Another question regarding rs232

[Alan]

Good day,
I need to use an rs232 at baud rates higher than 115200. As far as i see, the compiler doesn't allow me to exceed 115200, while, for what i know, writing in assembler, could have made it possible. Does anyone know a way to overcome this problem still using the ccs compiler?
Thank you in advance,
Alan

[asmallri]

If the restriction is real then it is still easy to do.

[Ttelmah]

It depend on your master crystal rate.
The following:

[Alan]

Thank you both very much. I'll try and let you know.
Alan

[Sigma]

Hi, Ttelmah,

If I am using a 4Mhz crystal and 16F76 device, the following statement is allowed by the compiler.

#use delay(clock=3790000)
#use rs232(baud=115200, parity=N, BITS=8, xmit=TXD, rcv=RXD) //Hardware UART

What will be the consequences as a result of inconsistency of frequency?

regards
Sigma

[Ttelmah]

It won't work.
The compiler will solve the divider as:

(3790000/115200)/4 = 8.22

Hence it'll select /8 as the divider (7 into the setup register).

With your real crystal, this will give:

4000000/(4*8) = 125000bps.

Error = 125000/115200 = 1.0857*.
8.57%

Now, the error that can exist without problems, depends on the sampling algorithm of the receive UART, the word length etc. etc., but in general, 3 to 4% is a 'good guide', with 5.5% being the absolute upper limit for most systems. This works out, because the simpler chips generally sample in the middle of the bit time. For a 10 bit transmission (1 start bit, 8 bit data, and one stop bit), with sampling only occurring on the data bits, if the system starts counting from the leading edge of the start bit, it'll go outside the data bits, if the total timing error exceeds this.
The CCS default, is not silly. It tells you if your hardware _can_ handle the speeds required, and though in a few small circumstances, 'tricking' it, may be worthwhile, in general, if the compiler says the rate won't work, it won't...

Best Wishes

[Sigma]

Thanks. Ttelmah.
That means if the baud rate is fixed, lets say, 115200bps. We had to choose the correct crystal?

regards
Sigma

[Ttelmah]