Fuses

The Nintendo Switch makes use of Tegra's fuse driver for a number of operations. This driver is mapped to physical address 0x7000F800 with a total size of 0x400 bytes and exposes several registers for fuse programming.

Registers from 0x7000F800 to 0x7000F800 + 0xFF can be used to directly program the hardware fuse array, while registers from 0x7000F800 + 0x100 (FUSE_CHIP_REG_START_OFFSET) to 0x7000F800 + 0x3FC (FUSE_CHIP_REG_END_OFFSET) represent cached values read from certain fuses.

Registers

Below is a list of fuse driver registers used by the Switch's bootloaders.

Driver registers

Name	Address
FUSE_CTRL	0x7000F800
FUSE_REG_ADDR	0x7000F804
FUSE_REG_READ	0x7000F808
FUSE_REG_WRITE	0x7000F80C
FUSE_TIME_RD1	0x7000F810
FUSE_TIME_RD2	0x7000F814
FUSE_TIME_PGM1	0x7000F818
FUSE_TIME_PGM2	0x7000F81C
FUSE_PRIV2INTFC	0x7000F820
FUSE_FUSEBYPASS	0x7000F824
FUSE_PRIVATEKEYDISABLE	0x7000F828
FUSE_DIS_PGM	0x7000F82C
FUSE_WRITE_ACCESS	0x7000F830
FUSE_PWR_GOOD_SW	0x7000F834

FUSE_CTRL

Bits	Description
0-1	Fuse command (1 = FUSE_READ; 2 = FUSE_WRITE; 3 = FUSE_SENSE)
26	Fuse power down mode flag (FUSE_CTRL_PD)

Before fuse reading/writing the power down mode must be disabled. FUSE_SENSE mode flushes programmed values into the cache registers.

FUSE_REG_ADDR

This register takes the address of the fuse to be read/written/sensed.

FUSE_REG_READ

This register receives the value read from the fuse.

FUSE_REG_WRITE

This register takes the value to be written to the fuse.

FUSE_TIME_PGM2

This register takes the fuse programming pulse (0xC0 == 19200 kHz).

FUSE_DIS_PGM

If set to 0x01, this register disables fuse programming.

FUSE_WRITE_ACCESS

If set to 0x01, this register disables software writes to the fuse driver registers.

Cache registers

Name	Address
FUSE_SKU_INFO	0x7000F910
FUSE_CPU_SPEEDO_0	0x7000F914
FUSE_CPU_IDDQ	0x7000F918
FUSE_FT_REV	0x7000F928
FUSE_CPU_SPEEDO_1	0x7000F92C
FUSE_CPU_SPEEDO_2	0x7000F930
FUSE_SOC_SPEEDO_0	0x7000F934
FUSE_SOC_SPEEDO_1	0x7000F938
FUSE_SOC_SPEEDO_2	0x7000F93C
FUSE_SOC_IDDQ	0x7000F940
FUSE_FA	0x7000F948
FUSE_TSENSOR_2	0x7000F988
FUSE_CP_REV	0x7000F990
FUSE_TSENSOR_0	0x7000F998
FUSE_FIRST_BOOTROM_PATCH_SIZE_REG	0x7000F99C
FUSE_VP8_ENABLE	0x7000F9C4
FUSE_RESERVED_ODM0	0x7000F9C8
FUSE_RESERVED_ODM1	0x7000F9CC
FUSE_RESERVED_ODM2	0x7000F9D0
FUSE_RESERVED_ODM3	0x7000F9D4
FUSE_RESERVED_ODM4	0x7000F9D8
FUSE_RESERVED_ODM5	0x7000F9DC
FUSE_RESERVED_ODM6	0x7000F9E0
FUSE_RESERVED_ODM7	0x7000F9E4
FUSE_SKU_USB_CALIB	0x7000F9F0
FUSE_SKU_DIRECT_CONFIG	0x7000F9F4
FUSE_VENDOR_CODE	0x7000FA00
FUSE_FAB_CODE	0x7000FA04
FUSE_LOT_CODE_0	0x7000FA08
FUSE_LOT_CODE_1	0x7000FA0C
FUSE_WAFER_ID	0x7000FA10
FUSE_X_COORDINATE	0x7000FA14
FUSE_Y_COORDINATE	0x7000FA18
FUSE_GPU_IDDQ	0x7000FA28
FUSE_TSENSOR_3	0x7000FA2C
FUSE_OPT_SUBREVISION	0x7000FA48
FUSE_TSENSOR_4	0x7000FA54
FUSE_TSENSOR_5	0x7000FA58
FUSE_TSENSOR_6	0x7000FA5C
FUSE_TSENSOR_7	0x7000FA60
FUSE_OPT_PRIV_SEC_DIS	0x7000FA64
FUSE_TSENSOR_COMMON	0x7000FA80
FUSE_TSENSOR_8	0x7000FAD4
FUSE_RESERVED_CALIB	0x7000FB04
FUSE_TSENSOR_9	0x7000FB1C
FUSE_USB_CALIB_EXT	0x7000FB50
FUSE_SPARE_BIT_0	0x7000FB80
FUSE_SPARE_BIT_1	0x7000FB84
FUSE_SPARE_BIT_2	0x7000FB88
FUSE_SPARE_BIT_3	0x7000FB8C
FUSE_SPARE_BIT_4	0x7000FB90
FUSE_SPARE_BIT_5	0x7000FB94
FUSE_SPARE_BIT_6	0x7000FB98
FUSE_SPARE_BIT_7	0x7000FB9C
FUSE_SPARE_BIT_8	0x7000FBA0
FUSE_SPARE_BIT_9	0x7000FBA4
FUSE_SPARE_BIT_10	0x7000FBA8
FUSE_SPARE_BIT_11	0x7000FBAC
FUSE_SPARE_BIT_12	0x7000FBB0
FUSE_SPARE_BIT_13	0x7000FBB4
FUSE_SPARE_BIT_14	0x7000FBB8
FUSE_SPARE_BIT_15	0x7000FBBC
FUSE_SPARE_BIT_16	0x7000FBC0
FUSE_SPARE_BIT_17	0x7000FBC4
FUSE_SPARE_BIT_18	0x7000FBC8
FUSE_SPARE_BIT_19	0x7000FBCC
FUSE_SPARE_BIT_20	0x7000FBD0
FUSE_SPARE_BIT_21	0x7000FBD4
FUSE_SPARE_BIT_22	0x7000FBD8
FUSE_SPARE_BIT_23	0x7000FBDC
FUSE_SPARE_BIT_24	0x7000FBE0
FUSE_SPARE_BIT_25	0x7000FBE4
FUSE_SPARE_BIT_26	0x7000FBE8
FUSE_SPARE_BIT_27	0x7000FBEC
FUSE_SPARE_BIT_28	0x7000FBF0
FUSE_SPARE_BIT_29	0x7000FBF4
FUSE_SPARE_BIT_30	0x7000FBF8
FUSE_SPARE_BIT_31	0x7000FBFC

