Clock Independence for Watchdogs & Resets (Automotive)

Q: How do I prove the watchdog clock is independent of the MCU PLL in production builds?

Show schematic and BOM evidence of separate clock and power domains, then run fault injection on production images: stop/derate the MCU PLL while the watchdog still asserts reset. Log missed‑reset rate ≤ 100 ppm across −40~+125 °C. Include scope/LA captures, build hashes, and a factory test script that toggles PLL while verifying watchdog behavior.

Q: What ppm/°C drift budget keeps window‑WDT feeds safe over −40~+125 °C?

Use Δt ≈ t_nom × (ppm_total/1e6), where ppm_total = initial tolerance + temp drift + aging + jitter margin. Center the window and keep ≥ 20–30% safety margin versus worst‑corner feed times. Validate with P1/P50/P99 statistics per corner and ensure the aggregate violation rate stays below 100 ppm across the full test matrix.

Q: When should I choose an RC‑WDT over an XTAL‑WDT for cold‑start reliability?

Prefer RC‑WDT when fast, guaranteed start is critical and crystal start‑up or low‑temperature oscillation is uncertain. Use a wider window and larger drift budget. Choose XTAL‑WDT for long‑term accuracy and tighter windows, but add POR/blanking to cover slow or failed crystal start. Always confirm independence and behavior under stop/derate faults.

Q: How do slow ramps and 20–50 mV ripple change the safe feed window?

Slow ramps and ripple can modulate internal thresholds and clock edges, behaving like an “effective slow clock.” Add a blanking/debounce period after PG rises and include jitter margin in the window calculation. Verify at min/max ramp rates and ripple levels that feeds remain inside the window with ≥ 20% residual margin across corners.

Q: What is a robust minimum reset pulse width across domains (OD vs PP)?

Enforce t_RST(min) ≥ 1.2 × datasheet under worst loading. For open‑drain resets crossing domains, check rise time t_rise ≈ 2.2·R_pullup·C_bus so the sampled pulse exceeds threshold across voltage and temperature. Use level shifting or buffering if domains differ. Confirm no back‑power paths and verify pulse width histograms meet the limit.

Q: Can I derive a 1 Hz from the RTC and still claim clock independence?

Yes, if the RTC’s timebase and supply are not common‑cause with the MCU PLL or its regulators. Document separate sources, then prove it: stop or derate the MCU clock while the RTC‑derived watchdog still asserts reset. Record missed‑reset rate ≤ 100 ppm and include captures showing the independent path continues to operate correctly.

Q: How do I interlock PG/FAULT semantics so a stuck WDT cannot mask faults?

Require PG_stable to overlap the watchdog feed window by ≥ 10–20% of t_feed and gate WDT feeds on power‑good status. Latch FAULT or limit power on watchdog triggers to prevent silent masking. Add a veto: if RTC/CLKOUT sanity checks fail, block reset release. Validate interlocks with scripted timing and log the overlaps.

Q: What blanking/debounce prevents chatter on pre‑biased rails?

After PG rises, apply a blanking time sized to the slowest rail: t_blank_after_PG ≈ 1–5 × RC_ramp of that domain. This filters transient thresholds and pre‑bias artifacts before enabling feeds or releasing resets. Verify empirically at min/max ramp rates and temperatures that no chatter occurs and windows remain respected.

Q: How do I inject “stopped clock / stuck high / jitter burst” and define pass criteria?

Gate or tri‑state the source for stopped clock, force a static level for stuck high, and modulate period or phase for 10–30% jitter bursts. Combine with slow ramps, ripple, and temperature corners. Pass criteria: missed‑reset rate ≤ 100 ppm, t_RST histograms above limit, and validated overlap between PG_stable and the feed window.

Q: What acceptance criteria make sense for ppm‑level window violations?

Use an aggregate violation rate ≤ 100 ppm across the entire test matrix, reported with P1/P50/P99 statistics per corner. Maintain ≥ 20–30% timing margin after accounting for ppm drift and jitter. Include evidence of interlock overlap and no‑chatter operation during ramps, ripple, and thermal extremes to support production readiness.

What It Solves

Clock independence removes common-cause failures (CCF) where the watchdog (WDT) depends on the same clock tree as the MCU. Typical triggers include PLL stalls, XTAL non-start, clock-distribution degradation, or frequency/jitter excursions that distort the feed window during slow ramps and 20–50 mV_pp ripple. The field symptoms are missed resets, non-recovering hangs, and lost logs.

