Registers

Old3DS	Name	Address	Width	Used by
Yes	I2C1_DATA	0x10161000	1	I2C bus 1 devices
Yes	I2C1_CNT	0x10161001	1	I2C bus 1 devices
Yes	I2C1_CNTEX	0x10161002	2	I2C bus 1 devices
Yes	I2C1_SCL	0x10161004	2	I2C bus 1 devices
Yes	I2C2_DATA	0x10144000	1	I2C bus 2 devices
Yes	I2C2_CNT	0x10144001	1	I2C bus 2 devices
Yes	I2C2_CNTEX	0x10144002	2	I2C bus 2 devices
Yes	I2C2_SCL	0x10144004	2	I2C bus 2 devices
Yes	I2C3_DATA	0x10148000	1	I2C bus 3 devices
Yes	I2C3_CNT	0x10148001	1	I2C bus 3 devices
Yes	I2C3_CNTEX	0x10148002	2	I2C bus 3 devices
Yes	I2C3_SCL	0x10148004	2	I2C bus 3 devices

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 Write 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).

Notice: These device addresses are used for writing to the respective device, for reading bit0 must be set (see I2C protocol). Thus, the actual device address is >> 1.

Device 3

REGISTER	WIDTH	INFO	DESCRIPTION
0x00	s	ro	Version high
0x01	s	ro	Version low
0x02	s	ro	?
0x03	s	rw	Top screen flicker
0x04	s	rw	Bottom screen flicker
0x05 - 0x07	s(3)	rw	Danger zone - MCU unlock sequence is written here
0x08	s	ro	Raw 3D slider position
0x09	s	ro	Volume slider state (0x00 - 0x3F) This is the same value returned by MCUHWC:GetSoundVolume
0x0A	s	ro	? (seems to be power management related?)
0x0B	s	ro	Battery percentage
0x0C	s	ro	? (changes to 0 for a second when the charger is plugged in then it resets to its previous value)
0x0D	s	ro	System voltage
0x0E	s	ro	?
0x0F	s	ro	Flags: bit7-5 are read via mcu::GPU. The rest of these are read via mcu::RTC: bit4 = BatteryChargeState. bit3 = AdapterState. bit1 = ShellState.
0x10 - 0x13	s	ro	Received interrupt bitmask, see register 0x18 for possible values If no interrupt was received this register is 0
0x14	s	ro	?
0x15	s	rw	?
0x16	s	rw	?
0x17	s	rw	?
0x18 - 0x1B	s	rw	Interrupt mask for register 0x10 (0=enabled,1=disabled) bit00: Power button press bit01: Power button held (the 3DS turns off regardless after a fixed time) bit02: HOME button press bit03: HOME button release bit04: WiFi switch button bit05: Shell close bit06: Shell open bit07: Fatal hardware condition(?) bit08: Charger removed bit09: Charger plugged in bit10: ??? (Power Management related?) bit11: ??? (HID related) bit12: HID update bit13: Battery dead(?) bit14: ??? (Power Management related?) bit15: ??? (Power Management related?) bit22: Volume slider position change bit24: ??? (???) bit25: ??? (???) bit26: ??? (???) bit27: ??? (???) bit28: ??? (???) bit29: ??? (???) bit30: forced off my mcu sysmodule bit31: forced off my mcu sysmodule
0x1C	s	rw	? (survives reboot and power off)
0x1D	s	rw	? (survives reboot and power off)
0x1E	s	rw	? (survives reboot and power off)
0x1F	s	rw	? (survives reboot and power off)
0x20	s	wo	System power control: bit0: power off bit1: reboot (unused?) bit2: reboot (used by mcu sysmodule and LgyBg) bit3: used by LgyBg to power off, causes hangs in 3DS-mode bit4: doesn't seem to do anything, but an mcu::RTC command uses this Other bits are unused, and seem to do nothing.
0x21	s	ro(?)	?
0x22	s	wo	Used to set LCD states bit0: don't push to LCDs bit1: push to LCDs bit2: bottom screen backlight off bit3: bottom screen backlight on bit4: top screen backlight off (does nothing on 2DS as the backlight is bottom only) bit5: top screen backlight on (see above)
0x23	s	ro(?)	?
0x24	s	??	Watchdog timer. This must be set before the timer is triggered, otherwise the old value is used. Value zero disables the watchdog.
0x25	s	rw	?
0x26	s	rw	?
0x27	s	ro	Raw volume slider state
0x28	s	rw	Brightness of the WiFi/Power LED
0x29	ds(0x64)	ro / rw	Empty battery pattern holder, repeats all written bytes
0x2A	s	rw	WiFi LED state, non-0 value turns on the WiFi LED, capped at 0x0F
0x2B	s	rw	Camera LED state, 0, 3, 6-0xF = off, 1 = slowly blinking, 2 = constantly on, 4 = flash once, 5 = delay before changing to 2
0x2C	s	rw	3D LED state, capped at 0x0F
0x2D	0x64	wo	This is used for controlling the notification LED (see MCURTC:SetInfoLEDPatternHeader as well), when this register is written. It's possible to write data here with size less than 0x64, and only that portion of the pattern data will get overwritten. Reading from this register only returns zeroes, so it's considered write-only. Writing past the size of this register seems to do nothing.
0x2E	s	ro	This returns the notification LED status when read (1 means new cycle started)
0x2F	s	ro(?)	?
0x30 - 0x37	s	rw	RTC time (system clock). 7 bytes are read from this. The upper nibble of each byte encodes 10s (BCD), so each byte is post-processed with (byte & 0xF) + (10 * (byte >> 4)). byte 0: seconds byte 1: minutes byte 2: hours byte 3: current day of the week (unused) byte 4: days byte 5: months byte 6: years byte 7: leap year counter (unused)
0x38 - 0x3C	s	rw	RTC alarm registers Register 0x3B could be used to upload MCU firmware if some conditions are met.
0x3D 0x3E	s (2)	ro	RTC tick counter (resets to 0 when the seconds increase)
0x3F	s	wo	Peripheral power related? bit0 seems to depower everything, pressing the power key afterwards instantly turns the whole 3DS off
0x40	s	rw	Pedometer state (?)
0x41	s	rw	Index selector for register 0x44
0x42	s	rw	?
0x43	s	rw	???, accelometer related
0x44	s	rw	???, accelometer related
0x45	s	rw	?
0x46	s	ro	Gyro Y(?) axis rotation (0x00 = flat, 0x40 = 3DS standing on right side, 0xBE = 3DS standing on left side)
0x48	s	ro	Gyro Z(?) axis rotation (0x00 = flat, 0x40 = 3DS standing horizontally, 0xBE = 3DS base is horizontally upside-down)
0x4A	s	ro	Gyro X(?) axis rotation (0x00 = 3DS base facing upwards, 0x40 = face-down flat, 0xBE = standing(?) flat)
0x4B	s	rw	PedometerStepCount (for the current day)
0x4C	s	rw	?
0x4D	s	rw	?
0x4E	s	rw	?
0x4F	0x156	ro	???
0x50	s	rw	???
0x51	s	rw	???
0x52 - 0x57	s	rw	?
0x58	s	rw	DSP volume slider 0% volume offset (setting this to 0xFF will esentially mute the DSP as it's the volume slider's maximum raw value)
0x59	s	rw	DSP volume slider 100% volume offset (setting both this and the above to 0 will disable the volume slider with 100% volume, setting this to a lower value than the above will make the volume slider have only 2 states; on and off)
0x5A	s	rw	?
0x5B - 0x5F	s	ro(?)	? (these seem to be invalid regsiters)
0x60	ds(1)	ro first byte is wo	Looping queue register Writing to first byte resets the queue position to the nth element Reading from this register causes the values to shift up by `readsize-1`(needs confirmation) bytes after returning `readsize-1` bytes from the top of the stack (first byte is read-only, so is always zero)
0x61	ds(0x100)	rw	Writing to this register pushes values on top of register 0x60's stack. Reading from this register doesn't advance the stack. The first byte is used to store flags for managing FIRM/NS state - bit0 = "WirelessDisabled", bit1 = "SoftwareClosed", bit2 = "PowerOffInitiated", bit4 = "LegacyJumpProhibited". This register survives power-off, but does not seem to be saved to non-volatile storage (does not survive battery pulls). This register doesn't seem to actually control MCU behaviour by itself, it just seems to be used for storing arbitrary data.
0x62 - 0x7E	s	invalid (ro)	These registers don't exist at all, thus reading them will yield 0xFF
0x7F	19(?)	ro	Various system state information. byte 06: battery related? (seems to decrease while charging and increase while discharging) byte 09: system model (see Cfg:GetSystemModel for values) byte 10: power LED related? 0 is off, 1 is red byte 13: RGB LED red intensity byte 14: RGB LED green intensity byte 15: RGB LED blue intensity byte 17: WiFi LED brightness byte 18: raw button states? bit0: unset while power button is held bit1: unset while home button is held bit2: unset while Wifi slider is held bit5: unset while the charging LED is active bit6: unset while charger is plugged in this byte is reset to 0 before an svcBreak takes effect
0x80 - 0xFF	s	invalid (ro)	These registers don't exist at all, thus reading them will yield 0xFF

