Hardware rs232 baud rate limit

[Anders Svensson]

Hi,
I am new to PIC, so would appreciate somebody can help.
I am using pic16c73 with 32.768Khz crystal. The pin C6, C7 connect to a maxim rs232 transceiver, then to a DB9 connector.
Can i get 115bps baud rate? What is the maximum baud rate i can get using this crystal and 115kbps rs232 transceiver?
Thanks.
Svensson

[Anders Svensson]

should be: "Can i get 115kbps baud rate?"
Sorry

[SherpaDoug]

With a 32Khz crystal you should be able to get 115 baud, but certaily not 115 kilo baud! You may be lucky to get 1200 baud. Look at the UART section of the datasheet and see what UART divisors are available.
The speed of the transciever only puts an upper limit on the baud rate. It can't help a slow uP clock.
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[Ttelmah]

The 'range', depends slightly on the chip. Latter chips, have a more complex baud rate generator, than the older processors, allowing larger divisors. The rates available on the normal chips, are the (crystal frequency/16)/(1 to 256)
and the same with a 64 for the first divisor.
So for a 32768Hz crystal, the maximum baud rate available, is:
(32768/16)/1 = 2048bps. The slowest rate available, is:
(32768/64)/256 = 2bps.
115bps, can be got 'close', with a primary divisor of 16, and a secondary of 18 (actual value=17 in the internal register), giving:
(32768/16)/18 = 113.7bps, which is only just over a 1% error.
Unfortunately most 'standard' rates are not even divisions from the 32768 crystal, which is generally designed/used for timekeeping functions, rather than serial comms.

Best Wishes