Is the ATMEGA328P's serial baud rate quantised?

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I'm using an ATMEGA328p, running from its internal oscillator (divided by 8 = 1MHz).



I've (roughly) measured the oscillator output, using my Salae logic analyser, as ranging from 960KHz to 1000KHz, so it's not awful. I did this using the "Clock output on PORTB0" fuse.



If I set the baud rate to 9,600, it outputs serial at 10,220 baud. (Is this because I'm not using a crystal, or because of quantisation?)



If I either increase F_CPU or decrease USART_BAUDRATE, gradually, the output serial baud does not decrease, until it jumps to 8,800 baud.



#define USART_BAUDRATE 9600
#define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) 

int main(void) {
 // serial port setup
 UCSR0B |= (1 << RXEN0) | (1 << TXEN0);
 UCSR0C |= (1 << UCSZ00) | (1 << UCSZ01);
 UBRR0H = (BAUD_PRESCALE >> 8);
 UBRR0L = BAUD_PRESCALE;
 ...


Is there some form of quantisation affecting the output baud?



P.S. I'm using GCC on Linux to compile code, and I'm not using Arduino code/IDE.






avr serial rs232 atmega328p







share|improve this question



























    up vote
    1
    down vote

    favorite












    I'm using an ATMEGA328p, running from its internal oscillator (divided by 8 = 1MHz).



    I've (roughly) measured the oscillator output, using my Salae logic analyser, as ranging from 960KHz to 1000KHz, so it's not awful. I did this using the "Clock output on PORTB0" fuse.



    If I set the baud rate to 9,600, it outputs serial at 10,220 baud. (Is this because I'm not using a crystal, or because of quantisation?)



    If I either increase F_CPU or decrease USART_BAUDRATE, gradually, the output serial baud does not decrease, until it jumps to 8,800 baud.



    #define USART_BAUDRATE 9600
    #define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) 

    int main(void) {
     // serial port setup
     UCSR0B |= (1 << RXEN0) | (1 << TXEN0);
     UCSR0C |= (1 << UCSZ00) | (1 << UCSZ01);
     UBRR0H = (BAUD_PRESCALE >> 8);
     UBRR0L = BAUD_PRESCALE;
     ...


    Is there some form of quantisation affecting the output baud?



    P.S. I'm using GCC on Linux to compile code, and I'm not using Arduino code/IDE.






    avr serial rs232 atmega328p







    share|improve this question

























      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      I'm using an ATMEGA328p, running from its internal oscillator (divided by 8 = 1MHz).



      I've (roughly) measured the oscillator output, using my Salae logic analyser, as ranging from 960KHz to 1000KHz, so it's not awful. I did this using the "Clock output on PORTB0" fuse.



      If I set the baud rate to 9,600, it outputs serial at 10,220 baud. (Is this because I'm not using a crystal, or because of quantisation?)



      If I either increase F_CPU or decrease USART_BAUDRATE, gradually, the output serial baud does not decrease, until it jumps to 8,800 baud.



      #define USART_BAUDRATE 9600
      #define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) 

      int main(void) {
       // serial port setup
       UCSR0B |= (1 << RXEN0) | (1 << TXEN0);
       UCSR0C |= (1 << UCSZ00) | (1 << UCSZ01);
       UBRR0H = (BAUD_PRESCALE >> 8);
       UBRR0L = BAUD_PRESCALE;
       ...


      Is there some form of quantisation affecting the output baud?



      P.S. I'm using GCC on Linux to compile code, and I'm not using Arduino code/IDE.






      avr serial rs232 atmega328p







      share|improve this question















      I'm using an ATMEGA328p, running from its internal oscillator (divided by 8 = 1MHz).



      I've (roughly) measured the oscillator output, using my Salae logic analyser, as ranging from 960KHz to 1000KHz, so it's not awful. I did this using the "Clock output on PORTB0" fuse.



      If I set the baud rate to 9,600, it outputs serial at 10,220 baud. (Is this because I'm not using a crystal, or because of quantisation?)



      If I either increase F_CPU or decrease USART_BAUDRATE, gradually, the output serial baud does not decrease, until it jumps to 8,800 baud.



