# Microcontroller UART - acting bizzarely

I'm using ATMEGA328P-PU and Pololu PGM03A (USB hardware programmer). I had a very simple code that was sending an 8-bit value back to my computer:

#include <avr/io.h>
#include <util/delay.h>

#define F_CPU 8000000UL
#define BAUD  9600

int main( void )
{
// BAUD RATE (bits per second)
UBRR0L = ( ( F_CPU / 8 / BAUD ) - 1 ); // 01100111

// CONFIGURATION REGISTERS
UCSR0B = 0b00001000; // enable transmitter (TXEN0)
UCSR0C = 0b00000110; // set data frame size (8-bit)

while( 1 )
{
UDR0 = 0b11110000;
_delay_ms(20);
}

return 0;
}


I can read the data using the following command:

xxd -c 2 -b /dev/ttyACM2


Every now and then a very strange thing happens - without any changes in the code, suddenly I'm getting zeros:

00019d40: 00000000 00000000  ..
00019d42: 00000000 00000000  ..
00019d44: 00000000 00000000  ..
00019d46: 00000000 00000000  ..
00019d48: 00000000 00000000  ..
00019d4a: 00000000 00000000  ..
00019d4c: 00000000 00000000  ..
00019d4e: 00000000 00000000  ..
00019d50: 00000000 00000000  ..
00019d52: 00000000 00000000  ..
00019d54: 00000000 00000000  ..
00019d56: 00000000 00000000  ..
00019d58: 00000000 00000000  ..
00019d5a: 00000000 00000000  ..
00019d5c: 00000000 00000000  ..
00019d5e: 00000000 00000000  ..
00019d60: 00000000 00000000  ..
00019d62: 00000000 00000000  ..
00019d64: 00000000 00000000  ..
00019d66: 00000000 00000000  ..
00019d68: 00000000 00000000  ..
00019d6a: 00000000 00000000  ..
00019d6c: 00000000 00000000  ..
00019d6e: 00000000 00000000  ..
00019d70: 00000000 00000000  ..
00019d72: 00000000 00000000  ..
00019d74: 00000000 00000000  ..


But there should be 1111000. I noticed that:

• This happens after reboot
• Switching USB ports take no effect
• Switching micro-controller produces the same problem. Even when I tried to write a simple code that simply blinks the light to a blank micro-controller, I can see the LED blink, but I still get 00000000's on serial port, but there should be no communication what so ever.
• Normally the data is sent at fairly slow rate (when it's sending the proper data), but now the output is very fast (when it sends the 00000000's )

Any tips on what's going on? Thanks!

• UBRR0L = ... doesn't look good; should be UBRR0 = .... – JimmyB Aug 3 '16 at 16:09
• You're not setting UCSR0A, specifically U2X0, so you're not using double-speed mode; hence, you need to divide UBRR by 16 instead of 8: UBRR0 = ( ( F_CPU / (16 * BAUD )) - 1 ); As it is, you would be outputting at 19200bps which may confuse the receiver (framing errors...). – JimmyB Aug 3 '16 at 16:15
• You can also do yourself a favor and replace things like UCSR0C = 0b00000110 with more meaningful (and less error-prone) statements, like UCSR0C = (1<<UCSZ01) | (1<<UCSZ00);. – JimmyB Aug 3 '16 at 16:17
• "but I still get 00000000's on serial port, but there should be no communication what so ever" - You need to make sure that the µC's TX pin is not floating while it is connected to the serial adapter. Configure it as output or turn on the internal pull-up to avoid "random" output from that pin. – JimmyB Aug 3 '16 at 16:20
• (No, I don't really think that UCSZ01 looks "meaningful", but at least one can refer to the datasheet to see what the intention is :)) – JimmyB Aug 3 '16 at 16:22

sudo stty -F /dev/ttyACM2 9600 raw -echo