Thanks for all the great responses.

I actually figure it out. I knew that my sensors were working because I could get the same code to work when using a ESP8266. I'm currently using GPIO25 for my 5 DS18B20 temperature sensors and I'm also using a 4.7 Ohm resistor as suggested by Dallas. At first, I could not see any sensor. After some thinkering, I was able to get the mac addresses of all 5 sensors with the Atom Lite but the temperature was ALWAYS 85. Googling for 85, someone said that we have to wait 750 ms after calling sensors.requestTemperatures();

  sensors.requestTemperatures();
  delay(750);  // This seems critical to get the sensors to work with AtomLite

Then I can proceed to get the temperatures like so. Note that this only illustrates the spirit of what needs to be done and I haven't tested the exact code that follows.

#include <M5Atom.h>
#include <OneWire.h>
#include <DallasTemperature.h>

// GPIO where the DS18B20 is connected to
const uint8_t oneWireBus = GPIO_NUM_25;

// Setup a oneWire instance to communicate with any OneWire devices
OneWire oneWire(oneWireBus);

// Pass our oneWire reference to Dallas Temperature sensor
DallasTemperature sensors(&oneWire);

void setup() {
  M5.begin(true, false, true);  // But I need to test without this line.
  sensors.begin();
}


void loop() {
  sensors.requestTemperatures();
  delay(750);  // This seems critical to get the sensors to work with AtomLite

  const unsigned int num_DS18B20 = sensors.getDeviceCount();

  for (unsigned int i=0; i<num_DS18B20; ++i) {
    DeviceAddress mac;
    sensors.getAddress(mac, i);
    delay(10);
    const float temperature = sensors.getTempCByIndex(i);
    send_MQTT(mac, temperature);
    }
  }
  delay(10000);
}

I hope that this information will be useful for someone else trying to hookup some DS18B20 to an Atom Lite.

@felmue Thanks for your help.
I've tried your suggestion of adding

#include <M5Atom.h>
...
M5.begin(true, false, true);

And I was getting 85 which, according to a google search, means there is an error but I can see the mac addresses of all my 5 probes. What bothers me is that looking at the code of M5.begin(true, false, true); and according to your previous comment, I skip the I2C initialization which I was already doing before adding that line since I wasn't calling Wire.begin(25,21,10000);. I really don't see what would help my I2C problem. By adding M5.begin(true, false, true);, the only thing I add is initializing the display which I don't have on my atom lite.

I've tried essentially the same code with my nodemcu, the same DS18B20 sensors and I can read the mac addresses and all 5 temperature no problem.

I also have a Feather Huzzah esp8266 and a Feather Huzzah 32 esp32 that I will try out later.

Could it be that the atom lite is not providing enough mA to the sensors?

Hi,
I'm trying to attach a DS18B20 (5 pcs, 3 meter), temperature sensors working with oneWire. I can't seem to get any reading on an Atom Matrix.

Here's a short version of my code.

#include <Arduino.h>
#include <OneWire.h>
#include <DallasTemperature.h>

// GPIO where the DS18B20 is connected to
const uint8_t oneWireBus = ONEWIREBUS;

// Setup a oneWire instance to communicate with any OneWire devices
OneWire oneWire(oneWireBus);

// Pass our oneWire reference to Dallas Temperature sensor
DallasTemperature sensors(&oneWire);

float DS18B20_temperature = -100.0;

void setup() {
  sensors.begin();
}

void loop() {
  sensors.requestTemperatures();
  const float temperature = sensors.getTempCByIndex(0);
  Serial.println(temperature);
  const unsigned int num_DS18B20 = sensors.getDeviceCount();
  Serial.printf("Number of DS18B20: %d\n", num_DS18B20);
  for (unsigned int i=0; i<num_DS18B20; ++i) {
    DeviceAddress mac;
    sensors.getAddress(mac, i);
    const float temperature = sensors.getTempCByIndex(i);
    Serial.println(temperature);
  }
}

I have tested my breadboard setup and the code with a NodeMCU and using a NodeMCU I can get the temperatures. When using my NodeMCU, I make sure to use the default SDA pin for it to work.

I googled around and I can find multiple posts that claim that we can use almost any pin as SDA so I wonder how I can change GPIO25 to be SDA on an Atom Lite?

In this post, EXT.IO Unit and UIFlow 1.4.5.1, there is comment hinting that 3 meter long wires could be too long and that 100cm is the longest at most.