I2C Registers

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_CNTEX

BIT	DESCRIPTION
0-1	? Set to 2 normally.

I2C_SCL

BIT	DESCRIPTION
0-5	?
8-12	? Set to 5 normally.

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"	Debug(?) gyroscope. 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 (old3DS)
11	3	0xd2	"i2c::HID"	Gyroscope (2DS, new3DSXL, new2DSXL)
12	3	0xa4	"i2c::HID"	DebugPad
13	3	0x9a	"i2c::IR"	IR
14	3	0xa0	"i2c::EEP"	HWCAL EEPROM (only present on dev units where SHA256 is used for HWCAL verification)
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

 ro = read-only (writing is no-op)
 rw = read-write
 wo = write-only (reading will yield 00, FF, or unpredictable data)

 d* = dynamic register (explaination below this table)
 s* = shared register (explaination below this table)
 ds = dynamic shared (explaination below this table)

REGISTER	WIDTH	INFO	DESCRIPTION
0x00	s	ro	Version high
0x01	s	ro	Version low
0x02	d	rw	For bit0 and 1 values, writing will mask away/"acknowledge" the event, set to 3 by mcuMainLoop on reset if reset source is Watchdog bit0: RTC clock value got reset to defaults bit1: Watchdog reset happened bit5: TWL volume mode (0: 8-step, 1: 32-step) bit6: Is in TWL mode
0x03	ds	rw	Top screen Vcom
0x04	ds	rw	Bottom screen Vcom
0x05 - 0x07	s	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	Battery temperature (in Celcius?)
0x0B	s	ro	Battery percentage
0x0C	s	ro	Battery percentage, fractional part (seems to have a resolution of around 0.1% according to tests)
0x0D	s	ro	System voltage
0x0E	s	ro	?
0x0F	s	ro	Flags: bit7-5 are read via mcu::GPU. The rest of them are read via mcu::RTC. bit01: ShellState bit03: AdapterState bit04: BatteryChargeState bit05: Bottom screen backlight on bit06: Top screen backlight on bit07: GPU on(?)
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	Unused and unwritable byte :(
0x15 - 0x17	s	rw	Unused and unreferenced free RAM! Good for userdata.
0x18 - 0x1B	s	rw	Interrupt mask for register 0x10 (0=enabled,1=disabled) bit00: Power button press (for 27 "ticks") bit01: Power button held (for 375 "ticks"; the 3DS turns off regardless after a fixed time) bit02: HOME button press (for 5 "ticks") bit03: HOME button release bit04: WiFi switch button bit05: Shell close bit06: Shell open bit07: Fatal hardware condition(?) (sent when the MCU gets reset by the Watchdog timer) bit08: Charger removed bit09: Charger plugged in bit10: RTC alarm (when some conditions are met it's sent when the current day and month and year matches the current RTC time) bit11: ??? (accelerometer related) bit12: HID update bit13: Battery percentage status change (triggered at 10%, 5%, and 0% while discharging) bit14: Battery stopped charging (independent of charger state) bit15: Battery started charging Nonmaskable(?) interrupts bit16: ??? bit17: ??? (opposite even for bit16) bit22: Volume slider position change bit23: ??? Register 0x0E update bit24: ??? (the off event for below bit) bit25: ??? (triggered when something related to the GPU is turned on, most likely backlight) bit26: ??? (???) bit27: ??? (???) bit28: ??? (???) bit29: ??? backlight on? bit30: bit set by mcu sysmodule bit31: bit set by mcu sysmodule
0x1C - 0x1F	s	rw	Unused and unreferenced free RAM! Good for userdata.
0x20	d	wo	System power control: bit0: power off bit1: full reboot (unused). Discards things like CFG9_BOOTENV bit2: reboot. Preserves CFG9_BOOTENV, etc. bit3: reset FCRAM (present in by LgyBg. Unused because a system reboot does the same thing & a PDN reg also implements this function. bit4: signal that sleep mode is about to be entered (used by PTM) Bit 4 sets a bit at a RAM address which seems to control the watcdog timer state, then this bit is immediately unmasked. This field has a bitmask of 0x0F.
0x21	d	wo	Used in legacy mode to signal events for TWL MCU "emulation" (written to REG[0x5D])? Software then asserts the TWL MCU IRQ pin via Legacy I/O registers. bit0: Signal TWL POWER button click bit1: Signal TWL reset bit2: Signal TWL power off bit3: Signal TWL battery low bit4: Signal TWL battery empty bit5: Signal TWL volume button click
0x22	d	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 bit5: top screen backlight on Bits 4 and 5 have no effect on a 2DS because the backlight source is the bottom screen. The rest of the bits are masked away.
0x23	d	wo	Writing 0x72 ('r') resets the MCU, but this is stubbed on retail?
0x24	s	rw	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	sd	rw	Raw volume slider state
0x28	s	rw	Brightness of the WiFi/Power LED
0x29	sd(5)	??	Power mode indicator state (read-write) 1 = forced default blue 2 = sleep mode animation 3 = "power off" mode 4 = disable blue power LED and turn on red power LED 5 = disable red power LED and turn on blue power LED 6 = animate blue power LED off and flash red power LED anything else = automatic mode The other 4 bytes (32bits) affect the pattern of the red power LED (write only)
