see how Atmel elected to By-Pass their uC's, and the specific type of caps they used.

 

Good luck with your project!

 

JC

 

 

 

 

 

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Thanks JC.

 

Go Cavs!!!