using UDP to receive ESP32 sensor data by json format

I want to using matlab to receive Esp32 data by UDP , UDP is fast.

I just code a test program which I send command to esp32 to return a sine wave , freq and sample is sent by matlab to esp32. My program works well when the total sample is under 160, it gives me the result I expected. But when I add the sample number, the UDP communication failed.

esp32 works seems correctly, I monitered the data by serial data port .

My program is following:

%% initialise UDP 
% ESP32  IP and port
global ALLOVER;
global buffer;
global bufferTotal;
esp32IP = '192.168.1.52'; %  ESP32  IP 
esp32Port = 12345;
udpObj = udpport("datagram", "IPV4");
configureCallback(udpObj,"datagram",1,@readMyData)
disp('UDP start');
% send command to esp32 
Freq = 10; % frequency
LengthSample = 150; % total sample number
command = struct('command', 'collect_data', 'length', LengthSample, 'frequency', Freq);
jsonCommand = jsonencode(command);
write(udpObj, jsonCommand, "string", esp32IP, esp32Port);
disp(['command sent out: ', jsonCommand]);
% receiving data
disp("reveive data...");
buffer = ""; % empty buffer 
bufferTotal=0;
ALLOVER=false;
while (true)
    if (ALLOVER==false)
        pause(0.1);
    else
        break;
    end
end
% 
% delete the  "END"
buffer = erase(buffer, "END");
%  get sample from JSON 
try
    receivedData = jsondecode(buffer);
    value = receivedData.values;
    
    % draw sine wave
    figure(1);
    samplingInterval = 5; % ms
    time = (0:length(value)-1) * samplingInterval;
    plot(time, value, 'LineWidth', 1.5);
    xlabel('Time (ms)');
    ylabel('Amplitude');
    title('Received Signal');
    grid on;
    disp('draw finish');
catch ME
    disp('JSON fail：');
    disp(ME.message);
end
function readMyData(u, ~)
%  UDP 
global buffer;
global ALLOVER;
global bufferTotal;
bufferTotal=bufferTotal+1;
packet = read(u, 1, "string");
buffer = buffer + string(packet.Data); % over 1 package
disp("receive data：");
disp(packet.Data); % 

% find "END"
if contains(buffer, "END")
    disp("Found 'END'，whole data ");
    ALLOVER=true;
end
end

esp32 program is following:

#include <WiFi.h>
#include <AsyncUDP.h>
#include <ArduinoJson.h>
const char* ssid = "ws0922902";         
const char* password = "13201604223"; 
AsyncUDP udp;
void setup() {
Serial.begin(115200);

//  WiFi
WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("conneting with WiFi ...");
    }
    Serial.println("WiFi is connected");
    Serial.println(WiFi.localIP());
    
    //  UDP
    if (udp.listen(12345)) {
        Serial.println("UDP has started");
        udp.onPacket([](AsyncUDPPacket packet) {
        Serial.print("command received: ");
        Serial.println((char*)packet.data());
        
        //  JSON 
        StaticJsonDocument<200> doc;
        DeserializationError error = deserializeJson(doc, packet.data());
        if (error) {
            Serial.println("JSON format failure");
            return;
            }
            String command = doc["command"];
            int length = doc["length"];
            int frequency = doc["frequency"];
            
            if (command == "collect_data") {
                // generate sine wave
                float data[length];
                for (int i = 0; i < length; i++) {
                    data[i] = sin(2 * PI * frequency * (i * 0.005)); // 5 ms sample
                    }
                    
                    // send by JSON 
                    StaticJsonDocument<2048> response;
                    //JsonArray timeArray = response.createNestedArray("time");
                    JsonArray valuesArray = response.createNestedArray("values");
                    for (int i = 0; i < length; i++) {
                         valuesArray.add(data[i]); // 
                        }
                        String responseData;
                        serializeJson(response, responseData);
                        responseData += "END";
                        packet.printf(responseData.c_str());
                        Serial.println("data has sent out");
                        Serial.printf(responseData.c_str());
                        }
                        });
                        }
                        }
                        void loop() {
                        // nothing
                        }

Answer 1

[jun]

I have found the problem that is when esp32 send the UDP package over 1500, some data is lost, so the END will never be detected in matlab . rewrite esp32 be sure max number of sent is less than 1400, if over, send several times.

...................................
    const int MAX_PACKET_SIZE = 1400; // Max payload size to fit within MTU      
............................
    responseData += "END";
// Send the response in smaller packets
int totalLength = responseData.length();
Serial.println("Sending data in chunks...");
for (int offset = 0; offset < totalLength; offset += MAX_PACKET_SIZE) {
    String chunk = responseData.substring(offset, offset + MAX_PACKET_SIZE);
    packet.printf(chunk.c_str(), chunk.length());
    delay(10); // Allow receiver to process
    }
    Serial.println("Data sent successfully.");        
    Serial.printf(responseData.c_str());
    }