FUSE_SKU_INFO

Stores the SKU ID (must be 0x83).

FUSE_FA

Stores if in factory mode (0 = factory).

FUSE_SOC_SPEEDO_1

Stores the bootrom patch version.

FUSE_RESERVED_ODM4

Bits	Description
0-1	Unit type (3 = debug; 0 = retail)
2	Unknown config (must be 1 on retail)
8	Unknown config mask (must be 0 on retail)
9	Unit type mask (0 = debug; 1 = retail)

This stores some device configuration parameters.

FUSE_RESERVED_ODM6

This register returns the value programmed into index 0x3A of the fuse array.

FUSE_RESERVED_ODM7

This register returns the value programmed into index 0x3C of the fuse array.

FUSE_SPARE_BIT_5

Must be non-zero on retail units, otherwise the first bootloader panics. On debug units it can be zero, which tells the bootloader to choose from two debug master key seeds. If set to non-zero on a debug unit, it tells the bootloader to choose from two retail master key seeds (only the last one matches the retail master key seed).

eFuses

The actual hardware fuses can be programmed through the fuse driver after enabling fuse programming.

Below is a list of common fuse indexes used by Tegra devices (and applicable to the Switch). Note that the indexes are relative to the start of the fuse array and each element is a 4 byte word. A single fuse write operation always writes the same word at both fuse_array + 0 (PRIMARY_ALIAS) and fuse_array + 1 (REDUNDANT_ALIAS).

Name	Index	Bits
jtag_disable	0x00	1
odm_production_mode	0x00	1
odm_lock	0x00	4
public_key	0x0C	256
secure_boot_key	0x22	128
device_key	0x2A	32
sec_boot_dev_cfg	0x2C	16
sec_boot_dev_sel	0x2C	3
sw_reserved	0x2E	12
ignore_dev_sel_straps	0x2E	1
odm_reserved	0x2E	256
pkc_disable	0x52	1

odm_reserved

The first bootloader only burns fuses in this region. Both fuse indexes 0x3A (odm_reserved + 0x0C) and 0x3C (odm_reserved + 0x0E) are used for anti-downgrade control. These fuses will have their values cached into FUSE_RESERVED_ODM6 and FUSE_RESERVED_ODM7.

Anti-downgrade

The first bootloader verifies FUSE_RESERVED_ODM7 to prevent downgrading. How many fuses are expected to be burnt depends the device's unit type as below.

System version	Expected number of burnt fuses (retail)	Expected number of burnt fuses (non-retail)
1.0.0	1	0
2.0.0-2.3.0	2	0
3.0.0	3	1
3.0.1-3.0.2	4	1

If too many fuses are burnt the bootloader will panic immediately.

If too few are burnt, the bootloader will enable fuse programming and write the expected value to fuse indexes 0x3A and 0x3C. Afterwards, fuse programming is disabled and a magic value (0x21 == TEGRA210) is written to PMC_SCRATCH200 register (0x7000EC40). Finally, the watchdog timer is initialized and programmed to force a reset.

On a subsequent boot, after the anti-downgrade fuses are checked again, the PMC_RST_STATUS register (0x7000E5B4) is checked and if set to 0x01 (watchdog reset) the PMC_SCRATCH200 register (0x7000EC40) will be checked for the magic value (0x21 == TEGRA210). PMC_RST_STATUS will only be set back to 0 (power on reset) if the fuse count matches the new expected value, otherwise the system will panic.