user3183: I try to send an "a" string via serial with
Serial.println("a");

and I receive "Oy=" in Ascii or {4F}{79}{3D} in Hexa.
Where should I start investigating? What could be wrong?  Baudrate is 9600 on all sides.
PS: I suspect the cable. Any hints how to check it?

user527: On your setup function
Serial.begin(115200);//<- 115200 is the baud rate, serial monitor must match this speed
while(!Serial);//<-- wait for Serial to be ready

see: https://electronics.stackexchange.com/questions/9264/what-standard-uart-rates-are-there for more baud rates

3183: <p>I try to send an "a" string via serial with</p>

<pre><code>Serial.println("a");
</code></pre>

<p>and I receive "Oy=" in Ascii or {4F}{79}{3D} in Hexa.</p>

<p>Where should I start investigating? What could be wrong?  Baudrate is 9600 on all sides.</p>

<p>PS: I suspect the cable. Any hints how to check it?</p>
 527: <p>On your setup function</p>

<pre><code>Serial.begin(115200);//&lt;- 115200 is the baud rate, serial monitor must match this speed
while(!Serial);//&lt;-- wait for Serial to be ready
</code></pre>

<p>see: <a href="https://electronics.stackexchange.com/questions/9264/what-standard-uart-rates-are-there">https://electronics.stackexchange.com/questions/9264/what-standard-uart-rates-are-there</a> for more baud rates</p>
