PIC16F877 Program for 1 KHz Square Wave Generation

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PIC16F877 Program for 1 KHz square  Wave Generation: Here is the steps for PIC16F877 Program for Generating a square wave of 1 KHz .

Square  Wave Generation using PIC16F877

Square Wave Generation Using PIC16F877
Square Wave Generation Using PIC16F877

1 kHz Square Wave with PIC16F877 (Assembly)
Assumptions:
• Oscillator Fosc=4 MHzF_{osc} = 4 \, \text{MHz}Fosc=4MHz → instruction cycle Tcy=1 μsT_{cy} = 1 \, \mu sTcy=1μs.
• We want 1 kHz square wave → Period = 1 ms → Half-period = 0.5 ms = 500 µs.
• Timer0 with prescaler 1:4 → tick = 4 μs4 \, \mu s4μs.
• Required ticks = 500/4=125500 / 4 = 125500/4=125.
• Preload value = 256−125=131(0x83)256 – 125 = 131 (0x83)256−125=131(0x83).
So → On every Timer0 overflow, we toggle the pin (RB0).

;===========================================
; PIC16F877 – 1 kHz square Wave, 50% duty
; Using Timer0 interrupt, output on RB0
; Fosc = 4 MHz
;===========================================

    LIST P=16F877
    INCLUDE <P16F877.INC>
    __CONFIG _HS_OSC & _WDT_OFF & _PWRTE_ON & _LVP_OFF

    CBLOCK 0x20
    ; No variables needed
    ENDC

;——————————————-
ORG 0x00
GOTO MAIN

    ORG 0x04          ; Interrupt vector
    GOTO ISR

;——————————————-
MAIN:
BSF STATUS, RP0 ; Bank1
CLRF TRISB ; PORTB as output
BCF STATUS, RP0 ; Back to Bank0
CLRF PORTB ; Clear PORTB

    ; Setup Timer0
    MOVLW b'00000001' ; PSA=0 (assign prescaler), PS=001 (1:4), T0CS=0 (internal clk)
    OPTION_REG = 0x81 ; Actually OPTION<2:0>=001 -> PS=1:4, bit PSA=0
                      ; T0CS=0 (internal clock)

    MOVLW 0x83        ; Preload value = 131
    MOVWF TMR0

    ; Enable interrupts
    BSF INTCON, T0IE  ; Enable TMR0 overflow interrupt
    BSF INTCON, GIE   ; Global interrupt enable

LOOP:
GOTO LOOP ; Main loop does nothing

;——————————————-
; Interrupt Service Routine
ISR:
BCF INTCON, T0IF ; Clear TMR0 interrupt flag

    MOVLW 0x83        ; Reload TMR0 with 131
    MOVWF TMR0

    ; Toggle RB0
    MOVF PORTB, W     ; Read PORTB
    XORLW 0x01        ; Toggle bit0
    MOVWF PORTB

    RETFIE

;——————————————-
END

  • Timer0 is configured with prescaler 1:4.
  • TMR0 is preloaded with 0x83 (131) so that after 125 ticks (500 µs), it overflows.
  • On each overflow, the ISR reloads TMR0 and toggles RB0.
  • Thus, RB0 goes HIGH for 500 µs, LOW for 500 µs → 1 kHz square  wave, 50% duty cycle.

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