Maybe this belongs in the microcontroller forum instead.
I am looking at a circuit board that uses an ATMega6224PA. All the pins available are used as digital output and PWM where supported. I notice the AVCC and AREF pins, they have it going to a capacitor and then to ground.
The 300+ page datasheet for the 6224PA says if it is not used, the AVCC pin should be routed to VCC. No where can I see that is says it is OK to send to ground. I guess this is OK because the board works...
I guess my questions are -
Does anyone else do this or have you seen this?
Any issues? I am assuming no since they did it and is a mass-produced board.
What value would the cap be? I am getting error trying to read it in-circuit.
A cap from AVcc to ground does not ground the pin as a cap will not conduct DC. It is there as a filter cap to bypass higher frequency noise to ground so that it will not get into AVcc from the power supply. The usual value is 0.1uF.
The same applies for the bypass cap from Vcc to ground and Vref to ground.
The bypass (or decoupling) caps from Vcc to ground and AVcc to ground are actually required. The cap from Vref to ground is optional, but recommended.
Still not clear why they aren't running AVCC to VCC like the datasheet says. But, maybe I read something they didn't. ?? If recreating the circuit board, should that be fixed?
Or does it really not matter since the Analog is not being used?
Here is a close up.. The 2 middle traces.. C15 is ARef and C14 is AVCC, via's are ground.
I would tie AVCC to VCC as well, thanks for the confirmation. Wait a sec. I bet they bridged the AVCC connection with the VCC connection under the chip.
Let me just check continuity real quick... AH Yes! Duh!!! There is continuity between the AVCC and VCC pins.
OK, what I am seeing is the filtering piece.
