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.

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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

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: Channel Numbers and Corresponding Sources

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-example). 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]{.title-ref} 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

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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 Continuous 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

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: Configuration Options

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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

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: Data Rates

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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

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: PGA Values

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

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: Comparator Queue Values

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.

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.

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

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.

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

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

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

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.

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.

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.