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registers.md

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Registers

Summary of the regs interface's registers

Name Offset Length Description
sram_ctrl.ALERT_TEST 0x0 4 Alert Test Register
sram_ctrl.STATUS 0x4 4 SRAM status register.
sram_ctrl.EXEC_REGWEN 0x8 4 Lock register for execution enable register.
sram_ctrl.EXEC 0xc 4 Sram execution enable.
sram_ctrl.CTRL_REGWEN 0x10 4 Lock register for control register.
sram_ctrl.CTRL 0x14 4 SRAM ctrl register.
sram_ctrl.SCR_KEY_ROTATED 0x18 4 Clearable SRAM key request status.

ALERT_TEST

Alert Test Register

  • Offset: 0x0
  • Reset default: 0x0
  • Reset mask: 0x1

Fields

{"reg": [{"name": "fatal_error", "bits": 1, "attr": ["wo"], "rotate": -90}, {"bits": 31}], "config": {"lanes": 1, "fontsize": 10, "vspace": 130}}
Bits Type Reset Name Description
31:1 Reserved
0 wo 0x0 fatal_error Write 1 to trigger one alert event of this kind.

STATUS

SRAM status register.

  • Offset: 0x4
  • Reset default: 0x0
  • Reset mask: 0x3f

Fields

{"reg": [{"name": "BUS_INTEG_ERROR", "bits": 1, "attr": ["ro"], "rotate": -90}, {"name": "INIT_ERROR", "bits": 1, "attr": ["ro"], "rotate": -90}, {"name": "ESCALATED", "bits": 1, "attr": ["ro"], "rotate": -90}, {"name": "SCR_KEY_VALID", "bits": 1, "attr": ["ro"], "rotate": -90}, {"name": "SCR_KEY_SEED_VALID", "bits": 1, "attr": ["ro"], "rotate": -90}, {"name": "INIT_DONE", "bits": 1, "attr": ["ro"], "rotate": -90}, {"bits": 26}], "config": {"lanes": 1, "fontsize": 10, "vspace": 200}}
Bits Type Reset Name
31:6 Reserved
5 ro 0x0 INIT_DONE
4 ro 0x0 SCR_KEY_SEED_VALID
3 ro 0x0 SCR_KEY_VALID
2 ro 0x0 ESCALATED
1 ro 0x0 INIT_ERROR
0 ro 0x0 BUS_INTEG_ERROR

STATUS . INIT_DONE

Set to 1 if the hardware initialization triggered via CTRL.INIT has completed.

STATUS . SCR_KEY_SEED_VALID

Set to 1 if the scrambling key has been derived from a valid key seed in OTP. If STATUS.SCR_KEY_VALID is set to 1, STATUS.SCR_KEY_SEED_VALID should be 1 except for cases where the scrambling key seeds have not yet been provisioned to OTP. In such a case, the scrambling key is still ephemeral (i.e., it is derived using entropy from CSRNG), but a default all-zero value is used as the key seed.

STATUS . SCR_KEY_VALID

Set to 1 if a new scrambling key has been successfully obtained from OTP. Note that if this is set to 0, the SRAM contents are still scrambled, but a default all-zero key and nonce are used to do so.

STATUS . ESCALATED

Set to 1 if the sram controller has received an escalate request. If this is set to 1, the scrambling keys have been reset to the default values and all subsequent memory requests will be blocked. This condition is terminal.

STATUS . INIT_ERROR

This bit is set to 1 if a the initialization counter has reached an invalid state. This error triggers a fatal_error alert. This condition is terminal.

STATUS . BUS_INTEG_ERROR

This bit is set to 1 if a fatal bus integrity fault is detected. This error triggers a fatal_error alert. This condition is terminal.

EXEC_REGWEN

Lock register for execution enable register.

  • Offset: 0x8
  • Reset default: 0x1
  • Reset mask: 0x1

Fields

{"reg": [{"name": "EXEC_REGWEN", "bits": 1, "attr": ["rw0c"], "rotate": -90}, {"bits": 31}], "config": {"lanes": 1, "fontsize": 10, "vspace": 130}}
Bits Type Reset Name Description
31:1 Reserved
0 rw0c 0x1 EXEC_REGWEN When cleared to zero, EXEC can not be written anymore.

