Raspberry Pi 3 UART with ATmega328p

Question

I'm trying to send data from ATmega328p to Raspberry Pi 3 with UART using Pis gpios.

The problem is Pi does not seem to be getting any data from the microcontroller.

Here is how I wired the microcontroller to Pi:

• ATmega328p VCC to Raspberry Pi 5 volt pin. changed to 3.3 volt pin
• ATmega328p GND to Raspberry Pi ground pin.
• ATmega328p RESET to Raspberry Pi GPIO #12.
• ATmega328p SCK to Raspberry Pi GPIO #24.
• ATmega328p MOSI to Raspberry Pi GPIO #23.
• ATmega328p MISO to Raspberry Pi GPIO #18.
• ATmega328p TXD to Raspberry Pi RX pin.
• ATmega328p RXD to Raspberry Pi TX pin.

Here is the C code for microcontroller:

#define F_CPU 1000000UL
#include <avr/io.h>
#include <util/delay.h>
#define BAUD 9600UL
#define BRC ((F_CPU/16/BAUD)-1)

int main(void){ 
 UBRR0H =(BRC >> 8);
 UBRR0L = BRC;

 UCSR0B = (1 << TXEN0);
 UCSR0C = (1 << UCSZ01) | (1 << UCSZ00);

 while(1){
  UDR0 = '8';
  _delay_ms(1000);

 }
 return 0;
}

And the python 2.7 script for Pi:

import serial


port = serial.Serial("/dev/ttyAMA0", baudrate = 9600, timeout=5)

port.flush()

while True:

    rcv = port.read(10)

    print "AVR is sending" + rcv

With sudo raspi-config I have enabled serial port hardware and made login shell to be not accessible over serial.

EDIT

I've tested if UART pins are any functional by hooking Pi pin 8(TX) to pin 10(RX) and running a python script which I copied from this tutorial http://www.elinux.org/Serial_port_programming .

Here is the script itself:

import serial

port = serial.Serial("/dev/ttyAMA0", baudrate=115200, timeout=3.0)

while True:
    port.write("\r\nSay something:")
    rcv = port.read(10)
    port.write("\r\nYou sent:" + repr(rcv))

The script is running but nothing is appearing on the screen. Seems like the script is unable to write to the port.

I've also double checked that serial is enabled in hardware and serial console is disabled.

In /boot/config.txt enable_uart=1 is set.

Answer 1

With a 1MHz clock speed on the AVR your selected baud rate will not work because the actual baud rate after integer clock division will be at too much of an error compared to your desired baud rate. It will still transmit but your receiver will not recognise it.

It is possible to use 9600 but you need to write a 1 to U2X0 in UCSR0A, you will then need to do BRC*2 as well.

See this site for a useful AVR baud rate calculator.

Edit - Configuring the Serial Port

Dug this out from some of my old documentation as well.

From command line type:

$sudo raspi-config

Go to Advanced Options > Serial interface and select yes to enable, exit settings menu and reboot if prompted.

Type:

$sudo nano /boot/cmdline.txt

Remove the part that reads:

console=serial0,115200

Then save and exit.

Now use a terminal program such as screen or minicom to try and talk to the Pi.