nesdev.org

Posted: **Tue Feb 04, 2020 11:58 am**

Why are interrupts failing on the real cartridge?

Also, you'll want to set the PRG-RAM size in that test ROM's NES 2.0 header to 8 KiB.

Posted: **Tue Feb 04, 2020 5:13 pm**

Ok, I added little error description to the test ROM. It looks like the read data on open-bus addresses is high byte of previous opcode, so maybe the pull-down are not strong as I previously thought.

Here are some waveform from the code:

Code: Select all

lda #1             ; just to make easy triggering in scope
sta $4016          ; just to make easy triggering in scope
lda #0             ; just to make easy triggering in scope
sta $4016          ; just to make easy triggering in scope
lda (zp_ptr1), y   ; this should read value from cell $4018
                   ; and the result is $40

CH1=M2, CH2=R/W, CH3=OUT0, CH4=D6
The voltage on D6 in cycle 5 is slowly decreasing, maybe too slow for read it as 0V (kazzo read operations are much slower)

~~BTW. It turns out that writing value to $4016 (joypad strobe) makes it appear on the end of NEXT cycle. Weird.~~
Lol no, it is random, in some power-ups t happens at the end of the write cycle and in some at the end of next one.

Why are interrupts failing on the real cartridge?

I expected the interrupt to happen after 17 loops of polling value set by IRQ interrupt routine, but on real cartridge it happened after 14 loops.

Code: Select all

	lda #0
	sta irq_arrived1
	
	ldx #0
	lda #10
	sta $7efd
	lda #0
	sta $7efe
	sta $7eff
	cli
	sta $7efe
	lda #3
	sta $7efe
	
test11_loop:
	inx 
	beq test11_fail_noirq   ;if no irq happends in 256 loops, consider it not working at all
	lda irq_arrived1
	beq test11_loop
	cpx #$14
	bne test11_fail_badirqcounter
	
	
...

irq_vector_handler:
	pha
	lda #1
	sta irq_arrived1
	lda #0
	sta $7efe
	pla
	rti

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