MCU Services

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Only one session can be open per service at a time. If a session is already open for a service, MCU module will wait for the thread handling the session to terminate(triggered by the session being closed by the user process), then it accepts the new session. The commands for each service are handled by separate threads.

MCU camera service "mcu::CAM"[edit]

Command Header Description
0x00010040 WriteCameraLedState (writes i2c register 0x2B)
0x00020080 ReadCameraLedState (reads i2c register 0x2B)

MCU GPU service "mcu::GPU"[edit]

Command Header Description
0x00010000 GetLcdPowerState. This writes the value of I2C-MCU register 0xf bit6 to u8 cmdreply[2], and the value of bit5 from that register to u8 cmdreply[3].
0x00020080 SetLcdPowerState. This writes the upper LCD bits of MCU register 0x22.
0x00030000 GetGpuLcdInterfaceState. This writes the value of I2C-MCU register 0xf bit7 to u8 cmdreply[2].
0x00040040 SetGpuLcdInterfaceState. This writes the lower two bits of MCU register 0x22.
0x00050040 SetTopScreenFlicker
0x00060080 GetTopScreenFlicker
0x00070040 SetBottomScreenFlicker
0x00080080 GetBottomScreenFlicker
0x00090000 GetMcuFwVerHigh. Called by GSP module
0x000A0000 GetMcuFwVerLow. Called by GSP module
0x000B0040 Set3dLedState
0x000C0000 Get3dLedState
0x000D0000 GetMcuGpuEventHandle. Event handle written to TLS+0x8c. MCU notifications 24 to 29 signal this.
0x000E0000 GetMcuGpuEventReason. Writes some value to TLS+0x88. Called by GSP module

MCU HID service "mcu::HID"[edit]

Command Header Description
0x00010040  ?
0x00020000  ??? test register 0x40 bit0, and writes result to IPC+8
0x00030040  ??? writes IPC+4 to register 0x41
0x00040000  ??? reads register 0x44 to IPC+8
0x00050080  ??? writes IPC+4 to register 0x43 and IPC+8 to register 0x44
0x00060000 ReadGyroscopeValues (reads gyroscopy with corrected values)
0x00070000 GetRaw3DSliderPosition
0x00080040  ?
0x00090000  ?
0x000A0040  ?
0x000B0000  ?
0x000C0000 GetMcuHidEventHandle. MCU notifications 11 and 12 signal this. Handle is written to IPC+12
0x000D0000 GetMcuHidEventReason. This reads an internal flield into IPC+8 and clears it.
0x000E0000 GetSoundVolume
0x000F0040 EnableAccelerometerInterrupt(int enable). 1 = enable, 0 = disable accelerometer

While before these functions are handled, the MCU interrupt with bitmask 0x800 is enabled, then after the commands were handled the MCU interrupt bits 0x1800 get cleared.

MCU service "mcu::RTC"[edit]

Command Header Description
0x00010080 SetRTC
0x00020000 GetRTC
0x00030040 SetRTCSeconds(u8 seconds)
0x00040000 u8 GetRTCSeconds
0x00050040 SetRTCMinutes(u8 minutes)
0x00060000 u8 GetRTCMinutes
0x00070040 SetRTCHours(u8 hours)
0x00080000 u8 GetRTCHours
0x00090040 SetRTCDayOfWeek(u8 dayno)
0x000A0000 u8 GetRTCDayOfWeek
0x000B0040 SetRTCDayOfMonth(u8 day)
0x000C0000 u8 GetRTCDayOfMonth
0x000D0040 SetRTCMonth(u8 month)
0x000E0000 u8 GetRTCMonth
0x000F0040 SetRTCYear(u8 year) where year = year - 2000
0x00100000 u8 GetRTCYear where year = result + 2000
0x00110040 SetRTCLeapYearCounter(u8 leap)
0x00120000 u8 GetRTCLeapYearCounter
0x00130080 SetRTCAlarm
0x00140000 GetRTCAlarm
0x00150040 SetRTCAlarmComponent[0] (u8 val)
0x00160000 u8 GetRTCAlarmComponent[0]
0x00170040 SetRTCAlarmComponent[1] (u8 val)
0x00180000 u8 GetRTCAlarmComponent[1]
0x00190040 SetRTCAlarmComponent[2] (u8 val)
0x001A0000 u8 GetRTCAlarmComponent[2]
0x001B0040 SetRTCAlarmComponent[3] (u8 val)
0x001C0000 u8 GetRTCAlarmComponent[3]
0x001D0040 SetRTCAlarmComponent[4] (u8 val)
0x001E0000 u8 GetRTCAlarmComponent[4]
0x001F0040 SetPedometerRecordingMode(u8 mode)
0x00200000 u8 GetPedometerRecordingMode
0x00210080 u8 GetStepCount (for the current day)
0x00220042 ReadRegister4Fh(u32 unused_size, translation_param size=0x156 << 4 | 0xC, u8[0x156] ptr)
0x00230000 void ??? writes 1 to register 0x4E which is not writable
0x00240000 Handle GetPowerEventHandle. MCU notifications 1, 8, 9, 10, 13, 14 and 15 signal this.

