CPU pinout: Difference between revisions

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(Clarify that φ1 and φ2 high denote the first and second phases of each CPU cycle)
 
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__TOC__
[[Category:Pinouts]]
[[Category:Pinouts]]
=== Pin out ===  
=== Pin out ===  
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=== Signal description ===  
=== Signal description ===  
Active-Low signals are indicated by a "/".
Active-Low signals are indicated by a "/". Every cycle is either a read or a write cycle.


* CLK is the 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 '''p'''hase (half-cycle) of each CPU cycle, while φ2 is high during the second '''p'''hase - hence the names.
* '''CLK''' : 21.47727 MHz (NTSC) or 26.6017 MHz (PAL) clock input. Internally, this clock is divided by 12 (NTSC [[CPU|2A03]]) or 16 (PAL [[CPU|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 (both pulse waves) and AD2 (triangle, noise, and DPCM) are the Audio Out pins.
* '''AD1''' : Audio out pin (both pulse waves).
* Axx is the address bus and Dx the data bus.
* '''AD2''' : Audio out pin (triangle, noise, and DPCM).
* OUT0-OUT2 are output pins used by the controllers ($4016 output latch bits 0-2). These 3 pins are connected to either the [[NES expansion port pinout|NES's]] or [[Famicom expansion port pinout|Famicom's]] expansion port, and OUT0 is additionally used as the "strobe" signal (OUT) on both [[controller port pinout|controller ports]].
* '''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).
* /OE1 and /OE2 also go to the controller ports, and each enable the output of their respective controller, if present.
* '''OUT0..OUT2''' : Output pins used by the controllers ($4016 output latch bits 0-2). These 3 pins are connected to either the [[NES expansion port pinout|NES]] or [[Famicom expansion port pinout|Famicom's]] expansion port, and '''OUT0''' is additionally used as the "strobe" signal (OUT) on both [[controller port pinout|controller ports]].
* R/W is the read/write signal, which is used to indicate operations of the same names. Low is write.
* '''/OE1''' and '''/OE2''' : Controller ports (for controller #1 and #2 respectively). Each enable the output of their respective controller, if present.
* /NMI and /IRQ are the two interrupt pins. See the 6502 manual and the primer in [[CPU interrupt quirks]] for more detailed explanation.
* '''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.
* 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.
* '''/NMI''' : Non-maskable interrupt pin. See the 6502 manual and [[CPU interrupts]] for more details.
** In the NTSC 2A03, M2 has a duty cycle (fraction of a cycle the signal stays high) of ⅝, or 350ns/559ns. Equivalently, a CPU read (which happens during the second, high phase of M2) takes 1⅞ PPU cycles. The internal φ2 duty cycle is exactly ½. (The duty cycle in the PAL 2A07 is not known, but suspected to be 19/32)
* '''/IRQ''' : Interrupt pin. See the 6502 manual and [[CPU interrupts]] for more details.
* TST (tentative name) (pin 30) is special: normally it is grounded in the NES, Famicom, PC10/VS. NES and other Nintendo Arcade Boards (Popeye and Donkey Kong 3). But if it is pulled high, extra diagnostic registers to test the sound hardware are enabled from $4018 through $401A, and the joystick ports $4016 and $4017 become open bus.
* '''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 2A03E, G, and H, '''M2''' has a [[Glossary|duty cycle]] of 15/24 (5/8), 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, '''M2''' has a duty cycle of 19/32, or 357ns/601ns, or 1.9 out of 3.2 pixels.
** In the original NTSC 2A03 (no letter), M2 has a duty cycle of 17/24, or 396ns/559ns, or 2 and 1/8th pixels.
* '''[[CPU Test Mode|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 the RP2A07 (and presumably also the RP2A03E), pulling pin 30 high instead causes the CPU to stop execution by means of activating the embedded 6502's RDY input.
* '''/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.

Latest revision as of 22:26, 28 April 2022

Pin out

        .--\/--.
 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
        `------'

Signal description

Active-Low signals are indicated by a "/". 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 2A03E, G, and H, M2 has a duty cycle of 15/24 (5/8), 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, M2 has a duty cycle of 19/32, or 357ns/601ns, or 1.9 out of 3.2 pixels.
    • In the original NTSC 2A03 (no letter), M2 has a duty cycle of 17/24, or 396ns/559ns, or 2 and 1/8th pixels.
  • 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 the RP2A07 (and presumably also the RP2A03E), pulling pin 30 high instead causes the CPU to stop execution by means of activating the embedded 6502's RDY input.
  • /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.
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