I manufactured PCB's where I used the Atmega328 AVCC pin to draw power for an LCD. It works however the voltage dropped to around 4.2v so I've connected the AVCC line to VCC meaning the LCD is now powered at 5v.
What is the AVCC pin and what's it used for?
What effect does it have to connect AVCC straight to VCC?
What effect does it have to connect AVCC straight to VCC?
You "can" connect the Analog Vcc to the Digital Vcc but good design should use a low-pass filter consisting of filter caps and a inductor- see the Uno schematic.
Good review of power issues: De-coupling
Ray
AVCC is used for the analog port and A to D converter.
"It should be connected to VCC along with a capacitor .1uF to ground ( even if not used )"
Look at the data sheet.
mrburnette:
You "can" connect the Analog Vcc to the Digital Vcc but good design should use a low-pass filter consisting of filter caps and a inductor- see the Uno schematic.
Umm, UNO doesn't have an LC-filter. It ties AVCC straight to VCC. Also, ARef should have a 100nF bypass cap - this is on the schematic.
http://arduino.cc/en/Main/ArduinoBoardUno
On my own boards, I use an RC-lowpass filter, 10 ohms and 100nF, to tie VCC to AVCC.
How about "AVCC powers part of the chip and must be connected to a Vcc-like rail. Some of those parts include the analog circuitry of the A-D converter, and A-D performance can be improved by using additional filtering on AVcc to keep digital noise from Vcc from getting into the chip through this power connection."
oric_dan:
Umm, UNO doesn't have an LC-filter. It ties AVCC straight to VCC. Also, ARef should have a 100nF bypass cap - this is on the schematic.http://arduino.cc/en/Main/ArduinoBoardUno
http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdfOn my own boards, I use an RC-lowpass filter, 10 ohms and 100nF, to tie VCC to AVCC.
Yes... My clone UNO's do have the filtering but the UNO does not. I stand corrected in pointing to the UNO schematic 
Reference: Atmel AVR126: Warning: PDF target!
Filtering should be used if the analog power supply (AVCC) is connected to digital
power supply. That is, the AVCC pin on the device should be connected to the
digital VCC voltage via an LC network. For more information on this LC network
connection and the L, C values (10μH, 100nF) please refer the respective mega
device datasheet. Please note that the AVCC and VCC are internally connected by
high impedance networks
No one ever said the UNO was well-designed. I would say, just adequate, IMO.
Ray