ESP8266 I2C doesn't respond

Question

I've got a huge problem using an ESP8266 from a Wemos D1 mini.

I am using an STM32L073RZ to send a request to an ESP8266 to send me the time it has acquired from a WiFi connection.

ESP code:

#include <ESP8266WiFi.h>
#include <WiFiUdp.h>
#include <NTPClient.h>
#include <Wire.h>
// Zastąp poniższe wartości danymi Twojej sieci WiFi
const char* ssid = ":>>>>>>>>>>>";
const char* password = "NOPE";
#define Status D4
WiFiUDP ntpUDP;
NTPClient timeClient(ntpUDP, "europe.pool.ntp.org", 3600, 60000);
void setup() {
  pinMode(D4, OUTPUT); // Pin odpowiedzialny za gotowość urządzenia do komunikacji przez I2C
  digitalWrite(D4, LOW); // Ustawiamy go na początku na "0".
Serial.begin(9600);
  WiFi.begin(ssid, password);
while ( WiFi.status() != WL_CONNECTED ) {
    delay ( 500 );
    Serial.print ( "." );
  }
timeClient.begin();
Wire.begin(8); // Odpala D2 jako SDA i D1 jako SCL jako slave
  Wire.onRequest(sendTime); // Rejestracja funkcji zwrotnej, która zostanie wywołana, gdy urządzenie nadrzędne zażąda danych
digitalWrite(D4, HIGH); // Tutaj konfiguracja z siecią jak i też konfiguracja z pinami i ustawienie I2C jest zakończone
                          // i sygnalizowane stanem wysokim na D4
}
void loop() {
  timeClient.update();
  Serial.println(timeClient.getFormattedTime());
  delay(1000);
}
void sendTime() {
  String time = timeClient.getFormattedTime();
  Wire.write(time.c_str()); // Wysłanie czasu do urządzenia nadrzędnego będzie to razem 8 bajtów bo jeszcze znaki oddzielające.
}

STM simple code :

xD = HAL_I2C_Master_Receive(&hi2c2, 0x08<<1, czas, 8, 1000);

I tried everything. Adding pull ups, changing the GPIO speeds, etc.

Here is what my logic analyser gave me:

Still nothing. I've also checked which pins are the SDA and SCL which are D2 and D1. I've checked is the connection was correct and it should be, but still no response.

I don't know anymore what is wrong or what is not.

Schematic :

It is already powered so I didn't need to add 5 V and G pins to it, but I also tested with adding G pin to ground of the STM32, and also with 5 V to the 5 V of the STM32, but it still didn't work.

Answer 1

I2C slave doesn't work on esp8266 because it doesn't have hardware support for I2C.

From the esp8266 Wire library documentation:

Wire library currently supports master mode ...

https://arduino-esp8266.readthedocs.io/en/3.1.2/libraries.html#i2c-wire-library

Answer 2

The devices must share a common ground, and also you may need 4.7k pull-up resistors on both SDA and SCL.

See http://gammon.com.au/i2c for a whole lot more detail.

Answer 3

Both other answers are correct, but there is yet another problem with your design.

As can be clearly seen from the logic analyzer output, the master runs its clock at 100kb/sec (the standard i2c rate). That leaves the slave ca. 5us to react. For a 80 MHz chip this might be quite a tight timing by itself and requires thorough inspection and optimization of code. But a UDP request into the big net? Seriously?!

The only way it might work is if both parties could agree on a clock stretching. However, the i2c spec never defines precise timings for the stretched clock, so, the chances are, any master would probably break the session assuming the slave hung up long before the UDP request could be served.

Therefore, such requests need to be split into two parts. First, the master asks the slave to initiate the process of measurement/exchange/computation/whatever. And second, after an appropriate delay, the master issues another request for the buffered result from slave.