Baud rate calculation

[ghostman11]

can anyone explain in idiot language how you go about calculating what baud rate to use with the usart on a pic. its something i just dont seem able to get my head around thanks guys

 

[ghostman11]

thought i should add an edit...... the code i have is

OpenUSART ( USART_TX_INT_OFF &
USART_RX_INT_OFF &
USART_ASYNCH_MODE &
USART_EIGHT_BIT &
USART_CONT_RX &
USART_BRGH_LOW,51);

i am using internal osc at 8mhz and would like 9600 baud rate.
i found this calculator online (its free) micromagic systems - home

but i dont seem to be doing something right as i just get rubish on the screen. hence the reason for asking someone clever how its done

 

[blueroomelectronics]

Nice link, I recall there is a table in may of the PIC datasheets and a formula too.

 

[ghostman11]

ive been looking at the datasheet but nothing is sinking in tonight so i am going to leave it untill tommorow! is frustrating when i get dense days like this

 

[blueroomelectronics]

Since there's a calculator I'd just use that. Who likes reading tables.

 

[ghostman11]

ive used the calculator but just get rubish on the terminal. probaly best wait till i have had some sleep

 

[Mr RB]

The calcs look scary in the datasheet but it's actually pretty easy.

Depending on bit BRGH you divide the xtal clock by 16 or 64.

So for a 4 Mhz xtal, BRGH=1, to get 9600 baud;
4000000 / 64 / 9600 = 6

For a 4 Mhz xtal, BRGH=0, to get 9600 baud;
4000000 / 16 / 9600 = 26

Obviously 26 is the better choice (better resolution). Then the last task is you always subtract 1 from the result;

so 6-1 = 5
or 26-1 = 25

That is the value you use in the BRG.

 

[ghostman11]

thanks RB thats alot clearer than a datasheet

 

[tunedwolf]

Common baud rates are usually listed in the tables in the device datasheets. For example, in the PIC16F877A datasheet, 9.6Kbaud SPBRG is listed as having a value of 25 with 0.16% error at 4Mhz in Asynchronous mode (SYNC=0).

RB has explained how to quickly and easily calculate the SPBRG value for any Asynchronous baud rate.

Obviously for Synchronous mode you simply divide the xtal freq by 4 then your desired baud rate and subtract 1 from the result (SPBRG = xtal freq / 4 / desired baud - 1).
In either case, you obviously want to calculate for the lowest error to guarantee framing stays put.

 

[ericgibbs]

thanks RB thats alot clearer than a datasheet

hi J,
Download the free PicMultiCalc program...

 

[ghostman11]

thanks for that one eric i will also add that to my other thread