I am trying to send a 50x50 bitmap bytearray from my python client to arduino where it should be displayed on a screen.

I need to receive 350 bytes to have the data to show the bitmap but as soon as i initialise the array with > 255 elements it does weird things (i guess because of the memory):

When initialising the array with size 254 it does the expected:

Transmission of 350 completed in 2.805466s (arduino) Received: 253 elements

with size 350 I only get

"Received: 93 elements"

How can i split the bytes i receive with the available memory and then build the bitmap accordingly or what is a better way to do it? I would like to have it transmitted via serial rather than building the qrcode in arduino (as i couldn't get the libraries required working in my setup).

My python client does basically this:

for element in qrCodeBytes:
    print("Sending " + str(element))
ser.write("$".encode()) #stop char

My arduino code is based on the popular Serial input basics guide https://forum.arduino.cc/index.php?topic=288234.0

 const byte numChars = 254;
 byte receivedChars[numChars]; // an array to store the received data

boolean newData = false;

void setup() {
 Serial.println("<Arduino is ready>");
 display.begin(SSD1306_SWITCHCAPVCC, 0x3C);  // initialize with the I2C addr 0x3D (for the 128x64) 

void loop() {

void recvWithEndMarker() {
 static byte ndx = 0;
 char endMarker = '$';
 char rc;

 // if (Serial.available() > 0) {
   while (Serial.available() > 0 && newData == false) {
     rc = Serial.read();
     if (rc != endMarker) {
       receivedChars[ndx] = rc;
       if (ndx >= numChars) {
         ndx = numChars - 1;
     else {
     receivedChars[ndx] = '$'; // terminate the string
     Serial.println("Received: " + String(ndx) + " elements");
     ndx = 0;
     newData = true;

void showNewData() {
 if (newData == true) {
 Serial.println("This just in ... ");
 newData = false;

void drawBitMap(unsigned char *img, int width, int height, int position){
  if (position ==0){
   display.drawBitmap(128/2-(width/2), 64/2-(height/2), img, width, height, 1);
  } else if (position == 1){
   display.drawBitmap(128-62,1, img, width, height, 1);
  } else if (position == 2){
   display.drawBitmap(0,5, img, width, height, 1);
  • what does this mean ? it does weird things
    – jsotola
    Jul 25, 2018 at 16:57
  • Sorry i will clarify in the post, if i initialise the array with size 254 it does the expected: Transmission of 350 completed in 2.805466s (arduino) Received: 253 elements if i initialise the array with 350 i get "Received: 93 elements" Jul 25, 2018 at 17:02
  • 1
    const byte numChars = 350;? max byte is 255. and you have ndx as byte too.
    – Juraj
    Jul 25, 2018 at 17:19
  • Thanks for pointing out the byte issue. I found out that the library i am using from adafruit does only support bitmap data located in program memory using the PROGMEM directive. I assume there is not much i can do now with this, i'll have to look for other means of creating this qrcode Jul 25, 2018 at 20:00

1 Answer 1


You use type byte for the variable ndx. Type byte is 8 bits, so the max value is 255. The variable ndx is incremented and after 255 it continues counting from 0.

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