The Texas Instruments ADS1115 is another multi-channel ADC that is easy to work with on the Raspberry Pi. AdaFruit sells a breakout board for the ADS1115.
The RPi 4.9 kernels have the ads1015.ko kernel module enabled.
Here is the help for the module
config SENSORS_ADS1015
tristate "Texas Instruments ADS1015"
depends on I2C
help
If you say yes here you get support for Texas Instruments
ADS1015/ADS1115 12/16-bit 4-input ADC device.
This driver can also be built as a module. If so, the module
will be called ads1015.
The ADS1015 and ADS1115 are I2C devices.
A device tree overlay is necessary to enable and configure the driver for use.
There are individual overlays for the two ADC chips
- ads1015-overlay.dts
- ads1115-overlay.dts
The two overlays have a similar syntax and usage. I only have an ADS1115 board so I will use that as the example.
Here is the relevant section from the overlays README
Name: ads1115
Info: Texas Instruments ADS1115 ADC
Load: dtoverlay=ads1115,<param>[=<val>]
Params: addr I2C bus address of device. Set based on how the
addr pin is wired. (default=0x48 assumes addr
is pulled to GND)
cha_enable Enable virtual channel a.
cha_cfg Set the configuration for virtual channel a.
(default=4 configures this channel for the
voltage at A0 with respect to GND)
cha_datarate Set the datarate (samples/sec) for this channel.
(default=7 sets 860 sps)
cha_gain Set the gain of the Programmable Gain
Amplifier for this channel. (Default 1 sets the
full scale of the channel to 4.096 Volts)
Channel parameters can be set for each enabled channel.
A maximum of 4 channels can be enabled (letters a thru d).
For more information refer to the device datasheet at:
http://www.ti.com/lit/ds/symlink/ads1115.pdf
NOTE: In the following config.txt changes, do not add spaces between values and the equal sign.
To use I2C on the RPis you need to enable it which you can do with this line in your config.txt
dtparam=i2c_arm=on
The default I2C speed is 100 kHz. You can increase it to 400 kHz like this
dtparam=i2c_arm_baudrate=400000
Then to use the ads1115-overlay add the following to config.txt
dtoverlay=ads1115
After that you need to provide parameters to the overlay to configure the driver.
Because the parameter names are rather long, I will be putting the overlay parameters on separate lines. (You can keep the params on a single line as long as you don’t exceed 80 characters total.)
To enable all 4 channels of the ADC in single-ended mode, add the following
dtparam=cha_enable
dtparam=chb_enable
dtparam=chc_enable
dtparam=chd_enable
This will use the default programmable gain setting of 4.096V and data sampling rate of 860 samples per second. (The ads1015 kernel module always drives the ADC in single-shot mode.)
If you boot the system with those changes to config.txt you will see the following kernel modules
root@rpi3:~# lsmod
Module Size Used by
ipv6 408710 26
joydev 9988 0
evdev 12359 0
ads1015 3728 0
hwmon 10616 1 ads1015
brcmfmac 224862 0
brcmutil 9220 1 brcmfmac
cfg80211 551440 1 brcmfmac
rfkill 21648 1 cfg80211
bcm2835_gpiomem 3900 0
i2c_bcm2835 7145 0
uio_pdrv_genirq 3923 0
uio 10396 1 uio_pdrv_genirq
fixed 3285 0
The hwmon sysfs interface will show up here
root@rpi3:~# ls /sys/class/hwmon/hwmon0/device
driver hwmon in4_input in5_input in6_input in7_input modalias name of_node power subsystem uevent
The devices show up as in4_input - in7_input because of the default chX_cfg settings from the ads1115-overlay.
The defaults are
cha_cfg = 4
chb_cfg = 5
chc_cfg = 6
chd_cfg = 7
The values come from the Config Register, the 3 MUX bits 12-14. See the datasheet.
Here are the possible MUX (chX_cfg) settings
0 : AINp = AIN0, AINn = AIN1
1 : AINp = AIN0, AINn = AIN3
2 : AINp = AIN1, AINn = AIN3
3 : AINp = AIN2, AINn = AIN3
4 : AINp = AIN0, AINn = GND
5 : AINp = AIN1, AINn = GND
6 : AINp = AIN2, AINn = GND
7 : AINp = AIN3, AINn = GND
So for instance if you wanted to use AIN0 and AIN1 in a differential configuration you could have this in your config.txt
dtoverlay=ads1115
dtparam=cha_enable,cha_cfg=0
You would then see this in sysfs
root@rpi3:~# ls /sys/class/hwmon/hwmon0/device
driver hwmon in0_input modalias name of_node power subsystem uevent
The in0_input would be the differential reading between AIN0 and AIN1.
To read the ADC values, read the inX_input value.
The following script reads the 4 channels configured in single-ended mode
#!/bin/sh
while true; do
for i in 4 5 6 7; do
echo -n "ch[$i]: "
cat /sys/class/hwmon/hwmon0/device/in${i}_input
done
echo ""
sleep 1
done
Here’s a sample of the script running while varying the same input 0.3V to 3.3V using a rheostat to all 4 channels
root@rpi3:~# ./poll_ads1115.sh
ch[4]: 1663
ch[5]: 1663
ch[6]: 1664
ch[7]: 1664
ch[4]: 1663
ch[5]: 1664
ch[6]: 1663
ch[7]: 1663
ch[4]: 1633
ch[5]: 1555
ch[6]: 1569
ch[7]: 1570
ch[4]: 563
ch[5]: 554
ch[6]: 535
ch[7]: 516
ch[4]: 308
ch[5]: 308
ch[6]: 308
ch[7]: 308
ch[4]: 482
ch[5]: 488
ch[6]: 515
ch[7]: 549
ch[4]: 3299
ch[5]: 3299
ch[6]: 3301
ch[7]: 3300
ch[4]: 3283
ch[5]: 3282
ch[6]: 3282
ch[7]: 3281
^C