Serial Interface

[el98650]

Posted on March 25, 2003 at 06:16

Serial Interface

[el98650]

Posted on March 24, 2003 at 12:25

Hello,

I am trying to set the baudrates for the st10f269 to something higher then 57600. Obvisouly since the st10 runs at 20 MHz the deviation is too high and i communication over the serial is erronous. Is there a way i can set the baudrate to 230400 or higher(anything higher) without having to use an external crystall.

Thanks

[maxime2399]

Posted on March 24, 2003 at 17:15

Hello Dimos,

Basically there is only two choices.

If you look at the baud rate formulas (chapter 4 of the user's manual) you have only two choises according to the bit (SOBRS):

Code:

(SOBRL) = (Fcpu / {16 x [ 2 + (SOBRS)] x Basync})- 1

with Fcpu=20MHz and Basync=57600bauds we have

either (SOBRL) = 9.85 rounded to 10 AND (SOBRS) = 0

or (SOBRL) = 6.23 rounded to 6 AND (SOBRS) = 0

If we use the rounded value in the backward formula (same page)

Code:

Basync = Fcpu / { 16 x [ 2 + (SOBRS)] x [ (SOBRL) + 1]}

we get:

for ( S0BRS=0; S0BRL=10) we have Basync = 56818Bauds ( ~57600 - 1.376% deviation) and

for ( S0BRS=1; S0BRL=6) we have Basync = 59524Bauds (~57600 + 3.232% deviation)

Those values are close enough to establish a communication with most of the UART compatible cell available on the market.

If your application require a very precise baud rate, you may have to use a specific crystal (baud rate crystals).

Using the same method, we can calculate the closer speed to 230400 bauds.

The closest we can go with a 20MHz clock is 208333

( - 9.6% frequency deviation) with either ( SOBRL=2; S0BRS=0) or ( S0BRL=1; S0BRS=1)

The higher the baud rate, the bigger the deviation can be...

The maximum speed of the cell at 20MHz is 625kBauds with ( SOBRS=0 ; S0BRL=0).

[ This message was edited by: Max on 24-03-2003 21:50 ]

[ This message was edited by: Max on 24-03-2003 21:54 ]

[ This message was edited by: Max on 24-03-2003 21:57 ]

[ This message was edited by: Max on 24-03-2003 21:59 ]

[el98650]

Posted on March 25, 2003 at 06:16

Thanks for the fast reply..

I am aming at the 230400 baudrate, and 9% seems too high , since the bits are intepreted as different characters...

Seems like i have to use an external crystall

Thanks again