Symptoms

Cold-start fails to reset; intermittent boot lockups.
Feed-window drift under slow ramp / pre-bias supply.
Door access / motor / battery systems hang without self-recovery.
Power-fail logs missing; root-cause hard to reconstruct.

Mechanism

Shared clock ⇒ WDT co-sourced with MCU. When the reference stalls, slows, or jitters, the watchdog timing collapses with it. Slow-ramp + 20–50 mV_pp ripple act like a pseudo-frequency error, shifting the window and causing false passes or missed resets.

Cost

Truck-rolls and field returns from non-recovering hangs.
Black-box analysis blocked by missing logs.
Safety cases weakened by latent common-cause dependencies.

Availability (coarse): Availability ≈ 1 − P(CCF) × Impact(CCF)
Reset success goal: missed-reset rate ≤ 100 ppm under combined “stopped-clock + temperature” stress.

Common Pitfalls

Using MCU internal WDT still tied to main XTAL/PLL.
Push-pull resets crossing voltage domains without level strategy → chatter or under-width pulses.

Decision Checks

Any shared-clock path to WDT?
Target environment: industrial/automotive (−40~+125 °C)? Need dual timebases (RC+XTAL)?
MTBF / escape-risk and reset/response-time requirements?

Architectures for Clock Independence

Four reusable patterns map to different environments and fault models. Select by start-up criticality, long-term accuracy, cross-domain reset fan-out, and functional-safety targets.

A · RC-WDT (On-Chip Independent Timebase)

Pros: survives main-clock loss; fast cold-start.
Trade-offs: higher ppm/°C → use wider windows; verify corners.
Use: cold-start critical, minimal BOM.

B · External µP Supervisor + Windowed WDT

Pros: truly independent device; OD/PP; cross-voltage domains; delay/debounce.
Trade-offs: area/cost; pull-up sizing and routing matter.
Use: multi-domain reset fan-out; ripple/slow-ramp sensitive.

C · Dual Timebases (RC ⇄ XTAL) with Interlock/Voting

Pros: mutual supervision; single-point tolerance ↑.
Trade-offs: complexity; interlock logic and diagnostics required.
Use: high availability / safety (ASIL paths).

D · RTC-Derived 1 Hz / CLKOUT Interlock

Pros: WDT cross-checks a low-drift reference.
Risks: loses independence if RTC power/clock shares sources.
Use: systems with RTC that can ensure power/clock isolation.

Drift aggregation: ppm_total = |ppm_RC| + |ppm_XTAL| + temp + aging → Δt ≈ t_nom × (ppm_total / 10^6)
Window margin: t_low < min_feed_across_corner and max_feed_across_corner < t_high (≥20% recommended).

Selection Hints

Cold-start-critical → A or C (include RC path).
Long-term accuracy → B or C (include XTAL).
Cross-domain reset fan-out → B.
High ASIL / availability → C (+ simple voting / interlocks).

Anti-Pitfalls

“Dual timebases” secretly share the same LDO/clock tree.
Interlock signals share domains and suffer back-powering.
Missing level strategy on OD/PP resets across domains.

Timing Budget & Drift Mapping

Convert ppm/°C, aging, and jitter into milliseconds of window shift so windowed-WDT feeds remain safe across temperature, voltage corners, and cold-start. Then add blanking/debounce to absorb slow-ramp and pre-bias artifacts.

Collect Parameters

Initial frequency error ppm_init
Temperature drift ppm_temp(T) (per °C curve)
Aging ppm_aging(L) at lifetime L
Jitter → jitter_margin (ms) inside window

Design the Window

Choose (t_low, t_high) around t_nom so min/max feed across corners stays inside, with ≥ 20% margin. RC-WDTs typically need wider windows or temperature-aware feeding.

Handle Slow Ramps

Slow dV/dt and 20–50 mV_pp ripple create a pseudo frequency error. Use PG-based t_blank and input debounce so the first feeds occur after rails are stable.

Drift aggregation (temperature & lifetime aware): ppm_total(T,L) = |ppm_init| + |ppm_temp(T)| + |ppm_aging(L)| + jitter_equiv_ppm
Time shift from ppm: Δt_ppm(T,L) = t_nom × (ppm_total(T,L) / 10^6)
High-side shift (incl. jitter band): Δt_feed_high = t_nom × (ppm_total / 10^6) + jitter_margin
Pass criteria: t_low < min_feed_across_corner and max_feed_across_corner < t_high (≥ 20% margin).

