advice needed for rs232 baudrate with #use rs232

fabien.casas · Joined: 28 Nov 2006 Posts: 6

Hi all,

I'm working on a 18F4550-based custom board, using CCS C Compiler v3.249, and I wanted to have some explanations on the #use rs232 directive, concerning the baudrate.

My "problem" is that, to achieve a 19200 bps baudrate, I have to set the rs232 directive like this :

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

I'm not positive of all the possible options on the 18F4550, but this will most likely fix your problem until you can learn more about what you're doing:

Change XTAL to a 20MHZ.
Change #use delay to 20000000 (should match the value of your xtal, unless using PLL)
Change HSPLL fuse to HS
Change baud=8100 to baud=19200 (or whatever the precise baud rate is, that you desire)
Look at some of the other options for the #use rs232 statement, you may want to include them... ERRORS, BITS, etc.

The pros will give more details....

John

Ttelmah · Guest

The 'clock' value, needs to be the frequency the CPU is running at, not the external oscillator. On the 4550, the external [spam] is divided, to generate 4MHz (PLLDIV5), and then multiplied by the internal USB PLL, to generate the 96MHz used by the USB circuitry. This is then fed to the CPU divider, and divided by 2 (CPUDIV1). So your CPU is actually running at 48MHz, not 20MHz. 19200/48*20 = 8000.
Change the clock statement to match the real frequency of your CPU, and things should be fine.
The oscillator settings on these chips, are _complex_!...

Best Wishes

fabien.casas · Joined: 28 Nov 2006 Posts: 6

Ok, thanks John and Ttelmah,

I've taken a closer look at the CPU clock chain.

My external oscillator provides a 24MHz frequency, so I configured
#fuses with:
- HSPLL and USBDIV: because I plan to use USB,
- PLL5: to scale my input frequency down to 4 MHz before the 96MHz PLL, as required,
- CPUDIV1: to scale that 96Mhz frequency down to 48MHz (so the PIC may work...)

I get a CPU frequency of 48MHz.
So I set my "clock" accordingly: