VAR-SOM-AM33 ADC
Introduction
An analog-to-digital converter (abbreviated ADC) is a device that uses sampling to convert a continuous quantity to a discrete time representation in digital form.
The TSC_ADC_SS (Touchscreen_ADC_subsystem) is an 8 channel general purpose ADC, with optional support for interleaving Touch Screen conversions. The TSC_ADC_SS can be used and configured in one of the following application options:
- 8 general purpose ADC channels
- 4 wire TS, with 4 general purpose ADC channels
- 5 wire TS, with 3 general purpose ADC channels
- 8 wire TS
ADC used is 12 bit SAR ADC with a sample rate of 200 KSPS (Kilo Samples Per Second). The ADC samples the analog signal when "start of conversion" signal is high and continues sampling 1 clock cycle after the falling edge. It captures the signal at the end of sampling period and starts conversion. It uses 12 clock cycles to digitize the sampled input; then an "end of conversion" signal is enabled high indicating that the digital data ADCOUT<11:0> is ready for SW to consume. A new conversion cycle can be initiated after the previous data is read. Please note that the ADC output is positive binary weighted data.
Driver Configuration
You can enable ADC driver in the kernel as follows.
Device Drivers --->
[*] Staging drivers --->
[*] Industrial I/O support --->
[*] Enable buffer support within IIO
<*> Industrial I/O lock free software ring
< > Industrial I/O buffering based on kfifo
-*- Enable triggered sampling support
(2) Maximum number of consumers per trigger
Analog to digital converters --->
<*> TI's ADC driver
Building as Loadable Kernel Module
- In-case if you want to build the driver as module, use <M> instead of <*> during menuconfig while selecting the drivers (as shown below). For more information on loadable modules refer Loadable Module HOWTO
Device Drivers --->
[*] Staging drivers --->
[*] Industrial I/O support --->
[*] Enable buffer support within IIO
<*> Industrial I/O lock free software ring
< > Industrial I/O buffering based on kfifo
-*- Enable triggered sampling support
(2) Maximum number of consumers per trigger
Analog to digital converters --->
<M> TI's ADC driver
- This step applies if the driver is built as module
- Do "make modules" to build the ADC driver as module. The module should be present in "drivers/staging/iio/adc/ti_adc.ko".
- Load the driver using "ti_adc.ko".
Platform data
ADC platform data is added in board file(arch/arm/mach-omap2/board-am335xevm.c) as shown below.
#include <linux/platform_data/ti_adc.h>
static struct adc_data am335x_adc_data = {
.adc_channels = 4,
};
static struct mfd_tscadc_board tscadc = {
.tsc_init = &am335x_touchscreen_data,
.adc_init = &am335x_adc_data,
};
The parameter "adc_channels" needs to hold data related to how many channels you want to use for ADC.
- If ADC and touchscreen are used together, add platform data as shown above.
- If ADC alone is being used, you will need to remove platform data for touch screen.
Example below. Notice adc_channels is increased to 8 in the adc initialization.
static struct adc_data am335x_adc_data = {
.adc_channels = 8,
};
/*
static struct tsc_data am335x_touchscreen_data = {
.wires = 4,
.x_plate_resistance = 200,
.steps_to_configure = 5,
}; */
static struct mfd_tscadc_board tscadc = {
/* .tsc_init = &am335x_touchscreen_data, */
.adc_init = &am335x_adc_data,
};
You can find the source code for ADC here
Usage
To test ADC, Connect a DC voltage supply to each of the AIN0 through AIN7 pins (based on your channel configuration), and vary voltage between 0 and 1.8v reference voltage.
On loading the module you would see the IIO device created
root@arago-armv7:~# ls -al /sys/bus/iio/devices/iio\:device0/ drwxr-xr-x 5 root root 0 Jan 1 00:00 . drwxr-xr-x 4 root root 0 Jan 1 00:00 .. drwxr-xr-x 2 root root 0 Jan 1 00:00 buffer -r--r--r-- 1 root root 4096 Jan 1 00:00 dev -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage0_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage1_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage2_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage3_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage4_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage5_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage6_raw -r--r--r-- 1 root root 4096 Jan 1 00:00 in_voltage7_raw -rw-r--r-- 1 root root 4096 Jan 1 00:00 mode -r--r--r-- 1 root root 4096 Jan 1 00:00 name drwxr-xr-x 2 root root 0 Jan 1 00:00 power drwxr-xr-x 2 root root 0 Jan 1 00:00 scan_elements lrwxrwxrwx 1 root root 0 Jan 1 00:00 subsystem -> ../../../../../../bus/iio -rw-r--r-- 1 root root 4096 Jan 1 00:00 uevent root@arago-armv7:~#
Modes of operation
When the ADC sequencer finishes cycling through all the enabled channels, the user can decide if the sequencer should stop (one-shot mode), or loop back and schedule again (continuous mode). If one-shot mode is enabled, then the sequencer will only be scheduled one time (the sequencer HW will automatically disable the StepEnable bit after it is scheduled which will guarantee only one sample is taken per channel). When the user wants to continuously take samples, continuous mode needs to be enabled. One cannot read ADC data from one channel operating in One-shot mode and and other in continuous mode at the same time.
One-shot Mode
To read a single ADC output from a particular channel this interface can be used.
root@arago-armv7:~# cat /sys/bus/iio/devices/iio\:device0/in_voltage0_raw 4095
This feature is exposed by IIO through the following files:
- in_voltageX_raw: raw value of the channel X of the ADC
Continuous Mode
Important folders in the iio:deviceX directory are:
- Buffer
- bytes_per_datum:
- enabled: get and set the state of the buffer
- length: get and set the length of the buffer.
