CPU pin out and signal description
From Nesdev wiki
.--\/--. AD1 <- |01 40| -- +5V AD2 <- |02 39| -> OUT0 /RST -> |03 38| -> OUT1 A00 <- |04 37| -> OUT2 A01 <- |05 36| -> /OE1 A02 <- |06 35| -> /OE2 A03 <- |07 34| -> R/W A04 <- |08 33| <- /NMI A05 <- |09 32| <- /IRQ A06 <- |10 31| -> M2 A07 <- |11 30| <- TST (usually GND) A08 <- |12 29| <- CLK A09 <- |13 28| <> D0 A10 <- |14 27| <> D1 A11 <- |15 26| <> D2 A12 <- |16 25| <> D3 A13 <- |17 24| <> D4 A14 <- |18 23| <> D5 A15 <- |19 22| <> D6 GND -- |20 21| <> D7 `------'
|Active-Low signals are prefixed by a / (slash) symbol. Every cycle is either a read or a write cycle.|
- CLK : 21.47727 MHz (NTSC) or 26.6017 MHz (PAL) clock input. Internally, this clock is divided by 12 (NTSC 2A03) or 16 (PAL 2A07) to feed the 6502's clock input φ0, which is in turn inverted to form φ1, which is then inverted to form φ2. φ1 is high during the first phase (half-cycle) of each CPU cycle, while φ2 is high during the second phase.
- AD1 : Audio out pin (both pulse waves).
- AD2 : Audio out pin (triangle, noise, and DPCM).
- Axx and Dx : Address and data bus, respectively. Axx holds the target address during the entire read/write cycle. For reads, the value is read from Dx during φ2. For writes, the value appears on Dx during φ2 (and no sooner).
- OUT0..OUT2 : Output pins used by the controllers ($4016 output latch bits 0-2). These 3 pins are connected to either the NES or Famicom's expansion port, and OUT0 is additionally used as the "strobe" signal (OUT) on both controller ports.
- /OE1 and /OE2 : Controller ports (for controller #1 and #2 respectively). Each enable the output of their respective controller, if present.
- R/W : Read/write signal, which is used to indicate operations of the same names. Low is write. R/W stays high/low during the entire read/write cycle.
- /NMI : Non-maskable interrupt pin. See the 6502 manual and CPU interrupts for more details.
- /IRQ : Interrupt pin. See the 6502 manual and CPU interrupts for more details.
- M2 : Can be considered as a "signals ready" pin. It is a modified version the 6502's φ2 (which roughly corresponds to the CPU input clock φ0) that allows for slower ROMs. CPU cycles begin at the point where M2 goes low.
- In the NTSC 2A03, M2 has a duty cycle of 5/8th, or 350ns/559ns. Equivalently, a CPU read (which happens during the second, high phase of M2) takes 1 and 7/8th PPU cycles. The internal φ2 duty cycle is exactly 1/2 (one half).
- In the PAL 2A07, the duty cycle is not known, but suspected to be 19/32.
- TST : (tentative name) Pin 30 is special: normally it is grounded in the NES, Famicom, PC10/VS. NES and other Nintendo Arcade Boards (Punch-Out!! and Donkey Kong 3). But if it is pulled high on the RP2A03G, extra diagnostic registers to test the sound hardware are enabled from $4018 through $401A, and the joystick ports $4016 and $4017 become open bus. On older revisions of the CPU, pulling pin 30 high instead causes the CPU to stop execution.
- /RST : When low, holds CPU in reset state, during which all CPU pins (except pin 2) are in high impedance state. When released, CPU starts executing code (read $FFFC, read $FFFD, ...) after 6 M2 clocks.