PIC Microcontroller Output Method

Expand 3 IO pins into 8 outputs with a 74HC595

Lawrence Glaister VE7IT says:

Code used to expand I/O on pic16xxx series. I used a 74hc595 to get 8 extra output bits on a pic16f627. The 595's can be cascaded for higher i/o counts with no extra lines used. In another project, I used 3 595's.

Also:

• PIC Input multiplexing, Microchip PIC, 74HC251, 4051, embedded keyboard, button debounce for the opposite

#DEFINE SRCLK   PORTB,4     ; 74hc595 clock output
#DEFINE SRDAT   PORTB,5     ; 74hc595 data output  
#DEFINE SRLAT   PORTB,3     ; 74hc595 output latch 

;----------------------------------------------------------------------
;  set relays to match global variable "relay"             
;  8 bits are clocked out to 74hct595 and then strobed to output register
;----------------------------------------------------------------------
set_relays
	movlw 	d'8'		    ; 8 hi bits to shift out
	movwf 	sr_tmp
    movf    relay,w
    movwf   sr_sr           ; save a temp copy of relay image

sr_001
	bcf	    SRDAT	        ; start with bit low
	rlf 	sr_sr,f		    ; rotate bits through c
	btfsc	STATUS,C
	bsf	    SRDAT   	    ; flip data if needed
	nop

	bsf	    SRCLK	        ; strobe active rising edge
	nop
	bcf	    SRCLK

	decfsz	sr_tmp,f
	goto	sr_001		    ; loop for rest of bits in byte	
    
	bsf	    SRLAT	        ; xfer data from sr to output latch
	nop
	bcf	    SRLAT

	return

Questions:

• Hi, I used that code and the part work fine. Good
• asks:

  I have seen this code written three different ways, what dose it do? and why show it ? I am new to programing Pic's and stuff like this confusses me. I s there anyway someone could give an example on how to use this or dose anyone know how to use this?

• deepdhillon@gmail.com asks:

  Hello,
  
  I am fairly new to this, and just have a question. When using this code to expand your outputs, is this code simply a subroutine that is to be called when the 8 bits are ready to be output, or is the code implemented into the main program code? Any details you can give me on the implementation of this type of code would be very helpful. Thanks.

  James Newton replies: It is to be used when the 8 bits are ready to be output.

Comments:

• martin@nnytech.net " This code uses BSF and BCF on port registers which may or may not cause read-modify-write errors."

Here is one possible C version of this code.

void OutPORT(unsigned char data) 

{ 
   unsigned char bits; 
   for (bits=0x80; bits!=0; bits >>= 1) 
   { 
      if ((bits & data) == bits)            // send Data 
         SRDAT = 1; 
      else 
         DRDAT = 0; 
      SRCLK = 1;                              // Clock the data
      SRCLK = 0; 
   } 
   SRLAT = 1;                                  // Latch the data 
   SRLAT = 0; 
}