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 channels4 wire TS, with 4 general purpose ADC channels5 wire TS, with 3 general purpose ADC channels8 wire TSADC 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.
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 driverADC 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
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.
CAUTION Make sure that the voltage supplied does not cross 1.8vOn 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:~#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.
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 4095This feature is exposed by IIO through the following files:
in_voltageX_raw: raw value of the channel X of the ADCNOTE Check ADC mode. root@arago-armv7:~# cat /sys/bus/iio/devices/iio\:device0/mode oneshot
To read a single ADC value, ADC has to be configured in one-shot mode. If not in one-shot mode, This can be set by:
root@arago-armv7:~# echo oneshot > /sys/bus/iio/devices/iio\:device0/modeImportant folders in the iio:deviceX directory are:
Buffer bytes_per_datum:enabled: get and set the state of the bufferlength: 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 chunksin_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>>0Where:
le represents the endianness, here little endianu 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 information32 is the actual number of bits used to store the datum0 is the number of right shifts needed.
To read ADC data continuously we need to enable buffer and channels to be used.
NOTE Check ADC mode. root@arago-armv7:~# cat /sys/bus/iio/devices/iio\:device0/mode oneshotTo read data continuously, ADC has to be configured in continuous mode. This can be done by:
root@arago-armv7:~# echo continuous > /sys/bus/iio/devices/iio\:device0/modeSet 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_enSet up the buffer length
root@arago-armv7:~# echo 100 > /sys/bus/iio/devices/iio\:device0/buffer/lengthEnable the capture
root@arago-armv7:~# echo 1 > /sys/bus/iio/devices/iio\:device0/buffer/enableNow, 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/enableThis 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.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 voltageEx: Read value on channel AIN4 for input voltage supplied 1.01:
Formula:
Vin = 1.01 * (2^12 -1 )/ 1.8 Vin = 2297.75Value read from sysfs:
root@arago-armv7:~# cat /sys/bus/iio/devices/iio\:device0/in_voltage4_raw 2298On 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
On BeagleBone platform, P9 expansion header can be used. For further information on expansion header layout please refer to the Beaglebone schematics here
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 patchMedia: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_bufferor
<path_to_cross-compiler/cross-compiler-prefix->-gcc --static generic_buffer.c -o generic_bufferThen 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_enCheck 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/modeFinally, 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 3The output of this application is directly printed on console.
转换公式
AM335x 使用的是 1.8V 的参考电压，ADC值与待测电压的转换关系： 待测电压 = (ADC值 * 1.8 / 4095) V
