TI ADS1115

Contributed by Jia Lin

The ADS1115 is a 16-bit, 4-channel ADC made by Texas Instruments which operates over I2C. It can measure voltages from each channel individually, or between different pairs of channels. The ADS1115 also supports a programmable gain amplifier (PGA) and a digital comparator.

Channel Numbers and Corresponding Sources
Channel number	AINP	AINN
0	AIN0	AIN1
1	AIN0	AIN3
2	AIN1	AIN3
3	AIN2	AIN3
4	AIN0	GND
5	AIN1	GND
6	AIN2	GND
7	AIN3	GND

Driver Interface

To register the ADS1115 device driver as a standard NuttX analog device on your board, you can use something similar to the below code for the RP2040.

#include <nuttx/analog/ads1115.h>
#include <nuttx/analog/adc.h>

/* Register ADS1115 ADC. */

struct adc_dev_s *ads1115 = ads1115_initialize(rp2040_i2cbus_initialize(0),
                                               CONFIG_ADC_ADS1115_ADDR);
if (ads1115 == NULL)
  {
    syslog(LOG_ERR, "Failed to initialize ADS1115\n");
  }

ret = adc_register("/dev/adc1", ads1115);
if (ret < 0)
  {
    syslog(LOG_ERR, "Failed to register ADS1115 device driver: %d\n", ret);
  }

If you have a measurement from the ADS1115, you can convert it into a voltage like so:

#define FSR (2.048)

struct adc_msg_s msg;

/* Some code here to read the ADC device, you can read the ADC driver docs */

double voltage = ((double)msg.am_data  * FSR) / (32768.0);

Once registered, this driver can be interacted with using the ADC example (adc Read from ADC). Be sure to enable the software trigger, since the ADS1115 driver does not support hardware triggers (interrupts). You can also change the number of samples per group up to 8 for all 8 channels of the ADC.

You may need to increase the CONFIG_ADC_FIFOSIZE value to something larger than 8 in order to be able to store all the ADC measurements after a measurement trigger (i.e 9).

Configuration

Configuration Options
Name	Description
CONFING_ADC_ADS1115_I2C_FREQUENCY	I2C frequency of the ADS1115
CONFIG_ADC_ADS1115_ADDR	I2C address of the ADS1115
CONFIG_ADC_ADS1115_CHANNEL	Default ADC channel to read
CONFIG_ADC_ADS1115_PGA	Gain of the ADS1115
CONFIG_ADC_ADS1115_CONTINOUS	Continous mode of the ADS1115
CONFIG_ADC_ADS1115_DR	Data rate of the ADS1115
CONFIG_ADC_ADS1115_COMP_MODE	Mode of the ADS1115 comparator, traditional or window
CONFIG_ADC_ADS1115_COMP_POL	Polarity of the ADS1115 comparator, active high or active low
CONFIG_ADC_ADS1115_COMP_LAT	Latching mode of the ADS1115 comparator, traditional or latching
CONFIG_ADC_ADS1115_COMP_QUE	Comparator queue of the ADS1115, which changes when the ALRT/RDY pin is asserted.
CONFIG_ADC_ADS1115_HI_THRESH	HIGH_THRESH register of the ADS1115
CONFIG_ADC_ADS1115_LO_THRESH	LOW_THRESH register of the ADS1115

Data Rates
Value in Kconfig	Data Rate
0	8 SPS
1	16 SPS
2	32 SPS
3	64 SPS
4	128 SPS
5	250 SPS
6	475 SPS
7	860 SPS

PGA Values
Value in Kconfig	Full Scale Range (FSR)
0	±6.144V
1	±4.096V
2	±2.048V
3	±1.024V
4	±0.512V
5	±0.256V
6	±0.256V
7	±0.256V

Comparator Queue Values
Value in Kconfig	Comparator Queue
0	Assert after one conversion
1	Assert after two conversions
2	Assert after four conversions
3	Disable comparator

Additional ioctl Commands

There are various additional ioctl() commands that can be used with the ADS1115 driver. These mostly allow for changes of configuration in runtime.

ANIOC_ADS1115_SET_PGA

This command changes the gain of the ADS1115 driver. The argument passed should be of type ads1115_pga_e, which corresponds to the gain seen above.

ANIOC_ADS1115_SET_MODE

This command changes the ADS1115 to operate in continous or single-shot mode.

ANIOC_ADS1115_SET_DR

This command changes the data rate of the ADS1115 driver. The argument passed should be of type ads1115_dr_e, which corresponds to the data rate seen above.

ANIOC_ADS1115_SET_COMP_MODE

This command changes the ADS1115 to operate in either traditional or window comparator mode.

ANIOC_ADS1115_SET_COMP_POL

This command changes the ADS1115 to operate in either active high or active low mode.

ANIOC_ADS1115_SET_COMP_LAT

This command changes the ADS1115 to operate in either traditional or latching comparator mode.

ANOIC_ADS1115_SET_COMP_QUEUE

This command changes the comparator queue feature of the ADS1115. The argument passed should be of type ads1115_comp_queue_e, which corresponds to the comparator queue seen above.

ANOIC_ADS1115_SET_HI_THRESH

This command changes the HIGH_THRESH register of the ADS1115. The argument passed should be of type uint16_t, which corresponds to the HIGH_THRESH register value.

ANOIC_ADS1115_SET_LO_THRESH

This command changes the LOW_THRESH register of the ADS1115. The argument passed should be of type uint16_t, which corresponds to the LOW_THRESH register value.