Espressif Systems ESP8266 Beginner's Manual page 47

In the setup, there's not much new either, we just start the temperature sensor, and check if we can communicate with it. If no
temperature sensor is found, the ESP resets.
Getting the temperature from the sensor may take some time (up to 750ms). We don't want our loop to take longer than a couple of
milliseconds, so we can't wait 750ms. If we did, the HTTP server etc. would start to misbehave.
The solution is to request the temperature first. The sensor will then start reading the analog temperature, and stores it in its memory.
In the meantime, the loop just keeps on running, the server refreshes etc. After 750ms, we contact the sensor again, and read the
temperature from its memory.
To tell the library that we don't want to wait for the analog to digital conversion of the sensor, we use setWaitForConversion.
Loop
const unsigned long intervalNTP = ONE_HOUR; // Update the time every hour
unsigned long prevNTP = 0;
unsigned long lastNTPResponse = millis();
const unsigned long intervalTemp = 60000; // Do a temperature measurement every minute
unsigned long prevTemp = 0;
bool tmpRequested = false;
const unsigned long DS_delay = 750; // Reading the temperature from the DS18x20 can take up to 750ms
uint32_t timeUNIX = 0; // The most recent timestamp received from the time server
void loop() {
unsigned long currentMillis = millis();
if (currentMillis - prevNTP > intervalNTP) { // Request the time from the time server every hour
prevNTP = currentMillis;
sendNTPpacket(timeServerIP);
}
uint32_t time = getTime(); // Check if the time server has responded, if so, get the UNIX time
if (time) {
timeUNIX = time;
Serial.print("NTP response:\t");
Serial.println(timeUNIX);
lastNTPResponse = millis();
} else if ((millis() - lastNTPResponse) > 24UL * ONE_HOUR) {
Serial.println("More than 24 hours since last NTP response. Rebooting.");
Serial.flush();
ESP.reset();
}
if (timeUNIX != 0) {
if (currentMillis - prevTemp > intervalTemp) { // Every minute, request the temperature
tempSensors.requestTemperatures(); // Request the temperature from the sensor (it takes some time to read it)
tmpRequested = true;
prevTemp = currentMillis;
Serial.println("Temperature requested");
}
if (currentMillis - prevTemp > DS_delay && tmpRequested) { // 750 ms after requesting the temperature
uint32_t actualTime = timeUNIX + (currentMillis - lastNTPResponse) / 1000;
// The actual time is the last NTP time plus the time that has elapsed since the last NTP response
tmpRequested = false;
float temp = tempSensors.getTempCByIndex(0); // Get the temperature from the sensor
temp = round(temp * 100.0) / 100.0; // round temperature to 2 digits
Serial.printf("Appending temperature to file: %lu,", actualTime);
Serial.println(temp);
File tempLog = SPIFFS.open("/temp.csv", "a"); // Write the time and the temperature to the csv file
tempLog.print(actualTime);
tempLog.print(',');
tempLog.println(temp);
tempLog.close();
}
} else { // If we didn't receive an NTP response yet, send another request
sendNTPpacket(timeServerIP);
delay(500);
}
server.handleClient(); // run the server
ArduinoOTA.handle(); // listen for OTA events
}
The loop looks a lot more complex, but it's actually pretty simple. It's all based on Blink Without Delay.
There's two things going on:
1. Every hour, the ESP requests the time from an NTP server. Then it constantly checks for a response, and updates the time if it
gets an NTP response. If it hasn't received any responses for over 24 hours, there's something wrong, and the ESP resets itself.
2. Every minute, the ESP requests the temperature from the DS18x20 sensor, and sets the 'tmpRequested' flag. The sensor will
start the analog to digital conversion.
750ms after the request, when the conversion should be finished, the ESP reads the temperature from the sensor, and resets the
flag (otherwise, it would keep on reading the same temperature over and over again). Then it writes the time and the
temperature to a file in SPIFFS.
By saving it as a CSV file in the filesystem, we can easily download it to the client (using the web server that is running), and it's
easy to parse with JavaScript.
If we miss the first NTP response, timeUNIX will be zero. If that's the case, we send another NTP request (otherwise, the next request
would be an hour later, and the temperature logging only starts when the time is known).
We also need to run the server and OTA functions to handle HTTP and OTA requests.
Setup functions, server handlers and helper functions
These functions haven't change since the previous example, so there's no need to cover them here. You do need them to get the
program running, though. Download the ZIP archive with examples for the full sketch.
HTML and JavaScript
There's some HTML and JavaScript files to plot the temperature using Google Graphs. I won't cover it here, but if you wan't to know