0x2A	s	rw	WiFi LED state, non-0 value turns on the WiFi LED, 4 bits wide
0x2B	s	rw	Camera LED state, 4bits wide, 0, 3, 6-0xF = off 1 = slowly blinking 2 = constantly on 3 = "TWL" mode 4 = flash once 5 = delay before changing to 2
0x2C	s	rw	3D LED state, 4 bits wide
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	wo?	??? The write function for this register is stubbed.
0x30 - 0x36	ds	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 week (unused) byte 4: days byte 5: months byte 6: years
0x37	s	rw	RTC time byte 7: leap year counter / "watch error correction" register (unused in code)
0x38 - 0x3C	s	rw	RTC alarm registers byte 0: minutes byte 1: hours byte 2: day byte 3: month byte 4: year
0x3B	s	rw	Could be used on extremely old MCU_FIRM versions to upload MCU firmware if reg 0xF == 0 and reg 0x10 == 1 (presumably major and minor version fields for mcufw 0.1 which largely predates factory firm).
0x3D 0x3E	ds	ro	RTC tick counter / "ITMC" (when resets to 0 the seconds increase) Only reading 0x3D will update the in-RAM value
0x3F	s	wo	2 bits bit0: turns off P00 and sets it to output mode (seems to kill the entire SoC) bit1: turns on a prohibited bit in an RTC Control register and turns P12 into an output
0x40	s	rw	Tilt sensor sampling mode. Bits 0 and 1 control the mode. If bits 0 or 1 are set then the tilt sensor is enabled and sampled.
0x41	s	rw	Index selector for register 0x44
0x42	s	rw	Unused?
0x43	s	rw	Unused???, accelometer related
0x44	s	rw	???, pedoometer related(?)
0x45 - 0x4A	s	ro	Tilt sensor 3D rotation from the 12bit ADC, left shifted 4 to fit in a 16bit signed short, relative to the 3DS bottom screen AXIS V=0x00 V=0x40 V=0xC0 X (left/right) held up vertically rotated left 90° like a steering wheel rotated right 90° like a steering wheel Y (forwards/backwards) laid flat on the desk with the screen facing up held up vertically held up vertically with screen facing upside-down Z (???) ??? ??? ???
0x4B	s	rw	PedometerStepCount (for the current day)
0x4C 0x4D	??	??	??
0x4E	d	rw	??? this = (0xFFE9E & 1) ? 0x10 : 0
0x4F	d(6)	ro
0x50	s	rw	???
0x51	s	rw	???
0x52 - 0x57	s	rw	?
0x58	s	rw	Register-mapped ADC register 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	Register-mapped ADC register 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	ro/rw	Invalid, do not use! On newer MCU_FIRM versions this is unused, but on older MCU_FIRM versions this is a read-only counter.
0x5B - 0x5F	s	-	These registers are out of bounds (0xFFC00 and up), they don't exist, writing is no-op, reading will yield FFs.
0x60	ds	rw	Free register bank address select Selects the index to read from in the free register bank, up to 200 (?). Used with reg 0x61. Reading from this register seem to read N bytes from the bank while incrementing the internal index by the same amount. byte 0: bit0 = "WirelessDisabled", bit1 = "SoftwareClosed", bit2 = "PowerOffInitiated", bit3 = "LgyNativeResolution", bit4 = "LegacyJumpProhibited" byte 1: Legacy LCD data bytes 2 and 3: Local Friend Code counter bytes 4 and 5: UUID clock sequence bytes 6 and 7: Unused bytes 8 to 175: Playtime data for legacy titles bytes 176 to 188: Playtime data bytes 188 to 199: Unused
0x61	ds(0x100)	rw	Free register bank, data is read from/written to here. Writing to here seems to increment the internal index?
0x62 - 0x7E	s	-	These registers don't exist, writing is no-op, reading will yield FFs.
0x7F	d(9-0x13)	ro	Various system state information (debug pointer table) byte 0x06: battery related? (seems to decrease while charging and increase while discharging) byte 0x09: system model (see Cfg:GetSystemModel for values) byte 0x0A: Red Power LED mode (0 = off, 1 = on) byte 0x0B: Blue Power LED intensity (0x00 - 0xFF) byte 0x0D: RGB LED red intensity byte 0x0E: RGB LED green intensity byte 0x0F: RGB LED blue intensity byte 0x11: WiFi LED brightness byte 0x12: 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 On MCU_FIRM major version 1 the size of this is 9, reading past the 9th byte will yield AA instead of FF.
0x80 - 0xFF	s	-	These registers don't exist, writing is no-op, reading will yield FFs.

Shared register: the letter "s" means that the given register is in a "shared register pool", meaning the resgister is in the register pool in RAM at address 0xFFBA4 + registernumber.

Dynamic register: these registers aren't in the shared pool, they just "pretend" to be there. These registers often don't retain their set value, change rapidly, or control various hardware.

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

On old versions of MCU_FIRM none of the invalid registers are masked away by the read handler function, but are still read-only. Newer MCU_FIRM versions return the hardcoded value FF instead.

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 11

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.