Cartridge

A Switch game cartridge, frontside	A Switch game cartridge, backside
Close-up of frontside PCB	Close-up of backside PCB
Close-up of stripped frontside PCB

Pinout

Pin	Name	Direction	Description
0	IRQ?	Output	Always wired to GND inside game cartridge; Possibly used for interrupt signaling
1	RCLK	Output	Return clock; Game cartridge sends back CLK signal delayed by a few ns
2	CLK	Input	Clock, 25MHz
3	CS	Input	Chip select; Switch pulls this LOW during a transfer
4	DAT0	Inout	Data bus pin 0
5	DAT1	Inout	Data bus pin 1
6	VCC 3.3v	Input
7	DAT2	Inout	Data bus pin 2
8	DAT3	Inout	Data bus pin 3
9	VCC 1.8v	Input
10	DAT4	Inout	Data bus pin 4
11	DAT5	Inout	Data bus pin 5
12	DAT6	Inout	Data bus pin 6
13	DAT7	Inout	Data bus pin 7
14	GND
15	RST	Input	Reset, active LOW.

All IO use 1.8V for logic HIGH and 0V for logic LOW.

Protocol

Switch game cartridges use a simple (but Nintendo proprietery) SPI-like bus with 8-bit width (DAT7..0). It is very similar to the bus interface of 3DS game cartridges, except with very different commands.

The Switch host starts a transfer by first pulling CS low, followed by clocking a byte each clock cycle. The bus data will always be ready before the rising edge of the CLK signal, so that it can be captured on the rising edge. After command bytes are written to the bus, the direction of the bus implicitly changes and the game cartridge responds. The Switch host keeps clocking while the game cartridge responds. After the transfer is ended, the CS line is pulled high again.

Commands are 16 bytes long, and followed immediately by a 4-byte CRC-32 over the command bytes. After this, the bus direction changes, and the bus will be 'floating' for a few cycles (TBD figure out if cartridge can respond with NAK over bad CRC during floating bus).

Before the game cartridge responds with the actual response data, it will send busy bytes (LSB will be zero) to the Switch host, until it is ready to send the real response. When the game cartridge is ready, it will send 2 acknowledgement bytes (LSB will be one) to let the Switch host know. After this, the game cartridge will send the actual response bytes.

The actual response bytes are also followed immediately by a 4-byte CRC-32 over the actual response bytes.

Commands

A typical boot up sequence of a game cartridge looks like this:

Command	Description
5B000000000000010000000000000000	Read sector 0, contains "HEAD" blob
5B000000000000010000000000000000	Read sector 0, contains "HEAD" blob
56000000000000000000000000000000	Read card id "AE F8 01 21"
28000000000000000000000000000000	Read ??? "02 00 00 00"
A5000000000000000000000000000000	Read ??? "00 00 00 00"
56000000000000000000000000000000	Read card id "AE F8 01 21"
28000000000000000000000000000000	Read ??? "02 00 00 00"
5B000000380000010000000000000000	Read sector 0x38, contains "CERT" blob
E2000000000000000000000000000000	Read ??? "01 00 00 00"
E0000000000000000000000000000000	Read crypto-challenge header
200838A25A344F818ABB6456694D4E8D	Enter crypto mode with seed "0838A25A344F818ABB6456694D4E8D"
7EE41FDF12C01C157CC899910673A0CF	Encrypted command
263C8230EC15FAE3CE79365BD850F4BD	Encrypted command
B6FDA6F37FFA29E18831D0B217DFBDBE	Encrypted command
7B97F7DF07240AA9870E1C974336FA8A	Encrypted command

The meaning of some these commands are currently unknown.

Encryption

After a few initial plaintext commands, the Switch instructs the game cartridge to enter into encrypted mode. From that point on, commands and responses are sent encrypted over the bus. The encryption algorithm used is currently unknown.

Manufacturers

Macronix (MX)

Uses package: BGA

Uses card id: 0xC2

OKI Semiconductor

Uses package: TSOP-48

Uses card id: 0xAE

SanDisk?

Uses package: ??

Uses card id: 0x45 ?