Temperature (°C)	ppm_total	Δt_ppm (ms)	jitter_margin (ms)	Δt_total (ms)
−40	…	t_nom × ppm_total/1e6	…	Δt_ppm + jitter_margin
25	…	t_nom × ppm_total/1e6	…	Δt_ppm + jitter_margin
125	…	t_nom × ppm_total/1e6	…	Δt_ppm + jitter_margin

Reset I/O & Level Domains (OD vs PP, Pull-ups, Back-Power)

Ensure that resets asserted by the independent timebase propagate across multiple voltage domains with clean edges and no back-powering. Choose OD or PP deliberately, size pull-ups to meet t_RST(min), and block reverse-current paths.

Open-Drain (OD) + Pull-up

Pros: natural cross-domain by pull-up rail; easy fan-out.
Risks: slow t_rise → effective pulse shorter than t_RST(min); too-strong pull-up → power/bias issues.
Rule: t_rise ≈ 2.2 × R_pullup × C_bus; verify t_pulse_effective ≥ t_RST(min).

Push-Pull (PP)

Pros: clean edges, controlled width.
Risks: needs level translation across domains; may overdrive absolute ratings; can create back-power paths.
Mitigation: use level shifters or switch to OD + per-domain pull-ups.

Minimum reset pulse: t_RST(min) ≥ datasheet × 1.2 (increase margin for slow ramps)
Rise-time with OD: t_rise ≈ 2.2 × R_pullup × C_bus
Cross-domain levels: stay within absolute ratings; add level shifting or OD-with-local-pull-ups.

Driver	Vpullup / Vout	Rpullup	Cbus	t_rise calc	t_RST(min)	Domains	Back-power risk	Mitigation
OD	…	…	…	2.2×R×C	≥ spec × 1.2	e.g., 5V_IO → 3V3_MCU	Y/N	Schottky, series R, buffer
PP	…	—	C_load	driver slew	≥ spec × 1.2	e.g., 3V3_MCU → 1V8_Core	Y/N	Level shifter or OD

Injection & Fault Modeling (Stopped/Slowed/Jittered Clock)

Define reproducible clock/power/temperature injections to validate reset effectiveness when the main clock is stopped, slowed, or jittered. Quantify field-relevant risks and decide mitigations (window, blanking, interlocks, domain fixes).

Clock Injections

Stopped clock (gated/Hi-Z)
Slowdown: −10% / −30% / −50%
Burst period jitter: 10–30%
Duty-cycle faults: 40/60, 30/70

Power Conditions

Slow ramp: 100 mV/s → 1 V/ms
Ripple: 20–50 mV_pp @ 10–200 kHz
Pre-bias on selected rails

Temperature Sweep

−40 / −20 / 25 / 60 / 85 / 125 °C (dwell ≥ thermal settle)

Injection methods: replace XTAL with programmable source or insert via clock MUX; gate/Hi-Z for stop; configure RIN/period jitter and duty; power ramp/ripple scripted on PSU with function-gen superposition; apply pre-bias on per-domain rails. Measure LeakResetRate, ResetPulseWidth histogram, and Back-power current per domain.

Metric	Acceptance	How to Measure
LeakResetRate	≤ 100 ppm (aggregate across corners)	Count missed/false resets over N cycles
ResetPulseWidth (min)	≥ 1.2 × datasheet requirement	Scope histogram P1/P99 vs. spec
Back-power (per domain)	< 1 µA with domain off	SMU/µA meter on off domain pins

Common Pitfalls

Only testing “stopped clock” but not slow/jitter/duty faults → false pass.
Room-temp only; too few ripple frequencies; missing pre-bias cases.
PP driven across domains causing back-power (see Ch.4 mitigations).

Remediation Hooks

Window re-centering / wider margins for RC paths
PG-based blanking and debounce tuning
OD + per-domain pull-ups, isolation diodes/buffers

Interlocks & Voting (PG/FAULT Semantics, N-of-M)

Make the independent timebase cross-validate with power-good and the watchdog window so no single signal can mask a true fault. Use PG-gated blanking, window overlap, and simple N-of-M voting with an RTC/CLKOUT veto.

PG/FAULT Semantics

PG valid = thresholds + dwell time (PG_stable window).
FAULT from UV/OV/window violations; latch/clear rules explicit.
Feeding is forbidden until PG_stable; FAULT may block reset release.

