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.

and also you could try use other pins. like 21, 22.

make sure the sensor sda and scl has pull up resistor. (because atom pins don't have pullup resistor)

you are absolutely correct - as long as you don't call Wire.begin(25,21,10000) GPIO25 or GPIO21 should be available for OneWire.

Unfortunately I only have one DS18B20 so I cannot fully recreate your setup. Did you try your M5Atom Lite setup with only one sensor?

Good luck!
Felix

If you don't need the I2C bus at all, you can disable it like this:

Wire.~TwoWire();

Try it to see if it solves your problem.

#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?

I think you missed enabling Serial on your M5Atom. Try including:

#include <M5Atom.h>

then in setup() enable Serial and disable I2C to have GPIO25 and GPIO21 freed up for use with OneWire:

// Enable Serial, disable I2C, enable Display
M5.begin(true, false, true);

and I would add some delay in your loop():

delay(1000);

BTW: In my M5Atom setup I've verified all available GPIOs (19, 22, 23, 33, 21 and 25) successfully to work with OneWire.

Good luck
Felix

For reference - full code using GPIO25:

#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);

float DS18B20_temperature = -100.0;

void setup() {
  // Enable Serial, disable I2C, enable Display
  M5.begin(true, false, true);
  sensors.begin();
}

void loop() {
  sensors.requestTemperatures();
  const float temperature = sensors.getTempCByIndex(0);
  Serial.println(temperature);
  delay(1000);
}