Sending >255 bytes array to arduino

Question

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:
 countElementsSent+=1
 ser.write(element)
 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.begin(114000);
 Serial.setTimeout(10000);
 Serial.println("<Arduino is ready>");
 display.begin(SSD1306_SWITCHCAPVCC, 0x3C); // initialize with the I2C addr 0x3D (for the 128x64) 
 display.clearDisplay();
 display.display();
}
void loop() {
 recvWithEndMarker();
 showNewData();
}
void recvWithEndMarker() {
 static byte ndx = 0;
 char endMarker = '$';
 char rc;
 // if (Serial.available() > 0) {
 while (Serial.available() > 0 && newData == false) {
 rc = Serial.read();
 if (rc != endMarker) {
 receivedChars[ndx] = rc;
 ndx++;
 if (ndx >= numChars) {
 ndx = numChars - 1;
 }
 }
 else {
 receivedChars[ndx] = '$'; // terminate the string
 Serial.println("Received: " + String(ndx) + " elements");
 ndx = 0;
 newData = true;
 }
 }
}
void showNewData() {
 if (newData == true) {
 Serial.println("This just in ... ");
 display.clearDisplay();
 drawBitMap(receivedChars,50,50,0);
 display.display();
 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);
 }
}

• jsotola: what does this mean ? it does weird things
• Nico Müller: 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"
• Juraj: const byte numChars = 350;? max byte is 255. and you have ndx as byte too.
• Nico Müller: 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

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.