Window Overlap

Ensure Overlap(t_feed_window, PG_stable) ≥ X ms (recommend 10–20% of t_feed), and do not erode t_RST(min).

Voting

1oo2: (RC_OK ∨ XTAL_OK) ∧ PG_OK
2oo2: (RC_OK ∧ XTAL_OK) ∧ PG_OK
2oo3: maj(RC_OK, XTAL_OK, RTC_OK) ∧ PG_OK
Veto: if RTC/CLKOUT abnormal ⇒ no reset release

Blanking after PG: t_blank_after_PG↑ = max(t_ramp_settle, k × τ_supply), with k≈3–5
Overlap rule: Overlap ≥ max(0.1 × t_feed, jitter_margin + Δt_ppm)

Selection Guide (RC vs XTAL vs Hybrid)

Map real use-cases to a timebase choice (RC / XTAL / Hybrid) and a window strategy (t_low, t_high, margin, interlocks). Use RC for cold-start, XTAL for long-term ppm, and Hybrid where EMI is harsh or ASIL evidence must be strengthened.

Environment × Requirement	≤ Startup time	ppm / drift priority	EMI robustness	Reset I/O across domains	Recommended scheme + window
Cold-start-critical (−40→room) × Quick recovery	< 10–50 ms	Moderate	Normal	OD + per-domain pull-ups	RC-WDT; centered window with ≥25–30% margin; PG-gated blanking
Long-term accuracy × Tight timing spec	> 50–100 ms acceptable	High (ppm + aging)	Normal	PP or OD (match domains)	XTAL-WDT; narrower window; verify temp/aging; overlap with PG_stable ≥10–20% t_feed
Harsh EMI × Field diagnostics	Flexible	Medium–High	High (burst/surge)	OD + level-shifters; no back-power	Hybrid (RC↔XTAL or RTC-CLKOUT interlock); centered window + interlocks; 1oo2/2oo3 voting + veto
Multi-domain resets × Cross-voltage fanout	App-specific	Medium	Normal/High	OD preferred; t_rise≈2.2·R_PU·C_bus; enforce t_RST(min)	RC or Hybrid depending on EMI; wide window if RC; per-domain pull-ups; isolation diodes

Cross-Brand Shortlist (Seven Vendors)

Trial-friendly, automotive/industrial capable picks. Each row includes a datasheet link (rel="nofollow") and one-line selection rationale. Use with Ch.4 I/O rules and Ch.6 interlocks.

Brand	Family / PN	Timebase	Windowed?	OD/PP	t_RST(min)	AEC-Q100	Pkg H (mm)	Second-source	Datasheet
TI	TPS3430-Q1	Independent WDT	Window	OD/PP options	DS spec	Yes (Q1)	Small (WSON/SOT)	Alt: STWD100, MLX80051 (SBC)	TI Datasheet
TI	TPS3435	Independent WDT	Timeout (non-window)	OD/PP options	DS spec	Q variants	Tiny (SOT/WSON)	Alt: STWD180	TI Datasheet
TI	TPS3890 / TPS3890-Q1 (Supervisor)	Voltage supervisor	—	OD (RESET), MR pin	DS spec (programmable delay)	Q1 option	SOT/SON	Alt: ISL88014	TI Product Page
ST	STWD100	Independent WDT	Timeout (OD/PP variants)	OD or PP (config)	DS spec (min reset widths)	Industrial/auto variants	SOT-23	Alt: TPS3435	ST Datasheet
ST	STM811/812 (Supervisor)	Voltage supervisor + MR	—	Active-low/-high options	DS spec (t_rec)	Industrial	SOT143-4	Alt: TPS3890	ST Product Page
NXP	VR5510 (PMIC)	Integrated Window WDT	Window (program.)	PG/FAULT semantics	Per DS	Automotive (ASIL docs)	QFN	Alt: onsemi NCV97400	NXP Docs
NXP	MC33907/33908 (SBC)	Advanced WDT + supervisor	Window / challenge WDT	SPI control + safety states	Per DS	Automotive	SBC packages	Alt: Melexis MLX80051	NXP Product Page
Renesas	ISL88015 (Supervisor + WDT)	Supervisor + WDT	Timeout (startup / normal)	Active-low/high options	DS (e.g., 1.6 s normal)	Industrial/auto variants	SOT-23	Alt: STWD100	Renesas (DS via Mouser)
Renesas	ISL88014 (Supervisor)	Voltage supervisor	—	Comp/OD variants	DS (e.g., 200 ms POR adj.)	Industrial	SOT-23	Alt: TPS3890	Renesas Product Page
onsemi	NCV97400 (PMIC)	Integrated Window WDT + monitors	Window	PG/FAULT tree	Per DS	Automotive	QFN	Alt: NXP VR5510	onsemi Datasheet
onsemi	CAT824 (Supervisor + WDT)	Supervisor + WDT + MR	Timeout	Active-low	DS (≥140 ms typical)	Industrial/auto options	SOT-23	Alt: MCP1316	onsemi Product Page
Microchip	MCP1316 (Supervisor, PP)	Voltage supervisor (WDI pin)	Timeout (WDT mode)	Push-pull, active-low	Per DS (−40~+125 °C)	Auto-capable options	SOT-23	Alt: CAT824	Microchip Datasheet
Microchip	MCP1321 (Supervisor, OD)	Voltage supervisor (WDI pin)	Timeout (WDT mode)	Open-drain, active-low	Per DS (AEC-Q options)	AEC-Q (select codes)	SOT-23	Alt: STM811	Microchip Product Page
Melexis	MLX80051 (LIN SBC: Reg + WDT + RESET)	Integrated Window WDT	Window (ext. settable)	NRES reset output	Per DS	Automotive (LIN)	SOIC/QFN	Alt: NXP MC33907/8	Melexis Datasheet

