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We set the LM3S9D92 UART1 as standard RS232 for communication, but find some baudrate are offset some from the value we set.

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Posted by huang hua

on Mar 19 2012 01:57 AM

And we do some calcuation and detection as follows:

MCU:	LM3S9D92/UART1
Work power supply:	+3.3V

Formula(from lm3s9d92.pdf page 692):
BRD = BRDI + BRDF=SYSTEM_CLK/(UART_DIV*BAUDRATE)

BRDF_REG_VALUE = INT(BRDF*64 + 0.5)

Some parameters:
SYSTEM_CLK	50000000	Hz
UART_DIV	16
ONE_BYTE_BITS	10
BRDF_DIVIDER	100
EXTERNAL_CLK	16000000	Hz

	Basic calculation									real value detected by oscilloscope
Standard baudrate	BRD	BRDI	BRDF	BRDF_REG_VALUE	BRD_Select	Get baudrate	Baudrate offset	Baudrate error(%)	one byte error(%)	Detected by oscilloscope	Oscillo – standard	Oscillo – standard(%)	One byte last bit error(%)
1200	2604.17	2604	0.1667	11	2604.1100	1200.03	0.03	0.0022	0.02	1163	-37	-3.08	-30.83
2400	1302.08	1302	0.0833	5	1302.0500	2400.06	0.06	0.0026	0.03	2380	-20	-0.83	-8.33
4800	651.04	651	0.0417	3	651.0300	4800.09	0.09	0.0018	0.02	4762	-38	-0.79	-7.92
9600	325.52	325	0.5208	33	325.3300	9605.63	5.63	0.0587	0.59	9616	16	0.17	1.67
19200	162.76	162	0.7604	49	162.4900	19231.95	31.95	0.1664	1.66	19230	30	0.16	1.56
38400	81.38	81	0.3802	24	81.2400	38466.27	66.27	0.1726	1.73	38460	60	0.16	1.56
57600	54.25	54	0.2535	16	54.1600	57699.41	99.41	0.1726	1.73	57480	-120	-0.21	-2.08
76800	40.69	40	0.6901	44	40.4400	77274.98	474.98	0.6185	6.18	76920	120	0.16	1.56
115200	27.13	27	0.1267	8	27.0800	115398.82	198.82	0.1726	1.73	114940	-260	-0.23	-2.26

The oscilloscope is TPS2014 from Tektronix

Does the offset is normal or not?

If normal, could you give me some reference data?

If not, could you tell me why? Or introduce some FAE in Shenzhen, China to contact us?

We are looking forwared for your reply!

Thanks!

All Replies

Posted by Stellaris Jordan

on Mar 19 2012 10:34 AM

Intellectual800 points

Huang,

To my knowledge, there should not be any offsets to the actual UART baud rate other than those caused by the fractional divider. Can you go into more detail concerning your process for measuring the baud via oscilloscope? It's been a bit of time since I've scoped out a UART line, but my experience has been that it tends to be very close to the calculated offset. I would be interested in taking some measurements myself to verify what you are saying, but I'd like to follow the same procedure for measurement as you are currently using.

I had an additional question about the table of data that you posted as well: I'm not certain what you mean in the last column, that you have labeled "One byte last bit error(%)." Could you go into detail as to what you mean by this?

Also, I'm curious about your intended use for this UART. It was my understanding that most RS232 devices had a tolerance of around 2 or 3%. With the exception of the results you saw on the 1200 baud rate, it seems like the deviations you're observing would be well within that range.

-Jordan

Posted by huang hua

on Apr 15 2012 21:02 PM

Prodigy20 points

Hi Jordan

First the "One byte last bit error(%)." we use the "N,8,1", so one byte actual transmit 10bits time, just forget it.

I use the UART1 and UART2 in one MCU to do communication itself directly, and the results as follows:

	parity/frame
Baudrate	0 = 8, E, 1	1 = 8, O, 1	2 = 8, N, 2	3 = 8, N, 1
0 = 1200	OK
1 = 2400	OK
2 = 4800	OK
3 = 9600	OK
4 = 19200	OK	OK	OK	OK
5 = 38400	NO
6 = 57600	OK*
7 = 76800	NO
8 = 115200	OK*
Note: * sometimes is failed

Connections: UART1_TX to UART2_RX; UART1_RX to UART2_TX;

Could you do the same test as above? Thanks!

By the way, we actual application is UART1+RS485 IC(ADM2587EBRWZ)<==> RS485 BUS <==>RS485 IC(ADM2587EBRWZ)+UART2