I'm working on a small project that requires a random number to be generated using the ATMEL AT89C2051 chip. Rather than going ahead and coding/finding an actual pseudo-random number generator I made a realization: I only needed a random number to be generated after a user turned on a pushbutton switch. Therefore, I thought, since the amount of time between each push of the switch will always vary, I could simply keep timer 0 on and read TL0 (only 3 random bits are actually needed) once the user pushes the button. To test my theory I wired up the microcontroller to 3 7-segment displays which would be used to display the random value once a user pushes a switch (P3.0). Here is the code that I made:

#include "8051equ.inc"

.org	00h
	ajmp	start

.org	0bh
	reti

.org	13h
	reti

.org	1bh
	reti

.org	23h
	reti

.org	25h

initialize:
	; start timer 0 - 16 bit timer mode
	mov	TMOD, #01h
	setb	TR0

	mov	PSW, #00h
	mov	IE, #00h
	ret

; delay for 1 ms (11.0592 MHz crystal)
delay_ms:
	mov	r7, #00h
loop_1:
	inc	r7
	mov	a, r7
	cjne	a, #0ffh, loop_1
	ret

; 8-bit binary to bcd converter
; a  - input value
; r2 - output digit 1
; r3 - output digit 2
; r4 - output digit 3
bin_to_bcd:
	mov	r2, #00h
	mov	r3, #00h
	mov	r4, #00h

	mov	B, #010d
	div	ab
	mov	r4, B
	mov	B, #010d
	div	ab
	mov	r3, B
	mov	B, #010d
	div	ab
	mov	r2, B
	ret

display_0:
	mov	P1, #00001001b
	ret

display_1:
	mov	P1, #11101011b
	ret

display_2:
	mov	P1, #01000101b
	ret

display_3:
	mov	P1, #01100001b
	ret

display_4:
	mov	P1, #10100011b
	ret

display_5:
	mov	P1, #00110001b
	ret

display_6:
	mov	P1, #00010001b
	ret

display_7:
	mov	P1, #01101011b
	ret

display_8:
	mov	P1, #00000001b
	ret

display_9:
	mov	P1, #00100011b
	ret

; displays 3 digit BCD number in A register
display_bcd:
	cjne	a, #00h, check1
	acall	display_0
	ret
check1:
	cjne	a, #01h, check2
	acall	display_1
	ret
check2:
	cjne	a, #02h, check3
	acall	display_2
	ret
check3:
	cjne	a, #03h, check4
	acall	display_3
	ret
check4:
	cjne	a, #04h, check5
	acall	display_4
	ret
check5:
	cjne	a, #05h, check6
	acall	display_5
	ret
check6:
	cjne	a, #06h, check7
	acall	display_6
	ret
check7:
	cjne	a, #07h, check8
	acall	display_7
	ret
check8:
	cjne	a, #08h, disp9
	acall	display_8
	ret
disp9:
	acall	display_9
	ret
		
start:
	acall	initialize

	; set P3.0 to be used as an input
	setb	P3.0	

	; wait until switch is pressed
	jb	P3.0, $

random:
	mov	a, TL0

	; wait for switch to be released
	jnb	P3.0, $
	acall	bin_to_bcd

display_loop:
	mov	a, r2
	acall	display_bcd
        
        ; turn on 7-segment display 1
	setb	P3.3
	acall	delay_ms
	clr	P3.3

	mov	a, r3
	acall	display_bcd

        ; turn on 7-segment display 2
	setb	P3.4
	acall	delay_ms
	clr	P3.4

	mov	a, r4
	acall	display_bcd

        ; turn on 7-segment display 3
	setb	P3.5
	acall	delay_ms
	clr	P3.5

        ; display same number until switch is pressed
	jb	P3.0, display_loop
	ajmp	random
	
.end

This actually worked. Every time I hit the pushbutton switch connected to P3.0, a new number (between 0 and 255) appeared on the 3 7-segment displays. However, I realized that there was something wrong after pushing the button several (hundred) times. EVERY single number that was displayed was an even number. The first and second 7-segment displays showed both even and odd values but that last one (the LSB) was always 0,2,4,6, or 8. Why is this? Is it just a horrible coincidence (I have pushed the damn thing hundreds of times and it is ALWAYS an even number), or is there something wrong?

Thanks in advance for the help and keep in mind that I have only been working with 8051 for ~4 days so go easy.