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Using 74HC595 to expand output pins of a PIC microcontroller: Assembly code

This is actually a continuation of my blog entry https://www.edaboard.com/blog/1628/ where I have used C language routine. Now its the time to use Assembly Language for PIC. The Code is tested on PIC16F84A with 4MHz crystal. Compiler used MCS-51 under M-IDE Studio.

The associated program shows how to transfer a byte data serially to the 74HC595 and then sets the output accordingly.

According to page 5 of Datasheet, the outputs of 74HC595 provide 8 mA. Though I have used 330E resistors to drive LEDs , this should not be done! It is therefore recommended to connect a higher resistance (620E). LEDs light up very, very little, but enough to test the circuit.

I will mainly cover the technical details to connect a 74HC595 with a microcontroller. I will avoid the theory of a shift register and all those theoretical details.

The data are transferred into the shift register 74HC595 pin through the PIN 14 (Serial date).
PIN 11 (Shift register clock). Once finished sending data, just bring a high level on the pin 12 (Storage Clock or Latch) to transfer data from the shift register to the latch. The pin 13 (Output Enable - active low) enables the transfer of data from the latch to the output ports when it is brought low: since we connected the drive pin to ground, they meet again on the data outputs as soon as we give the shot clock on the latch. The pin 10 is the master reset: placing it at a low level resets the contents of the shift register, for which we keep it up for safety. The pin 9 we keep it disconnected because it is not used in this application.

You can use any microcontroller but be careful, in case of open collector pins, add pull up resistors.

extpin.png




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;Following is the code to switch on alternate LEDs for 1 seconds
 
 
list p=16F84A
#include <p16F84A.inc>
__CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC
ERRORLEVEL -302
 
 
CBLOCK 0X0C
TX ;
COUNTSPI
d1
d2
d3
ENDC
 
#define DATA1 PORTA,0
#define CLOCK PORTA,1
#define LATCH PORTA,2
 
ORG 0X00
GOTO MAIN
 
#include "HC595.INC" ;;Thanks to Nebojsa Matic of mikroElektronika for idea
 
;;;;;;;;;;;;;;;;;1 sec delay;;;;;;;;;;;;;;;;;;;;;;;
DELAY1s
 
 
    ;999990 cycles
    movlw 0x07
    movwf d1
    movlw 0x2F
    movwf d2
    movlw 0x03
    movwf d3
Delay_01
 
    decfsz d1, f
    goto $+2
    decfsz d2, f
    goto $+2
    decfsz d3, f
    goto Delay_01
 
    ;6 cycles
    goto $+1
    goto $+1
    goto $+1
 
    ;4 cycles (including call)
    return
 
MAIN
 
    BSF STATUS,RP0
    MOVLW B'00000'
    MOVWF TRISA
    BCF STATUS,RP0
    CLRF PORTA
LUPHR
 
    CLRF PORTA
    MOVLW B'10101010'
    MOVWF TX
    HC595 TX,COUNTSPI
    CALL DELAY1s
    CLRF PORTA
    MOVLW B'01010101'
    MOVWF TX
    HC595 TX,COUNTSPI
    CALL DELAY1s
    GOTO LUPHR
    END



And the HC595.inc library

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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;HC595.inc;;;;;;;;;;;;;; ;;;;;;;;;;;;;;;;;
;Thanks to Nebojsa Matic of mikroElektronika for idea
;Var variable whose contents is transferred to outputs of shift register.
;Var1 loop counter
;Here 8bit data is used, so VAR1 gets decimal 8 to continue the loop for 8 times
 
HC595 MACRO VAR,VAR1
 
    LOCAL LOOP
    MOVLW .8
    MOVWF VAR1
LOOP 
 
    RLF VAR,F
    BTFSS STATUS,C
    BCF DATA1
    BTFSC STATUS,C
    BSF DATA1
 
    BSF CLOCK
    NOP
    BCF CLOCK
 
    DECFSZ VAR1,F
    GOTO LOOP
 
    BSF LATCH
    NOP
    BCF LATCH
 
    ENDM

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