PIC Specific RS232

There has been a lot of confusion on how the RX or TX pins should be set with reguard to TRIS on the PICs with hardware USARTs especially with reguard to makeing them work in interrupt mode.

First, keep in mind that MPLAB does not (correctly or otherwise) simulate the USART Hardware.

Second, read the datasheets.

From DS30292B-page 33 (the '873 datasheet section 3.3 PORTC and TRISC Registers):

When enabling peripheral functions, care should be taken in defining TRIS bits for each PORTC pin. Some peripherals override the TRIS bit to make a pin an output, while other peripherals override the TRIS bit to make a pin an input. Since the TRIS bit override is in effect while the peripheral is enabled, read-modify-write instructions (BSF, BCF, XORWF) with TRISC as destination should be avoided. The user should refer to the corresponding peripheral section for the correct TRIS bit settings.

From DS30292B-page 95 (the '873 datasheet section 10.0n on USART):

Bit SPEN (RCSTA<7>) and bits TRISC<7:6> have to be set in order to configure pins RC6/TX/CK and RC7/RX/DT as the Universal Synchronous Asynchro-nous Receiver Transmitter.

In Microchip lingo "set" means make the bit a 1.

The bottom line, don't do read-modify-write operations on a TRIS register that is controlling pins with another active peripheral, such as the TRISC register when using the UART or CCP.

Finally, PICUART.ZIP from http://redrival.com/mcgahee/  or http://mcgahee.freeservers.com is known to work and explains most of the common problems.

Samples:

• Etienne's version
• Sony IR remote to serial via 16F84
• Tony Nixon's version 16C74
• Tony Kübek's version 16F876 (usart hardware, interrupts)
• Claudio Rachiele's version 16F877
• from B. Knudsen Data (CC5X Version 3.0E asm and C) with parity support.
• Ken Websters PIC16C74 serial port multiplexer. This includes a packet system with addressing, control, checksums, etc... Actually almost a network (OSI 3) system.
• Thomas C. Sefranek's 16C73 USART Serial Port Gotcha List and Source
• Lea's code for 9600Baud with a 4Mhz crystal and no interrupts
• Mike Harrisons code snippet for 115.2kBaud with a 4.096Mhz crystal
• Regulus Berdins code snippet for 500kBps with a 4Mhz crystal

rs232@ TTL to/from RS232@ TI Calc@ PIC C IO@

See also:

• http://www.certi.ufsc.br/~cae/irv01.c
• http://www.microchip.com/10/Faqs/tSupport/PICmicro/USART/index.htm
• http://redrival.com/mcgahee/  or http://mcgahee.freeservers.com PICUART.ZIP contains a heavily commented assembly language program/document in MPLAB format that details how to use the PIC16C74's built-in UART

"Serial PIC'n : PIC Microcontroller Serial Communications" by David Benson

Code:

• In the clock, UART, etc. VP's, the assumption is that the processor is
  running at 50 Mhz.  The constants that are used are:
  
  int_period = 163		;period between interrupts
  ;
  tick_lo = 80			;50000 = msec instruction count
  tick_hi	= 195			; for 50MHz, turbo, prescaler=1 
  
  To convert these for use at 4 Mhz, change the constants to:
  
  int_period = 104		;period between interrupts
  ;
  tick_lo = 160			;4000 = msec instruction count
  tick_hi = 15			; for 4MHz, turbo, prescaler=1
  
  That's all you need for the clock/timer VP's.  For the UART VP, here
  are the settings for 2400 and 1200 BPS.  (at 4Mhz you can't go higher
  than 2400, and I didn't bother to figure out anything lower than 1200)
  
  ; *** 2400 baud (at 50MHZ)
  ;baud_bit       =       7                       ;for 2400 baud
  ;start_delay    =       128+64+1                ; "    "    "
  ;int_period     =       163                     ; "    "    "
  ;
  ; *** 2400 baud (at 4MHZ)
  baud_bit        =       4                       ;for 2400 baud
  start_delay     =       16+8+1                  ; '    '     '
  int_period      =       104                     ; '    '     '
  ;
  ; *** 1200 baud (at 4MHZ)
  ;baud_bit       =       5                       ;for 1200 baud
  ;start_delay    =       32+16+1                  ; '    '    '
  ;int_period     =       104                     ; '    '    '
  
  I have tested the 2400 baud settings and it works.  I've not tested
  the 1200 baud settings but the math follows and they should work.
  
  Note that the time allowed in the ISR is significantly shorter with
  these settings.  If you need more time in the ISR you could use 204
  for int_period and adjust everything else accordingly.
  
  William Phelps
  
  

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