Driver for the ADS1015/ADS1115 Analogue-Digital Converter

This driver consists mostly of the work of Radomir Dopieralski (@deshipu). I added a few functions and changed the existing ones so it matches better my needs for a project. Especially were the functions time-optimized and made IRQ-proof (no allocation of RAM in the IRQ-prone methods). It was tested with ESP8266 Micropython

Features

Control the operation of the ADS1x15 ADC and read back the data

Connection

The ADS1X15 use a I2C interface. So SCL and SDA have to be connected as minimum. If in continuous mode the CPU shall be triggered, the ALERT/RDY pin has to be connected too, and obviously VDD, GND and the analogue input(2). You might also set the address pin to low (address = 72) or high (address = 73).

Class

The driver contains the ADS1115 class and the derived ADS1114, ADS1113 and ADS1015 classes. Since the these devices only differ by the minor parameters link the number of channels or conversion size, the same methods can be applied, with different interpretation of the parameters.

adc = ADS1115(i2c, address, gain)

or

adc = ADS1114(i2c, address, gain)

or

adc = ADS1113(i2c, address)

or

adc = ADS1015(i2c, address, gain)

The default value for the address is 72, for gain is 0. Gain is an index into a table. It defines the full range of the ADC. Acceptable values are:

0 : 6.144V # 2/3x
1 : 4.096V # 1x
2 : 2.048V # 2x
3 : 1.024V # 4x
4 : 0.512V # 8x
5 : 0.256V # 16x

Methods

adc.read()

value = adc.read([rate, [channel1[, channel2]]])

Start a conversion on channel at speed rate and return the value. Channel1 is the single input channel (0 .. 3). If channel2 is supplied, the difference between channel1 and channel2 is taken. Rate is the conversion rate. Suitable values are (ADS1015 / ADS1115/ ADS1114):

0 :  128/8      samples per second
1 :  250/16     samples per second
2 :  490/32     samples per second
3 :  920/64     samples per second
4 :  1600/128   samples per second (default)
5 :  2400/250   samples per second
6 :  3300/475   samples per second
7 :  - /860     samples per Second

The first value applies to the ADS1015, the second to the ADS1115 and ADS1114. The time required for a single conversion is 1/samples_per_second plus the time needed for communication with the ADC, which is about 1 ms on an esp8266 at 80 MHz. Slower conversion yields in a less noisy result. The data sheet figures of the ads1x15 are given for the slowest sample rate. The value returned is a signed integer of the raw ADC value. That value can be converted to a voltage with the method raw_to_v().

adc.set_conv and adc.read_rev()

Pair of methods for a time optimized sequential reading triggered by a time. For using, you would first set the conversion parameters with set_conv() and then get the values in a timer callback function with read_rev().

adc.set_conv([rate, [channel1[, channel2]]])
value = adc.read_rev()

The definition of channel1, channel2 and rate are the same as with adc.read(). The methods read_rev() reads first the last conversion value back, and the starts a new conversion. Care has to be taken, that the time needed for conversion and communication is shorter than the timer period plus the time needed to process the data. A sample code is shown below. The timing jitter observed on an esp8266 was about 1 ms, but the time period is defined by the micro's timer, which has it's own issues. The value returned by read_rev is a signed integer of the raw ADC value. That value can be converted to a voltage with the method raw_to_v().

adc.alert_start() and adc.alert_read()

Pair of methods to start a continuous sampling on a single channel and trigger an alert once a certain threshold is reached.

adc.alert_start([rate, [channel1[, channel2]]][, threshold_high, threshold_low, latched])
value = adc.alert_read()

The values of channel1, channel2 and rate are the same as for adc.read(). Threshold_high tells upper value of the threshold register, and threshold_low the lower value. Boths must be be within the range of the ADC, 0..32767 for ADC1115 and 0..2047 for ADS1015. Rate should be chosen according to the input signal change rate and the precision needed. The mode set is the traditional comparator mode, with the lower threshold set to 0. latched tells whether thge aler is latched or not. If False, only one alter is generated when the threshold is reached until the threshold condition is not valid, if True, alert signals are generated as long as the treshold conditions match. The value returned by alert_read is a signed integer of the raw ADC value. That value can be converted to a voltage with the method raw_to_v().

adc.conversion_start() and adc.alert_read()