TI TPS3430-Q1

Automotive window WDT (programmable window/reset delay). Works well with Ch.6 interlocks; a good independent timebase for ASIL projects.

ST STWD100

Independent WDT with OD/PP options and easy drop-in. Combine with OD pull-ups to cover multi-voltage-domain resets.

NXP VR5510

PMIC-class window WDT with PG/FAULT semantics; suited to domain controllers/gateways. Pair with voting/interlocks to build non-common-cause evidence.

Renesas ISL88015

Supervisor + WDT combo, small and low power; a solid starter for board-level watchdog and robust reset.

onsemi NCV97400

Multi-rail PMIC with window WDT; good for multi-rail systems—manage WDT together with power-health telemetry.

Microchip MCP1316/1321

Accurate voltage supervision with WDI. Pair with an external/internal WDT to form the classic “supervisor + independent timebase” setup.

Melexis MLX80051

LIN SBC: regulator + window WDT + RESET. For lighting/body nodes; still recommend an external independent supervisor to ensure non-common-cause coverage.

BOM & Procurement Notes

Provide the fields below so we can return a two-option shortlist (primary/backup) within 48 hours. This page focuses on clock independence for watchdogs, cross-domain reset integrity, and small-batch readiness.

Field	Definition	Engineering note
`V_rail`, `n_rails`	Target rail voltage(s) and count	Affects supervisor thresholds and reset fanout domains
WDT type / Timebase	Window vs one-shot; RC vs XTAL vs Hybrid	Choose for startup speed vs ppm/aging, see Selection Guide
t_feed window & drift budget	(t_low, t_high) with ppm + jitter margin	Center window, keep ≥20–30% safety margin
t_RST(min), Output type	Minimum reset width; OD vs PP	t_RST ≥ 1.2× DS; OD + per-domain pull-ups across levels
AEC-Q100 / Package height / Second-source	Compliance & mechanical / sourcing	Prepare primary + backup vendor up front
Optional hooks	RTC 1 Hz/CLKOUT, I²C/PMBus, PG/FAULT semantics, temp grade, ESD/Surge	Enable interlocks and diagnostics without common-cause paths

Submit your BOM (48h)

We will return a primary + backup shortlist, window & interlock advice, and reset I/O guidance.

Verification Plan & Acceptance (DOE)

A copy-ready DOE: how to inject faults, what to log, and how to accept. Reuse the injection matrix from Ch.5; add statistics and histogram views for watchdog timing and reset width.

Item	Plan	Notes
Sample size	n ≥ 30 per condition; 6 temperature points (−40/−20/25/60/85/125 °C)	Covers startup, drift, aging corner
Clock injection	Stopped / −10% / −30% / −50% / jitter 10–30% / duty anomalies	MUX/gate source; RIN/period modulation for jitter
Power injection	Ramp 100 mV/s…1 V/ms; ripple 20–50 mVpp; pre-bias per domain	Scripted PSU + AWG; isolate domains to avoid back-power
Logging	Feed timestamps, violation rate (ppm), reset width histogram, back-power current, cross-domain delay	Scope + LA; SMU for leakage; store CSV per condition
Acceptance	Violation ≤ 100 ppm; t_RST ≥ 1.2× datasheet; Overlap(PG_stable, t_feed_window) ≥ 10–20% of t_feed; back-power < 1 µA	Use P1/P50/P99 to summarize distributions

Stat rules

Report P1/P50/P99 for t_RST and feed timing; compute violation rate as ppm over the entire matrix.

