A/D converter lines

This article explains how to use the 4-channel 10 bit A/D converters built into the Atmel CPU used on the Acme Boards

The A/D converter lines are available as listed below:

Signal	Description	ARIAG25	TERRA	DAISY-1	FOXG20
3.3V	3.3 volt power line	W24	D14.1	D4.1	J6.1
AVDD	Clean 3.24V out for A/D circuitry		D14.2	D4.2	J6.33
VREF	A/D voltage reference input	W19	D14.3	D4.3	J6.34
AGND	Analog GND		D14.4	D4.4	J6.35
AD0	Analog input 0	W20	D14.5	D4.5	J6.30
AD1	Analog input 1	W21	D14.6	D4.6	J6.29
AD2	Analog input 2	W22	D14.7	D4.7	J6.28
AD3	Analog input 3	W23	D14.8	D4.8	J6.27
GND	Digital GND	W1	D14.10	D4.10	J6.40

A Daisy module called DAISY-20 is available to use these lines in the range of 0 to 10 volt elsewhere use this schematic to test quickly the A/D lines:

By wiring VREF to AVDD we can read a max voltage of 3.24 volt with a resolution of 10 bit (1024 sample points). In this way the max sample resolution we obtain is about 3.24V/1024 ~= 3mV.

On VREF cannot be applied voltage higher than 3.3 Volt. If you need to read a higher signal use a schematic like this:

This is an example to read signal in a range of 0 to 10 volt.

Install the adc driver

A ready to use binary version of the A/D converter Linux driver is available on this site. You can download it directly on your Acme board by typing:

FOXG20 or NETUSG20 with Kernel 2.6.38

debarm:~# wget http://www.acmesystems.it/www/adc/g20/at91-adc.ko

TERRA or ARIAG25 with Kernel 2.6.39

debarm:~# wget http://www.acmesystems.it/www/adc/g25/at91-adc.ko

Load the driver module inside the kernel by typing:

debarm:~# insmod at91-adc.ko
Major: 250; minor: 0                                                            
Registered device at91_adc.

now move in /sys/bus/platform/devices/at91_adc/ and read the sample from A/D channel 0 by typing:

debarm:~# cd /sys/bus/platform/devices/at91-adc/
debarm:~# cat chan0
512

Turn the trimmer and read the value again.

Python example

This simple Python code continuously reads the samples from all the four A/D channel and shown the voltage read.

read.py

debarm:~/playground/python/adc# python read.py
Channel 0 = 2.32 volt
Channel 1 = 1.69 volt
Channel 2 = 1.64 volt
Channel 3 = 1.57 volt
-------

C example

This is the same example written in C instead of Python:

read.c

debarm:~/playground/c/adc# gcc read.c -o read 
debarm:~/playground/c/adc# ./read
Channel 0 = 2.31
Channel 1 = 1.70
Channel 2 = 1.64
Channel 3 = 1.57
-------

The examples source on this article are available on the Playground repository. Click here to learn how to install them on your FOX Board G20

Compile the ADC driver from the sources:

The at91-adc driver source is available on this GitHub repository: https://github.com/tanzilli/at91-adc

To compile the driver from sources you need install the Linux Kernel tree on your Linux PC and the toolchain requested to compile it for ARM architecture.

Compile it and copy the module of your Acme board by typing:

$ git clone git://github.com/tanzilli/at91-adc.git
$ cd at91_adc
$ make
$ scp at91_adc.ko root@[ip address of your acmeboard]:/root

Credits

Many thanks to Claudio Mignanti, Stefano Barbato, Antonio Galea, Mark Richards, Douglas Gilbert for their contributes on this article.

DAISY-20 : Four channel A/D converter EURO 22.00

Front-end for the 4 channel A/D 10bits converter built-in the Atmel CPU.

Note: the daisy flat cable to wire this board is not included. Don't forget to buy it (code FLAT-20CM)

| Product description | Daisy catalog |

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