Rapid data transfer

Getting Started
1

I would like to get rapid (at least 60 hz) data from an IMU shield attached to a photon. It seems that the cloud function protocol is not the way to go - I’ve only managed at most 4 hz. I can verify that neither the latency of the sensor or the photon is the issue, since I get good performance over serial. Does anyone have advice about what the right way to do this would be? Thanks.

2

On what sort of client do you want your data received?
TCPClient or UDP seem the best fit for WiFi transfer and for wired any of the onboard ports.

3

Yes the cloud limits you to 1 update per second (or 4 per second in short bursts) as you have seen.

One option would be to make your photon be a TCP server (listens for connections, when connected sends the IMU data every 1/60th of a second). The TCPServer documentation will have an example of this.

Another option would be to have your computer act as the server and have the Photon open a connection and send the data. This recent thread Quickest way to stream data from Photon has examples of this.

2 Likes
4

You could setup a node.js UDP server and have the photon send UDP packages, no connection overhead, but no delivery insurance either.

Nodejs server

const coap        = require('coap')
    , server      = coap.createServer();   

server.on('request', function(req, res) {
    var url = req.url.split('/')[1];

    //  console.log(req.rsinfo.address+":"+req.rsinfo.port+" >> "+req.payload.toString('utf-8'));

      if (url == "incoming")
      {
        var payload = req.payload.toString('utf-8');
        try
        {
            var js = JSON.parse(payload.toString('utf-8'));

         }
         catch (ex) {
              console.log("Json parse failed >> ",payload.toString('utf-8'),ex);
       }
})

// the default CoAP port is 5683
server.listen(function() {
  console.log("Server up ...");
})

Photon code

#include <application.h>
#include <CoapSender.h>

UDP Udp;

void CoapSender::init()
{
    Udp.begin(5683);
}

void CoapSender::send(char* data)
{
    char tempbuf[430];
    
    Udp.beginPacket({192,168,1,15}, 5683);

    uint8_t coapid = 1;
    uint8_t p = 0;
    //          VVTTLLLL
    tempbuf[p++] = 0b01010000; //V=01 T=01 (No confirm) 0 token length
    tempbuf[p++] = 0b00000010; //0.2=POST req
    tempbuf[p++] = (coapid & 0xFF00) >> 8; //Message id
    tempbuf[p++] = (coapid & 0xff); //Message id 1
    //tempbuf[p++] = 0xff; //no options
    
    //coap_server seems to require a hostname, although the contents does not matter
    char host[13] = "abc.dk";
    //Option 3 - delta 3 - URI_Host
    uint8_t hostlen = strlen(host);

    if (hostlen > 12) //Hostname is limited to 99chars, so we dont need to support type 14
    {
        tempbuf[p++] = 0b00110000 | 13; //1101 = 13
        tempbuf[p++] = hostlen - 13;
        } else {
        tempbuf[p++] = 0b00110000 | hostlen;
    }
    
    strncpy(&tempbuf[p], host, hostlen);
    p += hostlen;

    //Option 11 - delta 8 - URI_PATH - /incoming
    //88 69 6e 63 6f 6d 69 6e 67
    tempbuf[p++] = 0x88;
    tempbuf[p++] = 0x69;
    tempbuf[p++] = 0x6e;
    tempbuf[p++] = 0x63;
    tempbuf[p++] = 0x6f;
    tempbuf[p++] = 0x6d;
    tempbuf[p++] = 0x69;
    tempbuf[p++] = 0x6e;
    tempbuf[p++] = 0x67;
    
    //Option 12 - delta 1 - format json
    //11 32
    tempbuf[p++] = 0x11;
    tempbuf[p++] = 0x32;
    
    tempbuf[p++] = 0xff; //end of options    

    strncpy(&tempbuf[p], data, strlen(data));
    Udp.write((uint8_t*)tempbuf, p+strlen(data));
    Udp.endPacket();
}