User:Ulfalizer

Misc. timing stuff brought together in one place

Reads and writes

Read (LDA $1234, NOP)

http://www.qmtpro.com/~nes/chipimages/visual2a03/?graphics=false&logmore=Execute,phi2&a=0&d=ad3412ea&steps=25

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)

http://www.qmtpro.com/~nes/chipimages/visual2a03/?graphics=false&logmore=Execute,phi2&a=0&d=a9ab8d3412ea&steps=29

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

• Wiring diagram
• Address decoder datasheet

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 db lines directly generates signals like r2002.
• _io_ce follows M2.
• vbl_flag is set via a vpos was 241 and vpos is not 241 comparison. Goes high at hpos = 1 during at pclk0.