Serial Out Queuing, Sending Old Inputs

Projects Programming
1

I'm working on a project where I have a Feather ESP-12S microcontroller running Arduino IDE sending data via serial to a 4D Systems Diablo display (which has an onboard microcontroller) to show graphics, but I've been running into a strange issue.

If I have the TX/RX hooked up when the system is powered up, the serial out is shifted back by one character. As an example, if I was to do:

Serial.write(1);
Serial.write(2);
Serial.write(3);

When I serial read on the Diablo, I get -1, then 1, then 2. If I were to serial read again, I would get a -1. However, the next time I write from the Feather to the Diablo, I would read a 3. The code on the Feather is not stopping anywhere and runs fine.

Interesting to note, if I plug the RX/TX cables in after powering up both screen and microcontroller, this problem doesn't happen.

The code below is what I was using to debug this. Essentially, the Neopixel ring should be green on even numbers and red on odds. My issue is causing these to be switched.

After a startup handshake (sending the value 9 back and forth), the Diablo just takes the value of i and prints it to the display.

Any help would be greatly appreciated, I'm completely stumped.

#include <Adafruit_NeoPixel.h>
#define PIN            14
#define NUMPIXELS      24
Adafruit_NeoPixel pixels = Adafruit_NeoPixel(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);
int i = 0;

void setup() {
    Serial.begin(19200);

//  Wait for serial
  while (! Serial) {
    delay(1);
  }

  delay(3000);
  while (Serial.read() != 9) {
    Serial.write(9);
    delay(100);
  }

  pixels.begin();
  for (int j = 0; j < NUMPIXELS; j++){
    pixels.setPixelColor(j,pixels.Color(0,0,0));
  }
  pixels.show();

}

void loop() {
  Serial.write(i);
  
  i = i + 1;

  if (i%2 == 1) {
    for (int j = 0; j < NUMPIXELS; j++){
      pixels.setPixelColor(j,pixels.Color(10,0,0));
    }
    pixels.show();
  }
  else if (i%2 == 0) {
    for (int j = 0; j < NUMPIXELS; j++){
      pixels.setPixelColor(j,pixels.Color(0,10,0));
    }
    pixels.show();
  }
  
  if(i>9) {
    i = 0;
  }
  delay(3000);
}
2

You didn't include the Diablo code, but...

When I serial read on the Diablo, I get -1, then 1, then 2. If I were to serial read again, I would get a -1. However, the next time I write from the Feather to the Diablo, I would read a 3. The code on the Feather is not stopping anywhere and runs fine.

Arduino (maybe Diablo is the same?) Serial.read() returns -1 if there is no data to read. Are you sure that you are not trying to read data that hasn't yet arrived and you are just getting -1?

3

You didn't show the reading code, but if this is any example...

  while (Serial.read() != 9) {
    Serial.write(9);
    delay(100);
  }

Then you're definitely reading when there's nothing to be read. That's when you get -1.

Learn to use Serial.available()

Also, you could drop the entire while(!Serial) loop. It does nothing, except on Arduinos with native USB such as the Leonardo and Due.

4

At 19200 baud, it takes roughly 500 us for a byte to be transferred. You however seem to read back within that time.

Not familiar with the feather but I assume that you can use Serial.available() to check if data is available to be read.

In a very simple form

void loop()
{
  static byte counter = 0;

  Serial.write(counter);
  while(Serial.available() == 0);
  int reply = Serial.read();

}

Note that this code blocks till a reply is received. The code assumes a single byte to be returned.

5

I'm working on a project where I have a Feather ESP-12S microcontroller running Arduino IDE

I do NOT believe that the ESP12S is capable of running the IDE OR that you have ported the IDE on the ESP12S.

Running code created using the IDE on a PC is a different story.