Serial not reading all inputs from monitor correctly - arduino

Question

Im trying to send inputs to Arduino MEGA using the monitor. Ive noticed that 1/6 times, the Serial skips 1 or more letters in my input. Ive tried Serial.read() and Serial.readStringUntil() but both skip input. I tried putting delay after the Serial.read() but it does nothing. I also tried doing a Serial.flush in the if(complete){...} code block but to no avail.
In this picture, "A**" was sent correctly (Ive noticed it always has as it is the 1st monitor input). But subsequent commands like the "s**100" was sent as "s*0". Is there misuse of the Serial class here? Is there a misconfiguration maybe in using the Serial monitor? (No line ending). Could it be that using other custom classes interfere with the Serial even if it didnt explicitly interact with it?
//===========================================THE IMPORTANT PART===============================================
void loop() {
digitalWrite(13, HIGH);
while (Serial.available()){
String temp = Serial.readStringUntil(';');
Serial.println(temp);
command = temp.substring(0,3);
param = temp.substring(3).toInt();
complete = true;
Serial.println("temp "+temp);
Serial.println("comm "+command);
Serial.println("para "+command);
}
//=============================================================================================================
if(complete){
Serial.println("complete");
if(command == MODE_SOLID || command == MODE_GRADIENT || command == MODE_OFF){
current_mode = command;
}
else if(command == COMMAND_BRIGHTNESS){
current_brightness = param;
}
else if(command == COMMAND_SATURATION){
current_saturation = param;
}
else if(command == COMMAND_COLOR){
current_color = param;
}
else if(command == COMMAND_RANDOM_BRIGHTNESS_UPPERLIMIT){
current_random_brightness_upper_limit = param;
}
else if(command == COMMAND_RANDOM_BRIGHTNESS_LOWERLIMIT){
current_random_brightness_lower_limit = param;
}
else if(command == COMMAND_RANDOM_SATURATION_UPPERLIMIT){
current_random_saturation_upper_limit = param;
}
else if(command == COMMAND_RANDOM_SATURATION_LOWERLIMIT){
current_random_saturation_lower_limit = param;
}
else if(command == COMMAND_RANDOM_COLOR_UPPERLIMIT){
current_random_color_upper_limit = param;
}
else if(command == COMMAND_RANDOM_COLOR_LOWERLIMIT){
current_random_color_lower_limit = param;
}
else if(command == COMMAND_COLOR){
current_color = param;
}
complete = false;
command = "";
param = 0;
}
if(current_mode == MODE_SOLID){
solid.setBrightness(current_brightness);
solid.setSaturation(current_saturation);
solid.setColor(current_color);
solid.runPattern();
}
else if(current_mode == MODE_GRADIENT){
blinkSolid.setCurrentRandomBrightnessLowerLimit(current_random_brightness_lower_limit);
blinkSolid.setCurrentRandomBrightnessUpperLimit(current_random_brightness_upper_limit);
blinkSolid.setCurrentRandomSaturationLowerLimit(current_random_saturation_lower_limit);
blinkSolid.setCurrentRandomSaturationUpperLimit(current_random_saturation_upper_limit);
blinkSolid.setCurrentRandomColorLowerLimit(current_random_color_lower_limit);
blinkSolid.setCurrentRandomColorUpperLimit(current_random_color_upper_limit);
blinkSolid.runPattern();
}
else if(current_mode == MODE_OFF){
strobePalette.runPattern();
//fill_solid( leds, NUM_LEDS, CRGB::Black);
//FastLED.show();
}
}

Answer 1

I've experienced similar issues with physical devices, most of the times in harsh environments or using defective wiring.
I tested your code using tinkercad with an Arduino Uno, only the communication part. This is the code I tested:
bool complete;
String command;
int param;
void setup()
{
Serial.begin(9600);
}
void loop()
{
digitalWrite(13, HIGH);
while (Serial.available()){
String temp = Serial.readStringUntil(';');
Serial.println(temp);
command = temp.substring(0,3);
param = temp.substring(3).toInt();
complete = true;
Serial.println("temp "+temp);
Serial.println("comm "+command);
Serial.println("para "+ String(param)); //I changed this line in order to see the value of "param" in the serial monitor
}
if(complete){
Serial.println("complete");
complete=false;
command="";
param=0;
}
}
I tested the code sending the string A**100;B**101;C**102;D**103; all of the parts arrived ok and they were well decoded. This is the TinkerCad output:
A**100
temp A**100
comm A**
para 100
B**101
temp B**101
comm B**
para 100
C**102
temp C**102
comm C**
para 100
D**103
temp D**103
comm D**
para 100
I also tested the code using a physical Arduino uno, and It worked as well as in Tinkercad. This is what I would do:
Let the code only with the communication part, as I did, and see how it behaves. If your code behaves ok only with the communication part, then you have something interfering in the rest of the code. It is hard to tell what could be without knowing the code of the custom clases.
If you are connecting a physical arduino to the usb port of PC, change that cable. Some defective cables may work weird and cause that kind of issues.
If you will be installing your device in a harsh environment, try to make your code resistant to those kind of errors, it is most likely that they will be present anyway. In such scenarios, I have used a starting character, an ending character and a fixed length, something like #A**100;#B**101; where # is the starting character, the semicolon (;) is the ending character, and all the commands are eight characters long. I discard all of the commands that does not fulfill those params. Using CRC is another option, but in most cases it is enough with the simple pattern I just described.