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Stop bits in RS232 from GPS receivers.

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Diver300

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When I use a USART in a PIC, the choice is 1 or 2 stop bits.

However, in the SiRF reference manual, this is shown:-

Code:
Message ID	$PSRF100	PSRF100 protocol header
Protocol    	0             	0=SiRF binary, 1=NMEA
Baud         	9600        	1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200
DataBits   	8               8,7
StopBits   	1              	0,1
Parity        	0               0=None, 1=Odd, 2=Even
Checksum	*0C
<CR> <LF>               	End of message termination

That seems to show that there is a choice of 0 or 1 stop bits.

I don't see how 0 stop bits would work. Also, on the two types of GPS modules that I am using, there seem to be two stop bits. In the image, you can see the $ and G from the start of a GPS string, and there is a high period of two bits at the end of each.

Can anyone explain why the number of stop bits seems wrong?
 

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  • GPS_NMEA.GIF
    GPS_NMEA.GIF
    22.9 KB · Views: 133
Guessing a bit here, but I reckon the comment should be clearer and should say something like:

Code:
StopBits   	1              	0 = 1 stop bit, 1 = 2 stop bits

I agree that 0 stop bits would make no sense for asynchronous serial communication.
 
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