see Register 0x18

0x00250000 u32 GetPowerInterruptHistory
0x00260000 bool CheckRegister02hBit0
0x00270000 void ClearRegister02hBit0 (does nothing since the register is not writable)
0x00280000 bool CheckRegister02hBit1
0x00290000 void ClearRegister02hBit1
0x002A0000 bool GetShellState. This writes the value of I2C-MCU register 0xf bit1 to u8 cmdreply[2].
0x002B0000 bool GetAdapterState. This writes the value of I2C-MCU register 0xf bit3 to u8 cmdreply[2].
0x002C0000 bool GetBatteryChargeState. This writes the value of I2C-MCU register 0xf bit4 to u8 cmdreply[2].
0x002D0000 GetBatteryLevel
0x002E0000 u8 GetBatteryEmptyPatternByte0
0x002F0000 u8 GetBatteryEmptyPatternByte0_safe™
0x00300040 SetLEDBrightness(u8 brightness) (see Register 0x28)
0x00310000 u8 GetLEDBrightness (see Register 0x28)
0x00320000 void PowerOff (writes 0x1 to i2c MCU device, reg 0x20)
0x00330000 void HardwareReboot (writes 0x4 to i2c MCU device, reg 0x20)
0x00340000 WriteRegister reg=0x23 value=0x72 (writing to read-only registers does nothing)
0x00350000 Writes 0x10 to i2c MCU device, reg 0x20 (this bit of the register is not writable)
0x00360040 SetWatchdogTimer(u8 timer)
0x00370000 u8 GetWatchdogTimer
0x00380042 ReadInfoRegister(u32 unused_size, translation_param size << 4 | 0xC, u8[0x13] ptr)
0x00390082 WriteLoop (translation parameters too complex)
0x003A0082 ReadLoop (translation parameters too complex)
0x003B0640 SetInfoLEDPattern
0x003C0040 SetInfoLEDPatternHeader
0x003D0000 GetInfoLEDStatus
0x003E0040 WriteRegister50h(u8 value)
0x003F0000 u8 ReadRegister50h
0x00400040 WriteRegister51h(u8 value)
0x00410000 u8 ReadRegister51h
0x00420040 SetBatteryEmptyLEDPattern
0x00430040 SetScreenFlickerTop(u8 flicker)
0x00440000 u8 GetScreenFlickerTop
0x00450040 SetScreenFlickerBottom(u8 flicker)
0x00460000 u8 GetScreenFlickerBottom
0x00470080 SetVolumeSliderBounds(u8 low, u8 high) (see Register 0x58)
0x00480000 u8 low, u8 high GetVolumeSliderBounds
0x00490040 SetInterruptMask(u32 mask) (see Register 0x18)
0x004A0000 u32 GetInterruptMask (see Register 0x18)
0x004B0000 void ExitExclusiveInterruptMode
0x004C0000 void EnterExclusiveInterruptMode
0x004D0000 ReadInterrupt (see Register 0x10)
0x004E0040 TriggerInterrupt
0x004F0040 SetMCUFirmUpdated(u32 flag) not used by anything
0x00500000 u32 IsMCUFirmUpdated
0x00510040 SetSoftwareClosedFlag
0x00520000 GetSoftwareClosedFlag
0x00530040  ?
0x00540000  ?
0x00550040  ?
0x00560000  ?
0x00570040  ?
0x00580000  ?
0x00590040 SetLegacyJumpProhibitedFlag
0x005A0000 GetLegacyJumpProhibitedFlag

