User:Ulfalizer
Misc. timing stuff brought together in one place
Reads and writes
Read (LDA $1234, NOP)
Tick-by-tick from http://nesdev.org/6502_cpu.txt interleaved with steps from Visual 2A03:
# address R/W description
--- ------- --- -------------------------------------------------
1 PC R fetch opcode, increment PC
ab db rw pc phi2
0000 ad 1 0000 0
0000 ad 1 0000 1
2 PC R fetch low byte of address, increment PC
ab db rw pc phi2
0001 34 1 0001 0
0001 34 1 0001 1
3 PC R fetch high byte of address, increment PC
ab db rw pc phi2
0002 12 1 0002 0
0002 12 1 0002 1
4 address R read from effective address
ab db rw pc phi2
1234 00 1 0003 0
1234 00 1 0003 1
Write (LDA #$AB, STA $1234, NOP)
Tick-by-tick from http://nesdev.org/6502_cpu.txt interleaved with steps from Visual 2A03:
# address R/W description
--- ------- --- ------------------------------------------
1 PC R fetch opcode, increment PC
ab db rw pc phi2
0002 8d 1 0002 0
0002 8d 1 0002 1
2 PC R fetch low byte of address, increment PC
ab db rw pc phi2
0003 34 1 0003 0
0003 34 1 0003 1
3 PC R fetch high byte of address, increment PC
ab db rw pc phi2
0004 12 1 0004 0
0004 12 1 0004 1
4 address W write register to effective address
ab db rw pc phi2
1234 12 0 0005 0
1234 ab 0 0005 1
Read/write observations
- Address bus and rw changes right away (during φ1).
- Values appear to be read during φ2 (dbx pins buffered on cclk), and db changes during φ2 for writes too.
- Clocks section of tutorial.
Interrupts
Sampled during φ1: irq inverts into 1599, which is driven low by cclk (i.e., always driven low during φ2). Link to irq pin.
Visual 6502 links
Smallest IRQ assertion interval that will trigger an IRQ for LDA #FF: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq&a=0&d=58A9FF1890FE&irq0=5&irq1=6&steps=20
Ditto for LSR $AB: http://visual6502.org/JSSim/expert.html?logmore=Execute,irq&a=0&d=5846AB1890FE&irq0=11&irq1=12&steps=20
Ditto for LSR $AB with NMI instead of IRQ: http://visual6502.org/JSSim/expert.html?logmore=Execute,nmi&a=0&d=58461890FE&nmi0=11&nmi1=12&steps=20
M2 duty cycle
The M2 duty cycle is 5/8 - forum post and CPU pin-out page. Low for 9/8 PPU cycles, high for 15/8 PPU cycles. Low for 4.5 master cycles, high for 7.5 master cycles. Low for ~210 ns, high for ~349 ns.
PPU interface
Input map:
Binding | Interpretation ----------------+-------------------------------------- 0 -> 15 | 0 -> /Enable second demultiplexer a15 -> 13 | a15 -> A_1b a14 -> 3 | a14 -> A_1a a13 -> 2 | a13 -> A_0a M2 -> 14 | M2 -> A_0b 11 -> 1 | /O_1b -> /Enable first demultiplexer 9 -> /ROM SEL | /O_3b -> /ROM SEL 5 -> /DBE | /O_1a -> /DBE 4 -> RAM CS | /O_0a -> RAM CS
Input/output map:
Inputs | Outputs ---------+---------------- E A0 A1 | O_0 O_1 O_2 O_3 ---------+---------------- 1 x x | 1 1 1 1 0 0 0 | 0 1 1 1 0 1 0 | 1 0 1 1 0 0 1 | 1 1 0 1 0 1 1 | 1 1 1 0
Hence:
ROM SEL = M2 && a15 <Enable first demultiplexer> = M2 && !a15 DBE = <Enable first demultiplexer> && !a14 && a13 RAM CS = <Enable first demultiplexer> && !a14 && !a13
Summary:
When M2 high,
A15-13 | Signal -------+-------- 1xx | ROM SEL 001 | DBE 000 | RAM CS
- rw signal together with ab lines directly generates signals like /r2002.
- _io_ce follows inverted M2.
- vbl_flag is set via a vpos was 241 and vpos is not 241 comparison. Goes high at hpos = 1 during at pclk0.
- The set_vbl_flag signal is high during pclk0 of vpos=241/hpos=1.
- spr0_hit goes high at tick x+2, during pclk1. Reading uses same _io_ce behavior as vbl_flag.
- spr_overflow reading behavior just like spr0_hit. TODO: set timing.
- $2005/$2006 and $2000 nametable bits writes use the same timing.
