User:Ulfalizer: Difference between revisions
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=== Read/write observations === | === Read/write observations === | ||
* Address bus | * Address bus and '''rw''' changes right away (during '''φ1'''). | ||
* Values appear to be read during '''φ2''' ('''db''x''''' pins buffered on '''cclk'''), and '''db''' changes during '''φ2''' for writes too. | |||
* Values appear to be read during '''φ2''' ('''db''x''''' pins buffered on '''cclk''') | |||
* [[Visual_circuit_tutorial#Clocks|Clocks section of tutorial]]. | * [[Visual_circuit_tutorial#Clocks|Clocks section of tutorial]]. | ||
Revision as of 20:22, 5 June 2013
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.
