Help connecting custom ATMega328PB using serial FTDI

Question

I built this ATMega328PB circuit. It runs at 3.3v from 1.5v battery using booster and 8MHz from external resonator (CSTCE8M00G52A-R0).

Using USBTinyISP I set fuses: Low-FF (for 8MHz), High-D8 (Boot Reset Vector Enabled), Ext-FD.

Using VisualStudio and vMicro plug-in I compiled a program for UNO. vMicro created a HEX file of my program WITH a bootloader which I uploaded over ISP and it runs well (blink the LED).

My final goal is to make a customized 8MHz bootloader but as intermediate steps I am trying to get a sign of life over serial using FT231X FTDI, and than upload a sketch over it.

I cut the 5v jumper on FT231X and connected 3.3v jumper instead. Then connected (without a battery):

ATmega328   FT321X
---------   ------
Vcc         Vcc 
GND         GND
GND         CTS
RST         DTR (over 0.1uF)
RxD         TX
TxD         RX

Since the default UNO bootloader UBRR register is set for 16MHz, I initialize it manually and try to push data over serial.

#define myubbr (8000000/16/9600-1)
void setup()
{
    pinMode(6, OUTPUT);
    // Init serial
    UBRR0H = (unsigned char)(myubbr >> 8);
    UBRR0L = (unsigned char)myubbr;
    UCSR0A = 0;//Disable U2X mode
    UCSR0B = (1 << TXEN0);//Enable transmitter
    UCSR0C = (3 << UCSZ00);//N81
}

void loop()
{
    digitalWrite(6, HIGH);
    delay(100);
    digitalWrite(6, LOW);
    delay(100);

    char * str = "Hello World";
    while (*str) {
        while (!(UCSR0A & (1 << UDRE0)));
        UDR0 = *str++; //send the data
    }
}

On the PC I run: avrdude -v -p m328p -c arduino -PCOM8 -b 9600 Avrdude recognizes the correct com port but gives:

avrdude: stk500_getsync(): not in sync: resp=0x00

The RX and TX LEDs on the FT231X never blink.

I don't even know if the problem is HW or SW. What am I doing wrong. What else can I try?

Answer 1

In such a case, start with a working setup and then work your way step-by-step to your final setup. Like:

• Start with 5V, 16 MHz, existing bootloader, Arduino IDE with 'hello world' sketch.
• Change to 3.3V (lab supply) and 8 MHz. verify (baudrate will be halved)
• use your FTDI-to-serial circuit. verify.
• change to your 3.3V supply. verify.
• change to using your IDE and ISP. verify.

If the step that causes things to beak is still to big, divide it into smaller steps. Divide and conquer! This is the same technique I use to find out what causes a compiler to crash on a 30k lines application. Takes some time, but you'll get there in the end.

Answer 2

Its working. It was ultimately a combination of switching between TX and RX and using half the baud rate of the bootloader on the PC. So first I loaded: setup { Serial.begin(9600); } loop { Serial.write("Hello"); }. hfuse=D9, opened Arduino Serial Monitor with baud rate 4800 and finally got printouts. Then I checked 115200 baud with 57600 on monitor in both TX and RX using read/write loop. Then I burned an UNO bootloader (default baud rate 115200) and set uno.upload.speed=57600 in boards.txt, set hfuse=D8 to run bootloader and now I can upload programs over serial.