Ytdyklly

I wrote some code for the ATmega328P, (Packaged in a module that clones the Arduino Nano).

Normally when I send UART data from my computer to these micros, it is received just fine. However I have started a new project and I am getting intermittent characters missing from the beginning of a message.

The code I wrote thus far reads the incoming data and prints it back to the computer. I am trying to send "<A2>". Something like 80% of the messages are received and printed back fine. The rest are missing characters. Oddly, they are at the beginning of the message, giving me "A2>" or "2>"

I am using a baud of 115200, though 9600 and even 600 appear to give the same ratio of bad messages. I tried swapping the micro and there was no change. I'm only sending messages once every 5 seconds or so.

The micro is being powered from the same USB cable going to the laptop that the data is sent over.

Below are oscilloscope/logic analyzer captures of two signals, one that worked fine and another that was missing characters. The probe is placed right at the "RX" pin and grounded right at "GND".

Update: As user duskwuff mentioned, the signals are actually not approaching 0V. The top of the logic decoder is located at around 1.5V. What can cause this if the ground of the board is grounded to the laptop?

Signal received as "<A2>" (correct):

Signal received as only "2>" (incorrect):

Those two signals look awfully close.

The purpose of my this code is to create a sort of protocol for received messages that processes them one at a time, even if multiples are sent all at once ("<A1><A2><A3>...") or if they are sent in only partial pieces ("<A" and then "2>").

Here's the code:

const int pinBuzzer = A5;      // the number of pin for the buzzer



unsigned long T = 0; // "T" value





//max input size expected for a single incoming msg

#define INPUT_SIZE 32

char serialCommandstr[INPUT_SIZE + 1];

char serialCommand_singleChar; //single char for itterating through incoming WX

byte serialCommandindex = 0; //index of RX buffer for itterating

bool serialCommand_msgPending = false; // if msg exists in buffer without termination ('>');

bool serialCommand_ready = false; // if terminated msg ready (a '>' is received)





void setup()

{

    delay(500);

    Serial.begin(115200);



} // END Setup()





void loop()

{

    Serial.println("Waiting for input in the form of "<XY>"");

    Serial.println("X=[A/B], Y=[1~8]");





    serialCommand_ready = false;

    while (serialCommand_ready == false)

    {

        serialPreParse();

    }

    if (serialCommandstr[1] == 'A')

    {

        T = (serialCommandstr[2] - 1) - char('0');

        Serial.print("A, ");

    }



    Serial.print("T=");

    Serial.print(T);

    Serial.print("us...");



    //Beep to indicate waveform start

    tone(pinBuzzer, 1319, 50);

    delay(51);



}



void serialPreParse()

{

    if (Serial.available() && serialCommand_msgPending == false)

    {

        // NEW incoming message (buffer previously empty)



        //reinitialize

        serialCommandstr[0] = '';

        serialCommand_singleChar = '';

        serialCommandindex = 0;

        serialCommand_msgPending = true; // if msg received without termination

        serialCommand_ready = false; // if terminated msg ready



    }



    while (Serial.available())

    {

        if (serialCommandindex == INPUT_SIZE)

        {

            //maximum size of C string reached

            break;

        }

        serialCommand_singleChar = Serial.read();

        //strcat(serialCommand_singleChar, serialCommandstr);

        serialCommandstr[serialCommandindex] = serialCommand_singleChar;

        serialCommandindex++;

        serialCommandstr[serialCommandindex] = ''; //null-termination



        if (serialCommand_singleChar == '>')

        {

            //End of single msg.

            serialCommand_ready = true;

            serialCommand_msgPending = false;

            break;

        }



    }





    if (serialCommand_ready == true)

    {

        if (serialCommandstr[0] != '<')

        {

            Serial.println("nIncomplete command!");

        }



        Serial.println();

        Serial.print("RX:"");

        Serial.write(serialCommandstr);

        Serial.println(""");



    }





}

What jumps out to me here is the mark (low signal) levels in your UART signal. Marks should be close to zero, but the oscilloscope output you've included looks like it's only dropping to ~1.6V, especially on the first few characters. This is well above V_IL for your microcontroller (0.2×V_CC = 1V), so the microcontroller will not reliably interpret it as a logical low.

What hardware are you using to generate UART output from the computer? Make sure it shares a ground with the computer. If this doesn't help, it may be faulty and need to be replaced.