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) |
(Add some more timing details for Axx, Dx, and R/W (confirmed in Visual 6502)) |
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* 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 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. | ||
* AD1 (both pulse waves) and AD2 (triangle, noise, and DPCM) are the Audio Out pins. | * AD1 (both pulse waves) and AD2 (triangle, noise, and DPCM) are the Audio Out pins. | ||
* Axx is the address bus and Dx the data bus. | * Axx is the address bus and Dx the data bus. 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 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]]. | * 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]]. | ||
* /OE1 and /OE2 also go to the controller ports, and each enable the output of their respective controller, if present. | * /OE1 and /OE2 also go to the controller ports, and each enable the output of their respective controller, if present. | ||
* R/W is the read/write signal, which is used to indicate operations of the same names. Low is write. | * R/W is the 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 and /IRQ are the two interrupt pins. See the 6502 manual and the primer in [[CPU interrupt quirks]] for more detailed explanation. | * /NMI and /IRQ are the two interrupt pins. See the 6502 manual and the primer in [[CPU interrupt quirks]] for more detailed explanation. | ||
* 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. | * 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 (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) | ** 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) | ||
* 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. | * 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. | ||
Revision as of 13:20, 22 May 2013
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 "/".
- 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 phase (half-cycle) of each CPU cycle, while φ2 is high during the second phase - hence the names.
- AD1 (both pulse waves) and AD2 (triangle, noise, and DPCM) are the Audio Out pins.
- Axx is the address bus and Dx the data bus. 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 are output pins used by the controllers ($4016 output latch bits 0-2). These 3 pins are connected to either the NES's or Famicom's expansion port, and OUT0 is additionally used as the "strobe" signal (OUT) on both controller ports.
- /OE1 and /OE2 also go to the controller ports, and each enable the output of their respective controller, if present.
- R/W is the 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 and /IRQ are the two interrupt pins. See the 6502 manual and the primer in CPU interrupt quirks for more detailed explanation.
- 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 (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)
- 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.
