Fuses

The Nintendo Switch makes use of Tegra's efuse 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 and other miscellaneous functions.

Registers

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

Name	Address	Width
FUSE_CTRL	0x7000F800	0x04
FUSE_REG_ADDR	0x7000F804	0x04
FUSE_REG_READ	0x7000F808	0x04
FUSE_REG_WRITE	0x7000F80C	0x04
FUSE_TIME_PGM2	0x7000F81C	0x04
FUSE_DIS_PGM	0x7000F82C	0x04
FUSE_WRITE_ACCESS	0x7000F830	0x04
FUSE_SKU_INFO	0x7000F910	0x04
FUSE_SPEEDO_1_CALIB_0	0x7000F938	0x04
FUSE_GCPLEX_CONFIG_FUSE_0	0x7000F9C8	0x04
FUSE_GCPLEX_CONFIG_FUSE_1	0x7000F9CC	0x04
FUSE_GCPLEX_CONFIG_FUSE_2	0x7000F9D0	0x04
FUSE_GCPLEX_CONFIG_FUSE_3	0x7000F9D4	0x04
FUSE_GCPLEX_CONFIG_FUSE_4	0x7000F9D8	0x04
FUSE_GCPLEX_CONFIG_FUSE_5	0x7000F9DC	0x04
FUSE_BURNT_FUSE_COUNT_0	0x7000F9E0	0x04
FUSE_BURNT_FUSE_COUNT_1	0x7000F9E4	0x04
FUSE_MASTER_KEY_VER	0x7000FB94	0x04

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_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 registers.

FUSE_SKU_INFO

Stores the SKU ID (must be 0x83).

FUSE_SPEEDO_1_CALIB_0

Seems to store the bootrom patch version.

FUSE_GCPLEX_CONFIG_FUSE_4

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_MASTER_KEY_VER

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).

FUSE_BURNT_FUSE_COUNT_0

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

FUSE_BURNT_FUSE_COUNT_1

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

eFuses

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

Below is a list of common fuse offsets used by Tegra devices (and applicable to the Switch). Note that offsets are relative to the start of the fuse array and a single fuse write operation always writes the same word at both (fuse_offset + 0) and (fuse_offset + 1).

Name	Offset	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 offsets 0x3A (odm_reserved + 0x0C) and 0x3C (odm_reserved + 0x0E) are used for anti-downgrade control.

Anti-downgrade

The first bootloader verifies FUSE_BURNT_FUSE_COUNT_1 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	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 offsets 0x3A and 0x3C. Afterwards, fuse programming is disabled and a magic value (0x21 == TEGRA210) is written to an unknown PMC 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, PMC_RST_STATUS (0x7000E5B4) is checked and if set to 0x01 (watchdog reset) the unknown PMC register (0x7000EC40) will be checked for the magic value 0x21. 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.