Registers

Old3DS	Name	Address	Width
Yes	I2C1_DATA	0x10161000	1
Yes	I2C1_CNT	0x10161001	1
Yes	I2C1_CNTEX	0x10161002	2
Yes	I2C1_SCL	0x10161004	2
Yes	I2C2_DATA	0x10144000	1
Yes	I2C2_CNT	0x10144001	1
Yes	I2C2_CNTEX	0x10144002	2
Yes	I2C2_SCL	0x10144004	2
Yes	I2C3_DATA	0x10148000	1
Yes	I2C3_CNT	0x10148001	1
Yes	I2C3_CNTEX	0x10148002	2
Yes	I2C3_SCL	0x10148004	2

I2C_CNT

BIT	DESCRIPTION
0	Stop (0=No, 1=Stop/last byte)
1	Start (0=No, 1=Start/first byte)
2	Pause (0=Transfer Data, 1=Pause after Error, used with/after Stop)
4	Ack Flag (0=Error, 1=Okay) (For DataRead: W, for DataWrite: R)
5	Data Direction (0=Write, 1=Read)
6	Interrupt Enable (0=Disable, 1=Enable)
7	Start/busy (0=Ready, 1=Start/busy)

I2C Devices

Device id	Device bus id	Device address	Accessible via I2C service	Device description
0	1	0x4a	"i2c::MCU"	Power management?(same device addr as the DSi power-management)
1	1	0x7a	"i2c::CAM"	Camera0?(same dev-addr as DSi cam0)
2	1	0x78	"i2c::CAM"	Camera1?(same dev-addr as DSi cam1)
3	2	0x4a	"i2c::MCU"	MCU
4	2	0x78	"i2c::CAM"	?
5	2	0x2c	"i2c::LCD"	?
6	2	0x2e	"i2c::LCD"	?
7	2	0x40	"i2c::DEB"	?
8	2	0x44	"i2c::DEB"	?
9	3	0xa6	"i2c::HID"	Unknown. The device table in I2C-module had the device address changed from 0xA6 to 0xD6 with 8.0.0-18.
10	3	0xd0	"i2c::HID"	Gyroscope
11	3	0xd2	"i2c::HID"	?
12	3	0xa4	"i2c::HID"	DebugPad
13	3	0x9a	"i2c::IR"	IR
14	3	0xa0	"i2c::EEP"	eeprom?
15	2	0xee	"i2c::NFC"	New3DS-only NFC
16	1	0x40	"i2c::QTM"	New3DS-only QTM
17	3	0x54	"i2c::IR"	Used by IR-module starting with 8.0.0-18, for New3DS-only HID via "ir:rst". This deviceid doesn't seem to be supported by i2c module on 8.0.0-18(actual support was later added in New3DS i2c module).

Device 3

REGISTER	WIDTH	DESCRIPTION
0x03	8	?
0x04	8	?
0x09	1	Sound volume: 0x0..0x3F.
0xB	1	Battery level: 0x0..0x40.
0x18	8	?
0x20	8	Writing u8 value 4 here triggers a hardware system reboot. Writing u8 value 1 (repeatedly) triggers a shutdown via power-off?
0x22	8	?
0x23	8	?
0x24	8	?
0x28	8	?
0x29	8	?
0x2A	8	?
0x2B	8	?
0x2C	8	?
0x2D	0x64	This is used for controlling the notification LED(see MCURTC:SetInfoLEDPatternHeader as well), when this register is written.
0x2E	1	This returns the notification LED status when read.
0x30	8	?
0x31	8	?
0x32	8	?
0x33	8	?
0x34	8	?
0x35	8	?
0x36	8	?
0x37	8	?
0x38	8	?
0x39	8	?
0x3A	8	?
0x3B	8	?
0x3C	8	?
0x41	8	?
0x43	8	?
0x4E	8	?
0x50	8	?
0x51	8	?
0x58	8	?
0x60	8	?

Device 10

See the datasheet linked to on the Hardware page for reference.

Device 12

REGISTER	WIDTH	DESCRIPTION
0x0	21	DebugPad state.

This is the DebugPad device, see here.

Device 13

Raw I2C register address	Internal register address	Width	Description
0x0	0x0	0x40	RHR / THR (data receive/send FIFO)
0x8	0x1	0x1	IER
0x10	0x2	0x1	FCR/IIR
0x18	0x3	0x1	LCR
0x20	0x4	0x1	MCR
0x28	0x5	0x1	LSR
0x30	0x6	0x1	MSR/TCR
0x38	0x7	0x1	SPR/TLR
0x40	0x8	0x1	TXLVL
0x48	0x9	0x1	RXLVL
0x50	0xA	0x1	IODir
0x58	0xB	0x1	IOState
0x60	0xC	0x1	IoIntEna
0x68	0xD	0x1	reserved
0x70	0xE	0x1	IOControl
0x78	0xF	0x1	EFCR

See the datasheet linked to on the Hardware page for reference. From that datasheet, for the structure of the I2C register address u8: "Bit 0 is not used, bits 2:1 select the channel, bits 6:3 select one of the UART internal registers. Bit 7 is not used with the I2C-bus interface, but it is used by the SPI interface to indicate a read or a write operation."

Device 15

This the New3DS NFC controller "I2C" interface. This device is accessed via the WriteDeviceRaw/ReadDeviceRaw I2C service commands.

Since the *Raw commands are used with this, this device has no I2C registers. Instead, raw data is transfered after the I2C device is selected. Hence, WriteDeviceRaw is used for sending commands to the controller, while ReadDeviceRaw is for receiving responses from the controller. Certain commands may return multiple command responses.

Command request / response structure:

Offset	Size	Description
0x0	0x1	Normally 0x10?
0x1	0x1	?
0x2	0x1	CmdID
0x3	0x1	Payload size.

Following the above header is the payload data(when payload size is non-zero), with the size specified in the header. The command response payload is usually at least 1-byte, where that byte appears to be normally 0x0. For command requests the payload data is the command parameters.

During NFC module startup, a certain command is sent to the controller which eventually(after various cmd-reply headers etc) returns the following the payload after the first byte in the payload:

000000: 44 65 63 20 32 32 20 32 30 31 32 31 34 3a 35 33  Dec 22 201214:53 
000010: 3a 35 30 01 05 0d 46 05 1b 79 20 07 32 30 37 39  :50...F..y .2079
000020: 31 42 35                                         1B5

Or that is: "Dec 22 201214:53:50<binary>20791B5". Therefore, this appears to return the part-number of the NFC controller(other command request(s) / response(s) use this part-number value too).

Commands

CmdRequest[1]	CmdID	Payload data for parameters	Description
0x2E	0x2F	Firmware image for this chunk, size varies.	This is used during NFC module startup to upload the firmware image to the NFC controller. This is used repeatedly to upload multiple chunks of the image.

I2C Registers

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