Atmega16U Atmega8U UGND and GND

[Charles Creations]

Hi,

I am curious if anyone knows the purpose of the jumper on the Arduino Uno between UGND (USB ground) and GND (main board ground). It looks like they are internally shorted within the Atmega8U/16U when I test them for continuity and the jumper is not shorted together. In the datasheet for the Atmega8U/16U, it shows UGND and GND leads shorted together, so why would Arduino place a jumper there?

Here is schematic, the jumper is in the bottom left quadrant:
http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf

Atmega Datasheet:
http://www.atmel.com/Images/doc7799.pdf

Thanks,
Charlie

[dannyf]

I wouldn't hold arduino as the gold standard of circuit designs.

[8086]

Presumably because USB ground tends to be at mains earth potential, as it is chassis earth connected usually at the PC end, which may or may not be ideal for your application.

[Jeroen3]

Presumably because USB ground tends to be at mains earth potential, as it is chassis earth connected usually at the PC end, which may or may not be ideal for your application.

Shield is supposed to be mains earth.

UGND is probably a dedicated bond wire to the USB transceiver driver to make sure there is a low impedance ground path to the correct part of GND.
Some parts also include an AGND for the analog circuitry on chip.

How many ohms do you measure between UGND and GND when you have the VCC enabled? Might want to keep the chip in reset.
Enabling VCC is to prevent you from measuring through the esd diodes. If you have low voltage ohms, that might work too.

[Charles Creations]

I measured 6MOhm between GND and UGND with a low voltage ohms.

This chip also has a AGND as well and reading through the data sheet it says to connect the AGND at a single point to GND to decrease effects of ground noise. Since UGND is digital, it shouldn't be nearly as sensitive.

Could I clarification on what you meant by with VCC enabled? Did you mean with power on?:

How many ohms do you measure between UGND and GND when you have the VCC enabled? Might want to keep the chip in reset.
Enabling VCC is to prevent you from measuring through the esd diodes.

[Jeroen3]

Each pad contains ESD diodes to ground and Vcc. If you have the chip powered off, you'll need <1V to enable these diodes to Vcc and you'll be phantom powering your circuit.
If you have the circuit turned on you'd need Vcc+ <1 Volts to do so. If you have two multimeters, measure the voltage of the other one in Ohms mode.

If you measure 6 Mega Ohms (both ways?) between GND and UGND it is a "isolated" ground for the USB driver stage.
USB consumes a huge amount of silicon area because of the line capacitance it is required to drive.
Atmel seems to have chosen for a separate ground for this area reducing in-chip currents. I would guess other manufactures do this too, but do not label the pin differently.

That jumper is a mystery. It is probably there to be able to measure current in the development stage.

You can always ask Atmel for more details. They cannot put everything in their datasheets.