From 0c6306e8d6ca3330e7ecf82a9463b4629af5760d Mon Sep 17 00:00:00 2001
From: Fisch <interfisch@gmx.net>
Date: Thu, 22 Oct 2020 23:00:33 +0200
Subject: [PATCH] reimplement mhz19 read function and add delay between cmd
 send and response reading

---
 src/main.cpp | 47 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 45 insertions(+), 2 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index ee4785e..27f3606 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -129,6 +129,8 @@ struct sensordata
   MHZ19 mhz19;
   bool mhz19_ready=false;
   int value_co2=-1; //[ppm]
+
+  int mhz19_readValue_reimplemented(Stream *_streamRef, MHZ19 *_mhz19Ref); //declare function
 #endif
 
 // data/homie/config.json hochladen mit platformio run --target uploadfs
@@ -517,8 +519,9 @@ void loop_MHZ19()
   bool _changed=false;
   if (millis() >= (d.lastreadtime+d.readdelay)) {
     mhz19_ready=mhz19.isReady();
-    value_co2=mhz19.readValue(); //[ppm]
-    //Homie.getLogger() << "co2 " << ": " << value_co2 << " status=" << mhz19_ready << endl;
+    //value_co2=mhz19.readValue(); //[ppm]
+    value_co2=mhz19_readValue_reimplemented(&mhz19_swSerial, &mhz19); //[ppm]  reimplemented function to fix no response issue
+    Homie.getLogger() << "read co2 " << ": " << value_co2 << " status=" << mhz19_ready << endl;
     if (fabs(d.lastsentvalue-value_co2)>=d.minchange){
       _changed=true;
     }
@@ -628,4 +631,44 @@ int get_lux(const unsigned int* _in, const unsigned int* _out, byte size)
  // interpolate in the right segment for the rest
  return map(val, _in[pos-1], _in[pos], _out[pos-1], _out[pos]);
 }
+#endif
+
+
+#ifdef SENSOR_MHZ19
+byte mhz19_getCheckSum(byte* packet) {
+  byte checksum = 0;
+  for(uint8_t i = 1; i < 8; i++) {
+    checksum += packet[i];
+  }
+  checksum = 0xff - checksum;
+  checksum += 1;
+  return checksum;
+}
+
+int mhz19_readValue_reimplemented(Stream *_streamRef, MHZ19 *_mhz19Ref) { //same function as in mhz19 library from klevytskyi, but with delay between cmd send and response check
+  byte CMD_READ[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79}; // Read command
+  unsigned int co2 = -1;
+  unsigned char response[9];
+  _streamRef->write(CMD_READ, 9);
+
+  unsigned long _startwait=millis();
+  while (millis()-_startwait<100) { //wait for mhz19 to send response
+    //wait
+  }
+
+  if (_streamRef->available()) {
+    _streamRef->readBytes(response, 9);
+
+    byte crc = mhz19_getCheckSum(response);
+
+    if (response[0] == 0xFF && response[1] == CMD_READ[2] && response[8] == crc) {
+      unsigned int responseHigh = (unsigned int) response[2];
+      unsigned int responseLow = (unsigned int) response[3];
+      unsigned int ppm = (256*responseHigh) + responseLow;
+      co2 = ppm;
+    }
+  }
+
+  return co2;
+}
 #endif
\ No newline at end of file