What amount of baud rate error may be acceptable for long time-period on RS232 port?

[buenos]

Hi

What amount of baud rate error may be acceptable for a long time-period on an RS232 serial port?
[%]

In an earlier project, I have used 1% but it has worked rearly. the connection was faulty. after that, I had to replace the microcontroller's (PIC16F877) crystal to a "baud-rate-quarz". then it has worked fine.

Now, I cannot replace the quarz crystal. and I can set the baud rate with a 0.48% error for the required speed. this connection have to be between a TMS320F2811 DSP and a Bluetooth module. Debug is not easy.

Is it depends on the device type?

 

[XNOX_Rambo]

Re: UART baud rate

"Long time" is not really relevant as RS232 is an asynchronous interface - the
reception is synchronised to the start bit.
I know that some MCUs can handle a 3% deviation so you shouldn't have
a problem really.

 

[buenos]

UART baud rate

thanx.
i mean on long time: zero error during an hour.

Added after 28 seconds:

receiving all characters correctly during a time

 

[PaulHolland]

Re: UART baud rate

HI, You can calculate this easy. There are 10 bits in a ASYNC transmission. 8 data bits one stop and one start. The receiver (UART) is re-syncing everytime on the start of a START BIT. so if you want the stop bit to be correctly sampled in the middle of the bit you can see that the maximum error can be: 1/2 BIT in 10 BITS. is 5%. Bo be same stay below 4% + or -

regards,

Paul.

 

[creative_35]

Re: UART baud rate

Slight variation can be done in the crystel frequency by changing the value of the capacitor of CLKIN pin 12pf-33pf.

 

[Davood Amerion]

UART baud rate

In UART usualy we have 10 bits, 1 start bit 8 data bits and 1 stop bit.
First sampling point from high to low transition of stream is in the Tb/2 (half of baud rate period), and second bit sampling point is 3/2 Tb, ...
suppose 10 bits = 100%
If two system clock rates are different, maximum deviation is half of Tb or 5% (last sampling point).
5% value is marginal, and actually maximum value 2%-3% is tolerable.
but if both side clocks isnt precise, maximum 1%-1.5% can be accepted.

Regards,
Davood.

 

[buenos]

UART baud rate

thank you guys