EXEC

Sram execution enable.

  • Offset: 0xc
  • Reset default: 0x9
  • Reset mask: 0xf
  • Register enable: EXEC_REGWEN

Fields

{"reg": [{"name": "EN", "bits": 4, "attr": ["rw"], "rotate": 0}, {"bits": 28}], "config": {"lanes": 1, "fontsize": 10, "vspace": 80}}
Bits Type Reset Name
31:4 Reserved
3:0 rw 0x9 EN

EXEC . EN

Write kMultiBitBool4True to this field to enable execution from SRAM. Note that this register only takes effect if the EN_SRAM_IFETCH switch in the OTP HW_CFG1 partition is set to kMultiBitBool8True. Otherwise execution from SRAM cannot be enabled via this register.

CTRL_REGWEN

Lock register for control register.

  • Offset: 0x10
  • Reset default: 0x1
  • Reset mask: 0x1

Fields

{"reg": [{"name": "CTRL_REGWEN", "bits": 1, "attr": ["rw0c"], "rotate": -90}, {"bits": 31}], "config": {"lanes": 1, "fontsize": 10, "vspace": 130}}
Bits Type Reset Name Description
31:1 Reserved
0 rw0c 0x1 CTRL_REGWEN When cleared to zero, CTRL can not be written anymore.

CTRL

SRAM ctrl register.

  • Offset: 0x14
  • Reset default: 0x0
  • Reset mask: 0x3
  • Register enable: CTRL_REGWEN

Fields

{"reg": [{"name": "RENEW_SCR_KEY", "bits": 1, "attr": ["wo"], "rotate": -90}, {"name": "INIT", "bits": 1, "attr": ["wo"], "rotate": -90}, {"bits": 30}], "config": {"lanes": 1, "fontsize": 10, "vspace": 150}}
Bits Type Reset Name
31:2 Reserved
1 wo 0x0 INIT
0 wo 0x0 RENEW_SCR_KEY

CTRL . INIT

Write 1 to request memory init. The init mechanism uses an LFSR that is seeded with a part of the nonce supplied when requesting a scrambling key. Once seeded, the memory is initialized with pseudo-random data pulled from the LFSR. Note that CTRL.RENEW_SCR_KEY takes priority when writing 1 to both CTRL.RENEW_SCR_KEY and CTRL.INIT with the same write transaction. This means that the key request will complete first, followed by SRAM initialization.

CTRL . RENEW_SCR_KEY

Write 1 to request a new scrambling key from OTP. After writing to this register, SRAM transactions will be blocked until STATUS.SCR_KEY_VALID has been set to 1. If STATUS.SCR_KEY_VALID was already 1 before triggering a key renewal, hardware will automatically clear that status bit such that software can poll its status. Note that requesting a new scrambling key takes ~200 OTP cycles, which translates to ~800 CPU cycles (OTP runs at 24MHz, CPU runs at 100MHz). Note that writing 1 to this register while a key request is pending has no effect.

SCR_KEY_ROTATED

Clearable SRAM key request status.

  • Offset: 0x18
  • Reset default: 0x9
  • Reset mask: 0xf

Fields

{"reg": [{"name": "SUCCESS", "bits": 4, "attr": ["rw1c"], "rotate": -90}, {"bits": 28}], "config": {"lanes": 1, "fontsize": 10, "vspace": 90}}
Bits Type Reset Name
31:4 Reserved
3:0 rw1c 0x9 SUCCESS

SCR_KEY_ROTATED . SUCCESS

This status register is similar to SCR_KEY_VALID with the difference that the status is multibit encoded, SW clearable and sticky (i.e., HW does not auto-clear the register except during escalation). That way, SW can use this for a hardened acknowledgement mechanism where it clears the register before requesting a key.

kMultiBitBool4True indicates that a valid scrambling key has been obtained from OTP. Write kMultiBitBool4True to clear.

This interface does not expose any registers.