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
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