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Proper syntax for #use rs232 function

 
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jlucas



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Proper syntax for #use rs232 function
PostPosted: Sun May 08, 2011 7:43 pm     Reply with quote

This question is so simple that it seems no one has even had to ask it before. I'm trying to send 8-bit data from a PIC 18F4520 to a Laird/Aerocomm AC4790 RF transceiver module at 57,600 baud. The default state of both the send and receive pins is HIGH. The AC4790 module requires a low start bit, then 8 data bits, then a low stop bit. It sounds pretty straightforward, but I've searched the forum for about 3 hours, and I can't find the answer.

If I had designed this function, what I want to do would look like this:

Code:
#use rs232(baud=57600, xmit=PIN_D4, rcv=PIN_C2, start=0, bits=8, stop=0)


I know that this syntax is incorrect since there is no such option as "start." However, I believe my pidgin syntax makes it pretty clear what I'm trying to do. Could some kind soul please provide me with a line of code that will work?

Thanks in advance,

Jim
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PostPosted: Sun May 08, 2011 7:53 pm     Reply with quote

Quote:

The AC4790 module requires a low start bit, then 8 data bits, then a low
stop bit.

#use rs232() does this by default. You don't have to do anything extra
to get this.
jlucas



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PostPosted: Sun May 08, 2011 7:55 pm     Reply with quote

Thanks for the quick response. This sounds almost too good to be true, but I'll certainly give it a try! Very Happy
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temtronic



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PostPosted: Mon May 09, 2011 5:29 am     Reply with quote

Also be aware that USE RS232(...) requires a MAX232 or equal by default.

And.. add the 'errors' option to it as well.
SherpaDoug



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PostPosted: Mon May 09, 2011 6:15 am     Reply with quote

If you use a software UART there is the "invert" option.
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PostPosted: Mon May 09, 2011 11:50 am     Reply with quote

The AC4790 chip uses a direct, non-inverted, TTL-level connection to the PIC.
It doesn't need a Max232 chip and he doesn't need the INVERT option.

http://www.lairdtech.com/Products/Wireless-M2M-and-Telematics-Solutions/Proprietary-Radio-Modules/DataSheets/AC4790-0209/

http://www.lairdtech.com/DownloadAsset.aspx?id=2548

Also, he is using a software UART (since he's using pins D4 and C2) so
the ERRORS directive doesn't do anything. It's ignored by the compiler.

He didn't give the exact dash number of the AC4790 that he's using, and
different versions can run at 5v or 3.3v. So depending upon the Vdd
voltage of the PIC, he could have an issue with the logic voltage levels.
jlucas



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PostPosted: Mon May 09, 2011 11:57 am     Reply with quote

Embarassed Oops! I made a mistake in my post. The start bit is low, but the stop bit is high. If a low stop bit is the default, does that mean I need this?
Code:
#use rs232(baud=57600, xmit=PIN_A5, rcv=PIN_B3, bits=8, stop=1)

Will that still give me a low start bit if the default state of the line is high?

I'm using the AC4570-200-01 which runs on +5V. I checked the voltage output from the chip, and it's good.

Thanks again.
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PostPosted: Mon May 09, 2011 12:13 pm     Reply with quote

CCS uses the standard signaling methods. The AC4790 expects the
standard methods. You don't have to do anything.

Also, look on page 18 of the AC4790 manual. It shows the Stop bit is
at a high level, which is the standard.

And "Stop=1" just tells the PIC to transmit 1 Stop bit (it could be set to
send 2 stop bits).
jlucas



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PostPosted: Mon May 09, 2011 12:31 pm     Reply with quote

Thanks again! Wink
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