USB to RS485 Converter with PIC18F2455

[Fabri]

Hi To All,

I'm working on USB to 485 converter based on ex_usb_serial.c example.

There's no problem with UART baudrate of 9600 or more but the compiler give me Baud rate error when I set to speed to 2400. This happend with hardware UART, there's no problem with FORCE_SW option.

I use crystal of 16Mhz and this settings:

[kender]

[Neutone]

[Fabri]

Hi,

I supposed to use slow frequency crystal.

Thanks for help.

Regards,
Fabri

[Ttelmah]

This gets more complex than it appears.
On these chips, the baud rate, is _not_ directly determined by the crystal frequency. Instead it is determined by the CPU 'core' frequency, which can be derived from a whole series of sources (internal oscillator, secondary oscillator on T1, the USB PLL, or directly through a frequency divider from the master clock), while the crystal is at a different rate in many cases. The 'Fosc' used by the UART, is the line marked 'peripherals', coming out the right hand side of the clock diagram.
Then the available dividers are larger than on the older chips.The BRG, supports 16bit operation, and there is a /64 prescaller, as well as the normal /4, and /16 values. This allows (with the 48MHz maximum clock), rates down to 11.44bps...
What matters though, is the compiler version. The latest compilers 'know about' the extra features of the UART on these chips (4.051, compiles the USB sample, with the hardware UART rate set to 100bps fine), while 3.249, will complain about the baud rate range.
One answer, is to select a higher rate, and write a simple function to enable the 16bit divider yourself.

Best Wishes

[Fabri]

Thanks Ttelmah for explain me how compiler work.

My compiler version is 3,349 and, at the moment, I don't think to upgrade it to new version. I forced PIC to software UART and all work fine. I'll work around in second time.

Regards,
Fabri