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About RS232 9bit setting and question

 
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sindyme



Joined: 05 Aug 2009
Posts: 37

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About RS232 9bit setting and question
PostPosted: Sun Apr 25, 2010 8:24 pm     Reply with quote
morning everybody

I have question about RS232's 9Bit transmission.

How can I setting this syntax -->
Code:

#use rs232(UART1,baud=115200,parity=N,bits=8,stream=coma)

or
Code:
#use rs232(UART1,baud=115200,parity=1,bits=9,stream=coma) 

( X ) can't build
or
Code:
#use rs232(UART1,baud=115200,parity=odd,bits=9,stream=coma)

( X ) can't build

I need setting 9bit mode and odd.

And ~ if I finished the setting, and how can I get the odd bit only ?

I must get the odd bit to calculate.

Example :
start bit
1 data bit
0 data bit
1 data bit
0 data bit
1 data bit
0 data bit
1 data bit
0 data bit
1 odd bit < ------ I want to get it
stop bit
Ttelmah



Joined: 11 Mar 2010
Posts: 19506

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PostPosted: Mon Apr 26, 2010 2:20 am     Reply with quote
The reason is the hardware...
The PIC UART, supports 8bits, plus one 'extra' bit. Normally this bit is used for parity, _or_ it can be used to give an extra data bit. Problem is there is not a tenth bit available from the hardware to give 9bits plus parity.
So the hardware UART, cannot give 9bits plus parity.
This will work with the software UART, but with the limitations this brings on timings...
Crying or Very sad

In many cases, it doesn't matter. If (for example), you are receiving only, you can set up for 9bits without parity, and the parity bit will arrive when the stop bit ought to be present. Data will still be intact, and the 'OERR' bit may get set (depending on the polarity of the parity bit concerned).
Sending via the software UART, is not so problematical as receiving (especially at high rates), so a 'solution', is to use the hardware UART to receive, either just ignoring parity, using 'ERRORS', and letting the compiler automatically clear the OERR bit, or writing your own code, to work out whether the parity bit should be high or low, and testing if OERR is set, to 'test' for parity errors, then clearing this. Then use a software UART for the TX side.

So, the PIC hardware does not support 9bits plus parity (it is a rare combination).

Best Wishes
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