Standard controller

All NES units come with at least one standard controller - without it, you wouldn't be able to play any games!

Standard controllers can be used in both controller ports, or in a Four score accessory.

See also Controller Reading

Input ($4016 write)

7  bit  0
---- ----
xxxx xxxS
        |
        +- Controller shift register strobe

While S (strobe) is high, the shift registers in the controllers are continuously reloaded from the button states, and reading $4016/$4017 will keep returning the current state of the first button (A). Once S goes low, this reloading will stop. Hence a 1/0 write sequence is required to get the button states, after which the buttons can be read back one at a time.

(Note that bits 2-0 of $4016/write are stored in internal latches in the 2A03/07.)

Output ($4016/$4017 read)

7  bit  0
---- ----
xxxx xMxD
      | |
      | +- Serial controller data
      +--- On Famicom only, on $4016 only, microphone in controller 2

The first 8 reads will indicate which buttons are pressed (1 if pressed, 0 if not pressed); all subsequent reads will return D=1 on an authentic controller but may return D=0 on third party controllers.

Button status for each controller is returned as an 8-bit report in the following order: A, B, Select, Start, Up, Down, Left, Right.

Due to some of the bits being open bus when reading the controller, the actual value read back will be something like 0x40 | (shift_reg & 0x01) (the 0x40 comes from the last byte of the address, $4016/17, in little endian). Paperboy relies on this exact value being returned.

A Super NES controller can be wired to the NES controller port, and it returns a 16-bit report in a similar order: B, Y, Select, Start, Up, Down, Left, Right, A, X, L, R, then four 0 bits.

Hardware

The 4021 (or 74165) IC is an 8-bit parallel-to-serial shift register. It has a pin "serial input", ordinarily used to chain the output of one shift register into the next one as seen in the Four Score or the Super NES controller. The serial input on the tail end of such a chain (or the only one in the case of an NES) can be tied to ground or Vcc, which determines the state of the output after all bits have been shifted out. If this is grounded, the shift register produces a 0 after all bits have been shifted out; if it's tied to Vcc, it produces a 1.

In the NES controller, this input is grounded. But because the signals from the controllers pass through an inverter before reaching the CPU, the register produces a 1 for all reads after the first eight.

Evil Details

The CLK line for controller port is R/W nand (ADDRESS == $4016). When this transitions from high to low, the buffer inside the NES latches the output of the controller data lines, and when it transitions from low to high, the shift register in the controller shifts one bit.

This can cause glitches if the DMC DMA is running, and happens to start a read in the same cycle that the CPU is trying to read from $4016 or $4017. Since the address bus will change for one cycle, the shift register will see an extra rising clock edge (a "double clock"), and the shift register will drop a bit out. The program will see this as a bit deletion from the serial data, and games using DMC will reread the controller to make sure no bit deletion happened. See http://forums.nesdev.org/viewtopic.php?t=4116 for details and http://forums.nesdev.org/viewtopic.php?t=4124 for a reliable controller reading routine.

Turbo

A turbo controller such as the NES Max or NES Advantage is read just like a standard controller, but the user can switch some of its buttons to be toggled by an oscillator. Such an oscillator turns the button on and off at 15 to 30 Hz, producing rapid fire in games. A controller MUST NOT toggle the button states on each strobe pulse; that'll break any game that rereads the controller.