Note: the letter "s" in the size field means that the given register is in a "shared register pool", meaning if you read/write with size more than 1 you can read the next `readamount-1` of shared registers. It's possible to corrupt the shared value of a "non-shared" register by writing into a shared register with a size bigger than one. Writing more than 0x100 bytes into a shared register will corrupt all writable registers, including the shared portion of "non-shared" registers.

Non-shared register: it's a register separate from the shared register pool. Messing with these registers will not affect the shared register pool at all.

Device 5 & 6

LCD controllers for main/sub displays, most likely.

Register	Width	Name	Description
0x1	8	?
0x11	8	?
0x40	8	CMD_IN/CMD_RESULT1	Write to trigger a command? Seen commands: 0xFF=Reset?, 0x62=IsFinished?. Result is stored in CMD_RESULT1:CMD_RESULT0.
0x41	8	CMD_RESULT0	Read result
0x50	8	?
0x60	8	?
0xFE	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 14

Used by Cfg-sysmodule via the i2c::EEP service. This is presumably EEPROM going by the service name.

The Cfg-module code which loads the CCAL(nandro:/sys/{HWCAL0.dat/HWCAL1.dat}) file from NAND will load it from I2C instead, if a certain state flag is non-zero. Likewise for the function which writes CCAL to NAND. HMAC/hash verification after loading is skipped when the CCAL was loaded from I2C.

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	Command source / destination.
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.

For command requests sent to the NFC tag itself, Cmd[1]=0x0 and CmdID=0x0. The command request payload data here is the actual command request data for the NFC tag, starting with the CmdID u8 at payload+0.

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).

NFC controller 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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