Setting prescaler for 8MHz external oscillator in Atmega2560

Question

I am relatively new in Atmega MCU's (too used with graphical interfaces). I am using an external clock with 8MHz in my Atmega2560. My problem is that I cannot use UART communication with PIC18->Atmega2560 and STM32F4->Atmega2560 (no acknowledgements).

I thought that it is my baud rate that is not right, maybe the Atmgea2560 is using prescaler div by 8 in default (not familiar with Atmega). So, I forcely changed the prescaler to div by 1 using this little code:

void setup() {

  noInterrupts();
  CLKPR = 0x80; /*Enabling the clock prescaler function*/
  CLKPR = 0x00; /*Setting the prescaler to div by 1*/
  interrupts();
}

I also use this one in the bootloading/programming:

2560.menu.clock.8MHz_external=External 8 MHz
2560.menu.clock.8MHz_external.upload.speed=57600
2560.menu.clock.8MHz_external.bootloader.low_fuses=0xf7
2560.menu.clock.8MHz_external.build.clkpr=
2560.menu.clock.8MHz_external.build.clock_speed={build.f_cpu}
2560.menu.clock.8MHz_external.build.f_cpu=8000000L

Now, when setting the Atmega2560 Serial, Serial1, Serial2, and Serial3 to 9600 baud rate and PIC18/STM32F4 to 9600 baud rate, can my problem in UART be vanished (ack's will receive)? -theoretically

I cannot verify it now since it is weekend, things are in the office.

To those who are expert in these things, your help is much appreciated.

Regards, Tim

Answer 1

Datasheet recommends baud rate error below 2% under standard 16x oversampling mode. So with a 8 MHz clock, 57600 is not achievable, error is more than 3.5%, which is too close to the max total error of 4.5%.

Now, using U2X mode for only 8x oversampling, 8MHz clock can achieve 57600 with error of only 2.1%. Unfortunately it still exceeds the recommended error of 1.5% of U2X, even thouh it is less than max total error of 3.9%.

So, 57600 is not achievable with 8 MHz clock, it is not recommended by manufacturer.

However 38400 and many other baud rates are achievable with much below 1% error.

If you are able to change the clock to something more suitable like 7.372800 MHz, it is possible to achieve bit rate of 230400 with 0% error.