I/O integrity

Enforce t_RST ≥ 1.2× datasheet; use OD + per-domain pull-ups; verify no back-power (< 1 µA).

Overlap

Ensure PG_stable and the feed window overlap by ≥ 10–20% of t_feed across corners.

FAQs — Clock Independence & Common-Cause Isolation

12 practical questions with engineer-grade, copy-ready answers. Visible text exactly matches the JSON-LD below.

How do I prove the watchdog clock is independent of the MCU PLL in production builds?

Show schematic and BOM evidence of separate clock and power domains, then run fault injection on production images: stop/derate the MCU PLL while the watchdog still asserts reset. Log missed-reset rate ≤ 100 ppm across −40~+125 °C. Include scope/LA captures, build hashes, and a factory test script that toggles PLL while verifying watchdog behavior.

What ppm/°C drift budget keeps window-WDT feeds safe over −40~+125 °C?

Use Δt ≈ t_nom × (ppm_total/1e6), where ppm_total = initial tolerance + temp drift + aging + jitter margin. Center the window and keep ≥ 20–30% safety margin versus worst-corner feed times. Validate with P1/P50/P99 statistics per corner and ensure the aggregate violation rate stays below 100 ppm across the full test matrix.

When should I choose an RC-WDT over an XTAL-WDT for cold-start reliability?

Prefer RC-WDT when fast, guaranteed start is critical and crystal start-up or low-temperature oscillation is uncertain. Use a wider window and larger drift budget. Choose XTAL-WDT for long-term accuracy and tighter windows, but add POR/blanking to cover slow or failed crystal start. Always confirm independence and behavior under stop/derate faults.

How do slow ramps and 20–50 mV ripple change the safe feed window?

Slow ramps and ripple can modulate internal thresholds and clock edges, behaving like an “effective slow clock.” Add a blanking/debounce period after PG rises and include jitter margin in the window calculation. Verify at min/max ramp rates and ripple levels that feeds remain inside the window with ≥ 20% residual margin across corners.

What is a robust minimum reset pulse width across domains (OD vs PP)?

Enforce t_RST(min) ≥ 1.2 × datasheet under worst loading. For open-drain resets crossing domains, check rise time t_rise ≈ 2.2·R_pullup·C_bus so the sampled pulse exceeds threshold across voltage and temperature. Use level shifting or buffering if domains differ. Confirm no back-power paths and verify pulse width histograms meet the limit.

Can I derive a 1 Hz from the RTC and still claim clock independence?

Yes, if the RTC’s timebase and supply are not common-cause with the MCU PLL or its regulators. Document separate sources, then prove it: stop or derate the MCU clock while the RTC-derived watchdog still asserts reset. Record missed-reset rate ≤ 100 ppm and include captures showing the independent path continues to operate correctly.

How do I interlock PG/FAULT semantics so a stuck WDT cannot mask faults?

Require PG_stable to overlap the watchdog feed window by ≥ 10–20% of t_feed and gate WDT feeds on power-good status. Latch FAULT or limit power on watchdog triggers to prevent silent masking. Add a veto: if RTC/CLKOUT sanity checks fail, block reset release. Validate interlocks with scripted timing and log the overlaps.

What blanking/debounce prevents chatter on pre-biased rails?

After PG rises, apply a blanking time sized to the slowest rail: t_blank_after_PG ≈ 1–5 × RC_ramp of that domain. This filters transient thresholds and pre-bias artifacts before enabling feeds or releasing resets. Verify empirically at min/max ramp rates and temperatures that no chatter occurs and windows remain respected.

How do I inject “stopped clock / stuck high / jitter burst” and define pass criteria?

Gate or tri-state the source for stopped clock, force a static level for stuck high, and modulate period or phase for 10–30% jitter bursts. Combine with slow ramps, ripple, and temperature corners. Pass criteria: missed-reset rate ≤ 100 ppm, t_RST histograms above limit, and validated overlap between PG_stable and the feed window.

What acceptance criteria make sense for ppm-level window violations?

Use an aggregate violation rate ≤ 100 ppm across the entire test matrix, reported with P1/P50/P99 statistics per corner. