About "Baud rate out of range"

[mitice]

HI, everyone
I encountered a problem about setting the baud rate of rs232.

[temtronic]

I don't use that PIC but an option called FRC sounds like you're trying to use an internal Resistor-Capacitor module as the 'master oscillator', which has very tight limits of use.

Read the datasheet section that contains the oscillator information as well as the UART setup chapter.
There are several charts,diagram, text that will explain valid combinations of 'clock' vs. baudrates.

Your answer lies in there.

[Ttelmah]

The 'baud rate out of range' error, comes both when the baud rate is beyond the acceptable limits, but also when the baud rate that would be generated, has an error that is too large for reliable operation.

Now, look at equation 16-1 in the data sheet. Try solving with a 10MHz master clock, and 115200baud. Nearest BRG values are, 4 & 5, giving baud rates of:

= 10000000/(16*(4+1)) = 125000bps
and
= 10000000/(16*5+1)) = 104167bps

Errors of 8.5%, and 10.5% respectively!....

Maximum allowable error, is <5% maximum _total_ at both ends and CCS typically allows something like 3% at the PIC before complaining.

Do the same calculation with 7.37Mhz, and you get a BRG value of 3, giving:

= 7370000/(16*(3+1) = 115156bps

An error of less than 0.3%!....

The 115200 bps value, with a 10MHz clock is 'in range' (in the sense that the BRG, can generate clocks 'round' this), but out of acceptable tolerance, and this is what the compiler is telling you.

This is actually one reason for the odd sounding 7.37MHz clock value. It gives very good division ratios with standard baud rates.

A crystal like 9.216MHz, will give you nice accurate baud rates.

Best Wishes

[asmboy]

I use 14.7456 mhz ext oscillators all the time for high baud rate requirements. Gives perfect baud divisors at any standard rate
and is a benefit when I need to cram in an extra software based RS-232
channel too.

[mitice]

Ttelmah,
Thanks a lot for your answer. If I want this pic work under these condition:
UART baurd rate:115.2K & CAN BUS bit rate 500K.
What frequency of the master clock you recommend ?

[Ttelmah]

I think you will need to use 22MHz.
That gives perfect 500K Can bus timing, and on the UART, gives a BRG value of 11, giving just a 0.5% error (114583bps). Assuming you are running off 5v, and have the -30I version of the chip, this meets your needs. However if you need lower speed operation, you will need to think again. If you could accept 57600bps then 11MHz, would meet the timing requirements.

Best Wishes

[asmboy]

this is - ironic - and crazy coincidence?

i have several thousand surplus SMT HC149 low profile crystals on reels from a discontinued product that was designed for both canbus and RS-232 baud generation.

the frequency i selected was 22.1184 MHZ - 18pf phaser caps
such that 115200 baud was 0% error and
canbus 500000 was .55% error

it was the best compromise at the time of that design and i have several thousand of these on reels that will probably have to be buried with me LOL ;-))