interfaces.md

Interfaces

Referring to the Comportable guideline for peripheral device functionality, the module entropy_src has the following hardware interfaces defined

• Primary Clock: clk_i
• Other Clocks: none
• Bus Device Interfaces (TL-UL): tl
• Bus Host Interfaces (TL-UL): none
• Peripheral Pins for Chip IO: none

Inter-Module Signals

Port Name	Package::Struct	Type	Act	Width	Description
entropy_src_hw_if	entropy_src_pkg::entropy_src_hw_if	req_rsp	rsp	1
entropy_src_rng_enable	logic	uni	req	1	Signal through which entropy_src enables the noise source. entropy_src will keep this signal high as long as it expects the noise source to operate. This is not a flow control signal through which entropy_src would exert backpressure on the noise source; rather this signal stays high while entropy_src is enabled.
entropy_src_rng_valid	logic	uni	rcv	1	Acknowledgement signal from the noise source. When '1', it indicates that the entropy_src_rng_bit data is valid and ready to be consumed.
entropy_src_rng_bits	logic	uni	rcv	RngBusWidth	Output data bus carrying the raw entropy bits from the noise source. These bits are valid when entropy_src_rng_valid is asserted. The width is determined by RngBusWidth parametrization.
entropy_src_xht_valid	logic	uni	req	1	Valid signal for the external health test interface. When asserted, it indicates that entropy_src_xht_bits, entropy_src_xht_bit_sel, and entropy_src_xht_meta are valid for consumption by the external health test.
entropy_src_xht_bits	logic	uni	req	RngBusWidth	Carries the raw entropy data from the entropy source to be consumed by the external health test module. The data on this bus is valid when entropy_src_xht_valid is asserted. The width is determined by RngBusWidth parametrization.
entropy_src_xht_bit_sel	logic	uni	req	RngBusBitSelWidth	Provides bit selection information for the raw entropy data. It specifies which specific bit or subset of bits from entropy_src_xht_bit should be used. The width is determined by RngBusBitSelWidth parametrization.
entropy_src_xht_health_test_window	logic	uni	req	HealthTestWindowWidth	Provides the window size of the health in bits.
entropy_src_xht_meta	entropy_src_pkg::entropy_src_xht_meta	req_rsp	req	1
otp_en_entropy_src_fw_read	prim_mubi_pkg::mubi8	uni	rcv	1
otp_en_entropy_src_fw_over	prim_mubi_pkg::mubi8	uni	rcv	1
rng_fips	logic	uni	req	1
tl	tlul_pkg::tl	req_rsp	rsp	1

Interrupts

Interrupt Name	Type	Description
es_entropy_valid	Event	Asserted when entropy source bits are available for firmware for consumption via ENTROPY_DATA register.
es_health_test_failed	Event	Asserted whenever the main state machine is in the alert state, e.g., due to health tests failing and reaching the threshold value configured in ALERT_THRESHOLD.
es_observe_fifo_ready	Event	Asserted when the observe FIFO has filled to the configured threshold level (see OBSERVE_FIFO_THRESH).
es_fatal_err	Event	Asserted when an fatal error condition is met, e.g., upon FIFO errors, or if an illegal state machine state is reached.

Security Alerts

Alert Name	Description
recov_alert	This alert is triggered upon the alert health test threshold criteria not met.
fatal_alert	This alert triggers for any condition detected in the ERR_CODE register, which includes FIFO errors, COUNTER errors, FSM state errors, and also when integrity failures are detected on the TL-UL bus.

Security Countermeasures

Countermeasure ID	Description
ENTROPY_SRC.CONFIG.REGWEN	Registers are protected from writes.
ENTROPY_SRC.CONFIG.MUBI	Registers have multi-bit encoded fields.
ENTROPY_SRC.CONFIG.REDUN	Threshold register has an inverted copy to compare against.
ENTROPY_SRC.INTERSIG.MUBI	OTP signal used to enable software access to registers.
ENTROPY_SRC.MAIN_SM.FSM.SPARSE	The ENTROPY_SRC main state machine uses a sparse state encoding.
ENTROPY_SRC.ACK_SM.FSM.SPARSE	The ENTROPY_SRC ack state machine uses a sparse state encoding.
ENTROPY_SRC.RNG.BKGN_CHK	Random number generator is protected with continuous background health checks.
ENTROPY_SRC.FIFO.CTR.REDUN	The FIFO pointers of several FIFOs are implemented with duplicate counters.
ENTROPY_SRC.CTR.REDUN	Counter hardening for all health test counters.
ENTROPY_SRC.CTR.LOCAL_ESC	Redundant counter failures will cause a local escalation to the main state machine.
ENTROPY_SRC.ESFINAL_RDATA.BUS.CONSISTENCY	Comparison on successive bus values for the post-conditioned entropy seed bus.
ENTROPY_SRC.TILE_LINK.BUS.INTEGRITY	Tilelink end-to-end bus integrity scheme.