by Olin Lathrop [olin_piclist at COGNIVIS.COM]
[This is] a snippet from an include file containing a much larger collection of macros and such, so you need to set up some assembler state before calling the UART_BAUD macro. It looks like it needs FREQ_OSC, which is the oscillator frequency in Hz, and FAM_17 which is 1 on 17C family devices and 0 elsewhere.
I've used this a number of times with 16C devices successfully. I've just recently modified it to handle 17C devices but don't have target boards to test yet. I can't therefore absolutely promise I didn't introduce a bug, but you can't complain for the price...
;******************** ; ; Macro UART_BAUD <baud> ; ; This macro sets assembler variables to indicate the best baud rate ; generator configuration for the USART in asynchronous mode, given the ; desired baud rate and the oscillator frequency. No code is generated. ; The following assembler variables are set: ; ; VAL_SPBRG - Value for the SPBRG baud rate generator register. ; ; BAUD_REAL - Real (not desired) baud rate resulting from the ; selected settings. ; ; VAL_TXSTA - Value for the TXSTA regisIB4 In addition to ; selecting the proper baud rate generator configuration, this ; value also selects the following: ; ; Asynchronous mode ; 8 bits per character ; transmitter enabled ; ; This macro produces an assembly time warning if the closest available ; baud rate is more then 2.9% from the desired baud rate (off by 25% of ; a bit time in the middle of the last bit). It produces an assembly ; error if the baud rate error is 5.8% or higher. ; uart_baud macro baud local err ;baud rate error in parts per 1000 ; ; Init values assuming will use high speed mode. ; if fam_17 ;17Cxx family doesn't have high speed mode val_txsta set (1 << txen) ;set TXSTA register value else val_txsta set (1 << txen) | (1 << brgh) ;init TX config for high speed mode endif val_spbrg set freq_osc * 16 / baud - 256 ;find divider value, 8 fraction bits val_spbrg set (val_spbrg + 128) >> 8 ;round and scale to final SPBRG value baud_real set freq_osc / (16 * (val_spbrg + 1)) ;find actual selected baud rate ; ; Switch to low speed mode if the baud rate generator value would require ; more than 8 bits to represent, or this chip has not high speed mode. ; if (val_spbrg > 255) || fam_17 ;too slow for high speed mode or no HS ? if ! fam_17 val_txsta set val_txsta & ~(1 << brgh) ;disable high speed mode endif val_spbrg set freq_osc * 4 / baud - 256 ;find divider value, 8 fraction bits val_spbrg set (val_spbrg + 128) >> 8 ;round and scale to final SPBRG value if val_spbrg > 255 ;clip at largest allowable baud rate generator value val_spbrg set 255 endif baud_real set freq_osc / (64 * (val_spbrg + 1)) ;find actual selected baud rate endif err set (1000 * (baud_real - baud)) / baud ;baud rate err in parts/1000 if err < 0 err set -err ;make absolute value of error endif if err > 58 error "Baud rate error exceeds 5.8%" endif if err > 29 messg "WARNING: Baud rate error exceeds 2.9%" endif endm
|file: /Techref/microchip/autobaud-ol.htm, 9KB, , updated: 2013/7/22 17:08, local time: 2019/1/19 14:00,
|©2019 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?|
<A HREF="http://techref.massmind.org/techref/microchip/autobaud-ol.htm"> PIC Specific RS232 Setup Macro</A>
|Did you find what you needed?|
Ashley Roll has put together a really nice little unit here. Leave off the MAX232 and keep these handy for the few times you need true RS232!
Welcome to techref.massmind.org!