Pair of methods to start a continuous sampling on a single channel and trigger an alert at every sample. This function pair is provided for an IRQ-based set-up.

adc.conversion_start([rate, [channel1 [, channel2]]])
value = adc.alert_read()

The values of channel1, channel2 and rate are the same as for adc.read(). The timing jitter seen is about 200 ns. However the ADC's timer is not very precise. In applications where this is of importance some control and calibration of the returned timing pattern has to be done. The value returned by alert_read is a signed integer of the raw ADC value. That value can be converted to a voltage with the method raw_to_v().

adc.raw_to_v()

voltage = adc.raw_to_v(raw)

Convert the raw ADC result to a voltage that matches the gain setting of the constructor. It returns a float value of the voltage.

adc._read_register()

Read a register of the ADC.

value = adc._read_register(register)

Register is the number of the register according to the data sheet. Reading the conversion register returns the value of the most recent sampling. Bit 15 of the configuration register is set when a conversion is finished.

adc._write_register()

Write a register of the ADC.

value = adc._write_register(register, value)

Register is the number of the register according to the data sheet, value a 16 bit quantity coded accordingly.

Sample Code

Continuous sampling triggered by the timer

# Sample code for ESP8266 & ESP32, Micropython.org firmware
from machine import I2C, Pin, Timer
import ads1x15
from time import sleep_ms, ticks_ms, ticks_us
from array import array

addr = 72
gain = 1
_BUFFERSIZE = const(512)

data = array("h", (0 for _ in range(_BUFFERSIZE)))
timestamp = array("L", (0 for _ in range(_BUFFERSIZE)))
i2c = I2C(scl=Pin(5), sda=Pin(4), freq=400000)
# for the Pycom branch, use:
# i2c = I2C()
ads = ads1x15.ADS1115(i2c, addr, gain)

#
# Interrupt service routine for data acquisition
# called by a timer interrupt
#
def sample(x, adc = ads.read_rev, data=data, timestamp = timestamp):
    global index_put, irq_busy
    if irq_busy:
        return
    irq_busy = True
    if index_put < _BUFFERSIZE:
        timestamp[index_put] = ticks_us()
        data[index_put] = adc()
        index_put += 1
    irq_busy = False

irq_busy = False

index_put = 0
ADC_RATE = 5

# set the conversion rate to 860 SPS = 1.16 ms; that leaves about
# 3 ms time for processing the data with a 5 ms timer
ads.set_conv(7, 0) # start the first conversion
ads.read_rev()
sleep_ms(ADC_RATE)
tim = Timer(-1)
tim.init(period=ADC_RATE, mode=Timer.PERIODIC, callback=sample)

while index_put < _BUFFERSIZE:
    pass

tim.deinit()

# at that point data contains the sampled values, and
# timestamp the timer ticks which correlate to the conversion time
#

The timing jitter seen here was +/- 500 us, with 90% up to 50 µs and 5% each at about 450 and 550 µs. The timing interference occurred every second. At 160MHz clock, the Jitter was about +/- 50 µs

Continuous sampling trigged by the ADC

# Sample code for ESP8266 & ESP32, Micropython.org firmware
from machine import I2C, Pin, Timer
import ads1x15
from array import array

addr = 72
gain = 1
_BUFFERSIZE = const(512)

data = array("h", (0 for _ in range(_BUFFERSIZE)))
i2c = I2C(scl=Pin(5), sda=Pin(4), freq=400000)
# for the Pycom branch or Micropython, use:
# i2c = I2C()
ads = ads1x15.ADS1115(i2c, addr, gain)
#
# Interrupt service routine for data acquisition
# activated by a pin level interrupt
#
def sample_auto(x, adc = ads.alert_read, data = data):
    global index_put
    if index_put < _BUFFERSIZE:
        data[index_put] = adc()
        index_put += 1

index_put = 0

irq_pin = Pin(13, Pin.IN, Pin.PULL_UP)
ads.conversion_start(5, 0)

irq_pin.irq(trigger=Pin.IRQ_FALLING, handler=sample_auto)

while index_put < _BUFFERSIZE:
    pass

irq_pin.irq(handler=None)
#
# at that point data contains 512 samples acquired at the given rate
#

The sampling rate achieved in my test was 251.9 SPS or 3.97 ms/sample, as told by the ESP8266 clock, which may not be precise either.