===== Shift Register =====

Sourced from [[ http://romux.com/tutorials/pic-tutorial/shift-registers | Romux.com ]]

Output shift registers transform serial data into parallel data. On every rising edge of the clock, the shift register reads the value from data line, stores it in temporary register, and then repeats this cycle 8 times. On a signal from 'latch' line, data is copied from the shift register to input register, thus data is transformed from serial into parallel data.

{{ :electronics:pic:shift_reg_hc595_ovr.jpg?300 |}}

An outline of the 74HC595 shift register connections is shown on the diagram below:

{{ :electronics:pic:shift_reg_hc595.jpg?500 |}}

Macro HC595 has two parameters:

HC595 macro Var, Var1

**Var** variable whose contents is transferred to outputs of shift register.

**Var1** loop counter

Example: HC595 Data, counter

The data we want to transfer is stored in data variable, and counter variable is used as a loop counter.

<code asm>
HC595 MACRO VAR,VARL                    
    LOCAL LOOP                    ;Local  label
            MOVLW .8             ;Transfer  eight bits
            MOVWF VARL            ;Initialising  counter
 

            RLF    VAR,F            ;Rotate "Var" on place to the  left        
            
            BTFSS STATUS,C         ;Is  carry =  "1"   ?
            BCF DATAA            ;If not,   set Dataa line   to  "0"
            BTFSC STATUS,C        ;Is  carry =  "0"   ? 
            BSF    DATAA            ;If not,   set Dataa line   to  "1"
                      
            BSF CLOCK            ;Generate  one   clock    
            NOP 
            BCF CLOCK    
     
            DECFSZ VARL,F        ;Has  eight bits been sent  ?
            GOTO LOOP              ;If not,   repeat
        
            BSF LATCH            ;If  all  8 bits have been sent,
            NOP                    ;contents  of  SHIFT register  to    send the output latch    
            BCF     LATCH
            ENDM 
</code>

==== Example ====

<code asm>
/*  SOURCE : WWW.ROMUX.COM
    AUTHOR : romux team     */ 
    
    
			PROCESSOR  16f84
			#include "pl6f84.inc"
__CONFIG _CP_0FF  &  _TODT_0FF   &  _PWRTE_0N  &   _XT_0SC

			CBLOCK 0X0C				;RAM  starting address
			TX						;Belongs   to  the  function "HC595"
			COUNTSPI
			ENDC

#DEFINE DATAA PORTA, 0 
#DEFINE CLOCK PORTA, 1 
#DEFINE LATCH PORTA,2



			ORG  0X00				;Reset vector
			GOTO MAIN
			ORG 0X04				;Interrupt vector
			GOTO MAIN				;no   interrupt routine
			
			INCLUDE "HC595.INC"

MAIN								;Beginning of  the program
			BANKSEL TRISA 
			MOVLW  B'00011000'		;Initialising  port A 
			MOVWF TRISA  			;TRISA <-  0x18
			BANKSEL PORTA 
			CLRF PORTA 				;PORTA <-  0x00	
			MOVLW 0XCB				;Fill TX buffer
			MOVWF TX 				;TX <-   '11001011'
			HC595 TX, COUNTSPI
LOOP
     		GOTO     LOOP			;Remain  at this  line
			END						;End of program
</code>