Arduino code weird return with Serial.read()

Question

Having issues with the Serial.read() command on my Arduino code. I have it connected to two 74HC595's shift registers connected to LED's.

I check if there is Serial data, then read two bytes. Then pass these bytes to a method that shifts them both out. When I check the bytes with Serial.print to print them out to the serial moniter I get for example

Why am I getting the two 255's I have read the documentation on arduino.cc and it says it reads a single byte only. Any ideas?

The end goal is to read two bytes on the serial line and shift them out to the shift reg's IE is the byte values of decimal 5 & 6 were passed the 1st 3rd LED's would light on one shift register then the 2nd and 3rd would on the other shift register

const int dataPin = 8;
const int latchPin = 9;
const int clockPin = 10;

void setup() {
  Serial.begin(9600);
}

void loop() {
  if (Serial.available() > 0) {
    byte low = Serial.read();
    byte high = Serial.read();
    Serial.println(low);
    Serial.println(high);
    sendBytes(low,high);
  }
}

void sendBytes(byte l, byte h) {
  digitalWrite(latchPin,LOW);
  shiftOut(dataPin,clockPin,MSBFIRST,l);
  shiftOut(dataPin,clockPin,MSBFIRST,h);
  digitalWrite(latchPin,HIGH);
}

Hans Passant · Accepted Answer · 2013-07-09 23:31:20Z

7

    if (Serial.available() > 0) {
            byte low = Serial.read();
            byte high = Serial.read();
            //...

This is a bug in your code. Very likely to trip, serial ports are not that fast. So high odds that Serial.available() only returns 1. You'll read the low byte okay but Serial.read() will then return -1 if there is no data to read. Which makes high equal to 0xff. The simple fix is:

    if (Serial.available() >= 2)

edited Jul 9, 2013 at 23:31

answered Jul 9, 2013 at 12:32

Hans Passant

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Comments

hydronics · Accepted Answer · 2015-01-11 22:32:58Z

I had the same problem reading from a program...

I had luck both letting the code loop until filled...

and before taking on new Serial data I flush the serial to get the latest bytes

While(Serial.read() != -1);  //clears data in the PC Serial Port

Here's what it looks like all put together

int i = 0;
byte bytes[3];
interval = 150;  //allows for this amount of time to read from serial
long previousMillis = 0;

void loop() {
unsigned long currentMillis = millis();

if(currentMillis - previousMillis < interval && i < 2) {

   if(Serial.available() > 0) {
   bytes[i] = Serial.read(); 
   i++; }
}

if(currentMillis - previousMillis > 1000) {  //waits 1 second to get new serial data
previousMillis = currentMillis;  //resets the timer
i = 0;
While(Serial.read() != -1);  //clears data in the PC Serial Port
}

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