How to read temperature from multiple DS18B20 faster?

Question

I am writing an Arduino sketch to read temp data from multiple DS18B20 sensors and then write it to serial. The problem is, as I am connecting more sensors, the delay between my temp data is written to a serial increases (like 100ms for 1 sensor and 1s for 4 sensors). Can I somehow improve my sketch to read it faster?

Here is the sketch I am using: (I am using DallasTemperature to simplify it all)

#include <OneWire.h>
#include <DallasTemperature.h>
#define ONE_WIRE_BUS 10
#define TEMPERATURE_PRECISION 8
#define BAUD_RATE 9600
// Setup a oneWire instance to communicate with any OneWire devices (not just Maxim/Dallas temperature ICs)
OneWire oneWire(ONE_WIRE_BUS);
// Pass our oneWire reference to Dallas Temperature. 
DallasTemperature sensors(&oneWire);
// arrays to hold device addresses
DeviceAddress devices[10];
int devicesFound = 0;
void setup(void)
{
 // start serial port
 Serial.begin(BAUD_RATE);
 //Serial.println("Dallas Temperature IC Control Library Demo");
 // Start up the library
 sensors.begin();
 // locate devices on the bus
 Serial.print("Locating devices...");
 Serial.print("Found ");
 Serial.print(sensors.getDeviceCount(), DEC);
 Serial.println(" devices.");
 devicesFound = sensors.getDeviceCount(); 
 // report parasite power requirements
 Serial.print("Parasite power is: "); 
 if (sensors.isParasitePowerMode()) Serial.println("ON");
 else Serial.println("OFF");
 for (int i = 0; i < devicesFound; i++)
 if (!sensors.getAddress(devices[i], i)) 
 Serial.println("Unable to find address for Device" + i); 
 // show the addresses we found on the bus
 for (int i = 0; i < devicesFound; i++)
 { 
 Serial.print("Device " + (String)i + " Address: ");
 printAddress(devices[i]);
 Serial.println();
 }
 for (int i = 0; i < devicesFound; i++)
 sensors.setResolution(devices[i], TEMPERATURE_PRECISION);
}
// function to print a device address
void printAddress(DeviceAddress deviceAddress)
{
 for (uint8_t i = 0; i < 8; i++)
 {
 // zero pad the address if necessary
 if (deviceAddress[i] < 16) Serial.print("0");
 Serial.print(deviceAddress[i], HEX);
 }
}
// function to print the temperature for a device
String printTemperature(DeviceAddress deviceAddress)
{
 float tempC = sensors.getTempC(deviceAddress);
 if (tempC < 10)
 return "0" + (String)tempC;
 else
 return (String)tempC;
}
// function to print a device's resolution
void printResolution(DeviceAddress deviceAddress)
{
 Serial.print("Resolution: ");
 Serial.print(sensors.getResolution(deviceAddress));
 Serial.println(); 
}
// main function to print information about a device
void printData(DeviceAddress deviceAddress)
{
 Serial.print("Device Address: ");
 printAddress(deviceAddress);
 Serial.print(" ");
 printTemperature(deviceAddress);
 Serial.println();
}
void loop(void)
{ 
 if (devicesFound == 0)
 {
 Serial.println("No devices found.");
 return;
 }
 sensors.requestTemperatures();
 String sent = "";
 // print the device information
 for (int i = 0; i < devicesFound; i++)
 {
 sent += printTemperature(devices[i]);
 if (i != devicesFound - 1)
 sent += " "; 
 }
 Serial.println(sent);
}

• RufusVS: A trivial change, but I think it's cleaner: Where you are adding spacing to the string, I'd use if (i) sent += " "; as a statement before the statement adding the reading to the string, instead of the statement you currently have after

Answer 1

With 12 bits precision, DS18B20 may take up to 750 ms converting a temp.

Try using async reading, so you don't have to wait for the end of the process.

First you ask for a reading, later your read it

 DallasTemperature sensors(&oneWire);
 ...
 sensors.setWaitForConversion(false); // makes it async
 sensors.requestTemperatures();
 sensors.setWaitForConversion(true);
 ...
 // Do something else while temp is been reading by the sensor
 ...
 float temp = sensors.getTempCByIndex(0); 

• RufusVS: Does the getTempByIndex function build in the wait-for-complete or are you assuming your "do something else" section is longer than a conversion?

Answer 2

I would start by changing this section:

String sent = "";
// print the device information
for (int i = 0; i < devicesFound; i++)
{
 sent += printTemperature(devices[i]);
 if (i != devicesFound - 1)
 sent += " "; 
}
Serial.println(sent);

This section should read and print the temperature directly instead of creating a string. The string handling is a large overhead and it does not allow Serial (UART) to work very much in parallel if the buffer is filled. The printTemperature() function is using a lot of strings (and heap operations).

Otherwise you have done what is possible; 1) reduce the conversion resolution (9-bits is minimum), 2) use single conversion request command to all sensors.

The alternative is to use alarm triggers and only read sensors with alarms.

• serge1peshcoff: like that pastebin.com/NubeaW4M ?
• Mikael Patel: What are the results of that? I did some tests and it is possible to issue a conversion request, read and print the results from 48 sensors at 9-bits resolution and 14 sensors at 12-bits resolution (per second).
• serge1peshcoff: delay is still the same
• Mikael Patel: Can you scale it down to a single sensor and measure the conversion request and reading of scratch pad? We need to isolate the issue in the library that you are using.
• serge1peshcoff: So, I changed my sketch this way: pastebin.com/7EADdVAu, and it shows that around 110ms has passed while reading one sensor.