Nano, hwserial, JDY-31 - no Serial.available()

[mr0wl]

Hello All,
I use Nano v3 (At328p / pb) and a JDY-31 for Bluetooth serial communication, attached to hardware serial (RX0 TX1, see drawing). The JDY-31, when nothing is connected via Bluetooth, is in command mode where it accepts AT commands; if there is an active BT connection, it acts as a serial port.

My plan is to set up the BT parameters (name, baudrate) by the Nano via sending AT commands to JDY-31. In theory, it should work as it is in command mode.

1. I started with basic testing. I connected JDY-31 to software serial (RX=A0, TX=D8), then connected the Nano to my PC via the USB; then I used the below sketch that simply copies the terminal into softwareserial and vice versa, it works like a charm:

#include <SoftwareSerial.h>
SoftwareSerial btSerial(A0, 8); // RX, TX
char b;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  btSerial.begin(9600);
  Serial.println("Ready.");

}

void loop() {
  if(btSerial.available()) {
    b = btSerial.read();
    Serial.print(b);
  }
  if(Serial.available()) {
    b = Serial.read();
    btSerial.print(b);
    Serial.print(b);
  }
  delay(5);
}

2. I connected JDY-31 to hwserial (TXD to RX0, RXD to TX1) , and the AT commands did not work anymore, there is no reply to them. Note that in this setup there is nothing connected to the USB serial - I understand the consequences so I power the board from VIN + GND and let the USB port intact.

3. With the same hardware setup, If I connect to the Bluetooth module with my BT client, the communication is flawless, the Nano gets everything via SPP and it can also sends data without a problem and my BT client receives it well. So it shouldn't be a wiring problem.

Here is the sketch that does not work, the Serial.available() never true:

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);
  Serial.begin(9600);
  btWrite("AT+VERSION\r\n");
  delay(1000);
  if(Serial.available()){
      while(Serial.available()){Serial.read();} // flush
      btWrite("AT+BAUD7\r\n"); // 57600
  }else {
      while(1) {
          digitalWrite(LED_BUILTIN, HIGH);
          delay(1000);
          digitalWrite(LED_BUILTIN, LOW);
          delay(1000);
      }
  }
}

void loop() {
  // put your main code here, to run repeatedly:

}

void btWrite(const char *msg){
  const char *p = msg;
  while(*p){
    Serial.write(*p++);
    delay(20);
  }
}

There is no baud rate difference; since the JDY-31 board does not get the AT command, it stays at 9600 bps baud rate.

The question is that, why is this difference? If I send the same commands via RX0-TX1, the Bluetooth module does not send a reply to AT commands, if I use it via Swserial, it sends the replies for AT commands. I need to set up the JDY-31 via AT commands, using the hw serial setup.

Thank you!