      #define USART_BAUDRATE 9600
      #define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) 

      int main(void) {
       // serial port setup
       UCSR0B |= (1 << RXEN0) | (1 << TXEN0);
       UCSR0C |= (1 << UCSZ00) | (1 << UCSZ01);
       UBRR0H = (BAUD_PRESCALE >> 8);
       UBRR0L = BAUD_PRESCALE;
       ...


      Is there some form of quantisation affecting the output baud?



      P.S. I'm using GCC on Linux to compile code, and I'm not using Arduino code/IDE.






      avr serial rs232 atmega328p




      avr serial rs232 atmega328p






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      edited 3 hours ago

























      asked 3 hours ago









      fadedbee

      493214




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          2 Answers
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          It is a fully digital system. It must operate in discrete steps.



          This is the formula from the datasheet:
          enter image description here



          UBRR register is 16 bits wide (UBRRL + UBRRH), so it can only lead to 65536 possible baud settings.



          Look also at section "24.11. Examples of Baud Rate Setting" and table 24-4 of the datasheet.






          share|improve this answer



























            up vote
            2
            down vote













            If you own a precise logic analyzer you might want to calibrate the internal clock frequency via OSCCAL (Oscillator Calibration Register) in order to get further precision. Anyways, a crystal oscillator is almost always needed on a clean asynchronous communication.






            share|improve this answer




















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              2 Answers
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              2 Answers
              2






              active

              oldest

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              active

              oldest

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              active

              oldest

              votes








              up vote
              2
              down vote



              accepted










              It is a fully digital system. It must operate in discrete steps.



              This is the formula from the datasheet:
              enter image description here



              UBRR register is 16 bits wide (UBRRL + UBRRH), so it can only lead to 65536 possible baud settings.



              Look also at section "24.11. Examples of Baud Rate Setting" and table 24-4 of the datasheet.






              share|improve this answer
























                up vote
                2
                down vote



                accepted










                It is a fully digital system. It must operate in discrete steps.



                This is the formula from the datasheet:
                enter image description here



                UBRR register is 16 bits wide (UBRRL + UBRRH), so it can only lead to 65536 possible baud settings.



                Look also at section "24.11. Examples of Baud Rate Setting" and table 24-4 of the datasheet.






                share|improve this answer






















                  up vote
                  2
                  down vote



                  accepted







                  up vote
                  2
                  down vote



                  accepted






                  It is a fully digital system. It must operate in discrete steps.



                  This is the formula from the datasheet:
                  enter image description here



                  UBRR register is 16 bits wide (UBRRL + UBRRH), so it can only lead to 65536 possible baud settings.



                  Look also at section "24.11. Examples of Baud Rate Setting" and table 24-4 of the datasheet.






                  share|improve this answer












                  It is a fully digital system. It must operate in discrete steps.



                  This is the formula from the datasheet:
                  enter image description here



                  UBRR register is 16 bits wide (UBRRL + UBRRH), so it can only lead to 65536 possible baud settings.



                  Look also at section "24.11. Examples of Baud Rate Setting" and table 24-4 of the datasheet.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 2 hours ago









                  filo

                  5,7101930




                  5,7101930






















                      up vote
                      2
                      down vote













                      If you own a precise logic analyzer you might want to calibrate the internal clock frequency via OSCCAL (Oscillator Calibration Register) in order to get further precision. Anyways, a crystal oscillator is almost always needed on a clean asynchronous communication.






                      share|improve this answer
























                        up vote
                        2
                        down vote













                        If you own a precise logic analyzer you might want to calibrate the internal clock frequency via OSCCAL (Oscillator Calibration Register) in order to get further precision. Anyways, a crystal oscillator is almost always needed on a clean asynchronous communication.






                        share|improve this answer






















                          up vote
                          2
                          down vote










                          up vote
                          2
                          down vote









                          If you own a precise logic analyzer you might want to calibrate the internal clock frequency via OSCCAL (Oscillator Calibration Register) in order to get further precision. Anyways, a crystal oscillator is almost always needed on a clean asynchronous communication.






                          share|improve this answer












                          If you own a precise logic analyzer you might want to calibrate the internal clock frequency via OSCCAL (Oscillator Calibration Register) in order to get further precision. Anyways, a crystal oscillator is almost always needed on a clean asynchronous communication.







                          share|improve this answer












                          share|improve this answer



                          share|improve this answer










                          answered 2 hours ago









                          DrPepe

                          283




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