The buffer directory contains 3 files:
root@arago-armv7:~# ls -al /sys/bus/iio/devices/iio\:device0/buffer/ drwxr-xr-x 2 root root 0 Jan 1 00:00 . drwxr-xr-x 5 root root 0 Jan 1 00:00 .. -rw-r--r-- 1 root root 4096 Jan 1 00:01 bytes_per_datum -rw-r--r-- 1 root root 4096 Jan 1 00:01 enable -rw-r--r-- 1 root root 4096 Jan 1 00:01 length
- Scan_elements directory contains interfaces for elements that will be captured for a single sample set in the buffer.
root@arago-armv7:~# ls -al /sys/bus/iio/devices/iio\:device0/scan_elements/ drwxr-xr-x 2 root root 0 Jan 1 00:00 . drwxr-xr-x 5 root root 0 Jan 1 00:00 .. -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage0_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage0_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage0_type -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage1_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage1_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage1_type -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage2_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage2_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage2_type -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage3_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage3_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage3_type -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage4_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage4_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage4_type -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage5_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage5_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage5_type -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage6_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage6_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage6_type -rw-r--r-- 1 root root 4096 Jan 1 00:02 in_voltage7_en -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage7_index -r--r--r-- 1 root root 4096 Jan 1 00:02 in_voltage7_type root@arago-armv7:~#
Scan_elements exposes 3 files per channel:
- in_voltageX_en: is this channel enabled?
- in_voltageX_index: index of this channel in the buffer's chunks
- in_voltageX_type : How the ADC stores its data. Reading this file should return you a string something like below:
root@arago-armv7:~# cat /sys/bus/iio/devices/iio\:device0/scan_elements/in_voltage1_type le:u12/32>>0
Where:
- le represents the endianness, here little endian
- u is the sign of the value returned. It could be either u (for unsigned) or s (for signed)
- 12 is the number of relevant bits of information
- 32 is the actual number of bits used to store the datum
- 0 is the number of right shifts needed.
How to set it up
To read ADC data continuously we need to enable buffer and channels to be used.
Set up the channels in use (you can enable any combination of the channels you want)
root@arago-armv7:~# echo 1 > /sys/bus/iio/devices/iio\:device0/scan_elements/in_voltage0_en root@arago-armv7:~# echo 1 > /sys/bus/iio/devices/iio\:device0/scan_elements/in_voltage5_en root@arago-armv7:~# echo 1 > /sys/bus/iio/devices/iio\:device0/scan_elements/in_voltage7_en
Set up the buffer length
root@arago-armv7:~# echo 100 > /sys/bus/iio/devices/iio\:device0/buffer/length
Enable the capture
root@arago-armv7:~# echo 1 > /sys/bus/iio/devices/iio\:device0/buffer/enable
Now, all the captures are exposed in the character device /dev/iio:device0
To stop the capture, just disable the buffer
root@arago-armv7:~# echo 0 > /sys/bus/iio/devices/iio\:device0/buffer/enable
ADC Driver Limitations
This driver is based on the IIO (Industrial I/O subsystem), however this is the first release of this driver and it has limited functionality:
- No HW trigger Support. Currently only supporting software trigger.
- Limited number of samples in continuous capture mode. (Only 1528 samples per capture)
- Limited maximum sample rate in continuous mode: 8K samples / second.
- Simultaneous capture on multiple ADC channels is not supported. Currently only supports continuous capture on a single ADC input channel at a time.
- "Out of Range" not supported by ADC driver.
Formula Used for Calculation
To cross verify the digital values read use,
D = Vin * (2^n - 1) / Vref
Where: D = Digital value Vin = Input voltage n = No of bits Vref = reference voltage
Ex: Read value on channel AIN4 for input voltage supplied 1.01:
Formula:
Vin = 1.01 * (2^12 -1 )/ 1.8 Vin = 2297.75
Value read from sysfs:
root@arago-armv7:~# cat /sys/bus/iio/devices/iio\:device0/in_voltage4_raw 2298
Board setup
To test ADC on AM335x EVM
On top of EVM, on LCD daughter board, J8 connector can be used, where ADC channel input AIN0-AN7 pins are brought out. For further information of J8 connector layout please refer to EVM schematics here
To test ADC on Beaglebone:
On BeagleBone platform, P9 expansion header can be used. For further information on expansion header layout please refer to the Beaglebone schematics here
Sample Application
The source code is located under kernel sources "drivers/staging/iio/Documentation/generic_buffer.c". Since our driver is not trigger based we need to modify this application to bypass the trigger detection. Please apply patch Media:Generic_buffer.patch on top of the application generic_buffer.c in order to bypass the trigger conditions.
How to compile:
arm-arago-linux-gnueabi-gcc --static generic_buffer.c -o generic_buffer
or
<path_to_cross-compiler/cross-compiler-prefix->-gcc --static generic_buffer.c -o generic_buffer
Then copy the generic_buffer program on your target board and follow below sequence -
Enable the channels:
root@arago-armv7:~# echo 1 > /sys/bus/iio/devices/iio\:device0/scan_elements/in_voltage4_en
Check the mode:
root@arago-armv7:~# cat /sys/bus/iio/devices/iio\:device0/mode oneshot
root@arago-armv7:~# echo continuous > /sys/bus/iio/devices/iio\:device0/mode
Finally, the generic_buffer application does all the "enable" and "disable" actions for you. You will only need to specify the IIO driver. Application takes two arguments, buffer length to use (256 in this example) the default value is 128 and the number of iterations you want to run (3 in this example).
root@arago-armv7:~# ./generic_buffer -n tiadc -l 256 -c 3
The output of this application is directly printed on console.