MCU sound service "mcu::SND"[edit]

Command Header Description
0x00010080 GetSoundVolume, writes volume slider value (0-63) to IPC+8
0x00020040 Set...
0x00030000 GetRegister25h, cmdbuf[2] is 0 on n3ds

MCU wifi service "mcu::NWM"[edit]

Command Header Description
0x0001.... SetWirelessLedState
0x0002.... GetWirelessLedState
0x0003.... Sets GPIO 0x20 high/low?
0x0004.... Gets GPIO 0x20 high/low?
0x0005.... SetEnableWifiGpio
0x0006.... GetEnableWifiGpio
0x0007.... SetWirelessDisabledFlag
0x0008.... GetWirelessDisabledFlag

MCU service "mcu::HWC"[edit]

Command Header Description
0x00010082 ReadRegister
0x00020082 WriteRegister
0x00030042 GetInfoRegisters
0x00040000 GetBatteryVoltage
0x00050000 GetBatteryLevel
0x00060040 SetPowerLEDPattern
0x00070040 SetWifiLEDState
0x00080040 SetCameraLEDPattern
0x00090040 Set3DLEDState
0x000A0640 This is the same as MCURTC:SetInfoLEDPattern.
0x000B0000 GetSoundVolume
0x000C0040 SetTopScreenFlicker
0x000D0040 SetBottomScreenFlicker
0x000E0080  ?
0x000F00C0 GetRtcTime
0x00100000 GetMcuFwVerHigh
0x00110000 GetMcuFwVerLow

MCU service "mcu::PLS"[edit]

Beg the sysmodule to return the datetime registers in decimal form instead of as a Binary Coded Decimal

Command Header Description
0x00010000 GetDatetime (returns registers 0x30-0x36 in IPC+8)
0x00020000 u8 GetSeconds
0x00030000 u8 GetMinutes
0x00040000 u8 GetHour
0x00050000 u8 GetDayOfWeek
0x00060000 u8 GetDay
0x00070000 u8 GetMonth
0x00080000 u8 GetYear
0x00090000 u16 GetTickCounter

MCU codec service "mcu::CDC"[edit]

Command Header Description
0x00010000  ?


The Old3DS/New3DS MCU sysmodules are identical except that the MCU firmware binary written via I2C is different. The size of that binary is the same. The only different words in .text are for the version of that MCU fw binary.

MCU firmware versions[edit]

These reside in mcu-module .rodata, are uploaded to MCU register 0x05 and are usually 0x4003 bytes in size (the actual firmware is 0x4000 bytes preceeded by a 3 byte RL78 assembly opcode "jhl" (or 0xffe68, #108) to switch the I2C comms into flash write mode).

Before the upload could commence, WiFi interrupts are turned off via GPIO command 0x00020080(0, 0x40000), then after the upload completed, the sysmodule waits exactly one second for the MCU to reboot, then turns WiFi interrupts back on via gpio:MCU command 0x00020080(0x40000, 0x40000).

There exists an alternate code path where uploading is done using register 0x3B (if register 0x0F is zero meaning all peripherals are turned off, and 0x10 must be 1 (power button pressed/held)). This may be a "hack" around early versions of MCU? Register 0x3B is part of the RTC alarm registers on recent versions of MCU.

On dev-units, the user-facing representation of this firmware version is displayed by first subtracting 0x10 from the major field (raw register 0x00). It is these user-facing versions that are displayed in the table below. It is unknown what bit4 (0x10) actually represents, but it is seemingly always set.

Title version Firmware
New3DS v9216 (New2DSXL) 3.65
New3DS v8192/safe v9217 (latest) 3.56
Old3DS v6145 to v8192 (latest) 2.37
Old3DS v5122 2.35
Old3DS v4102 2.30
Old3DS v3072 2.16
Old3DS v2048 1.52
Old3DS v1026 1.51
Old3DS v0/safe v0 1.20
Old3DS factory 1.07