XMEGA ADC Selecting Channels

syndetic1 · Joined: 02 Jan 2007 Posts: 76

Hi guys,

I am having trouble understanding the MUX selection on the XMEGA.

Here is what I know. At the end is a question on how to measure the internal ground voltage.
Chip is ATXMEGA128A3U-AU

I have the ADCs configured for single ended operation.
1
Config Adca = Single , Convmode = Unsigned , Resolution = 12bit , Dma = Off , Reference = Int1v , Event_mode = None , Prescaler = 32 , Ch0_gain = 1 , Ch0_inp = Single_ended , Mux0 = &B0000000, Ch1_gain = 1 , Ch1_inp = INTERNAL , Mux1 = &B1_000 , Ch2_gain = 1 , Ch2_inp = Single_ended , Mux2 = &B10000000 , Ch3_gain = 1 , Ch3_inp = Single_ended , Mux3 = &B1100000

Config AdcB = Single , Convmode = Unsigned , Resolution = 12bit , Dma = Off , Reference = Int1v , Event_mode = None , Prescaler = 32 , Ch0_gain = 1 , Ch0_inp = Single_ended , Mux0 = &B0000000, Ch1_gain = 1 , Ch1_inp = INTERNAL , Mux1 = &B1_000 , Ch2_gain = 1 , Ch2_inp = Single_ended , Mux2 = &B10000000 , Ch3_gain = 1 , Ch3_inp = Single_ended , Mux3 = &B1100000

2 From
http://ww1.microchip.com/downloads/en/Appnotes/00002535A.pdf
"In devices with two ADCs, the inputs can only be connected to the corresponding port. Meaning that ADCA can be connected only to PORT A, and ADC B can be connected only to PORT B."
The XMEGA I am using has 2 ADCs

3 The GetADC command in BASCOM looks like this:
var = GETADC( ADC , channel ,MUX)
Where ADC is A or B
Channel is a virtual channel number 0, 1, 2, 3 - I am using channel 0.
MUX is as follows

4 MUX
MUX is an 8 bit register.
Bit 7 is always 0
Bits 6543 - 4 bits select the source to the positive side of the ADC
Bits 210 - 3 bits select the negative side. 000 selects Ground

So to read PortA.0 the BASCOM command is:
MyVar = GETADC(ADCA, 0 , &b00000000)
PortA.1 is GETADC(ADCA, 0 , &b00001000)
and to read PortB
PortB0 MyVar = GETADC(ADCB, 0 , &b01000000) ie channel 8
PortB1 MyVar = GETADC(ADCB, 0 , &b01001000) ie channel 9

5 The data sheet tells me that there are various internal voltage sources including ground.
QUESTION
How do I read the internal ground voltage?
What is the config statement I need and what is the info to put in the GETADC command

Thanks in advance

(BASCOM-AVR version : 2.0.8.4 )

MWS · Joined: 22 Aug 2009 Posts: 2262

JC · Joined: 15 Dec 2007 Posts: 586 Location: Cleveland, OH

Actually, I think it is very reasonable to read the Internal Ground Voltage.
This is the Ground against which the other inputs are measured, in single ended input mode.

The issues is that depending upon the PCB, there may be significant currents flowing through the ground trace / plane, causing it's voltage to be different from the voltage source (-), and different from the ground level inside the chip.

Remember that if you have the data sheet in front of you, you can always re-configure the ADC by setting the registers yourself.

JC

MWS · Joined: 22 Aug 2009 Posts: 2262

syndetic1 · Joined: 02 Jan 2007 Posts: 76

This bloke has a very interesting blog
https://blog.frankvh.com/2010/01/03/atmel-xmega-adc-problems-solutions/

MWS · Joined: 22 Aug 2009 Posts: 2262