I2C Registers

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Registers[edit]

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[edit]

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[edit]

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[edit]

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  ? (volatile;survives reboot and power off)
0x16 s rw  ? (volatile;survives reboot and power off)
0x17 s rw  ? (volatile;survives reboot and power off)
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  ? (volatile;survives reboot and power off)
0x1D s rw  ? (volatile;survives reboot and power off)
0x1E s rw  ? (volatile;survives reboot and power off)
0x1F s rw  ? (volatile;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[edit]

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[edit]

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

Device 12[edit]

REGISTER WIDTH DESCRIPTION
0x0 21 DebugPad state.

This is the DebugPad device, see here.

Device 13[edit]

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[edit]

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[edit]

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[edit]

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.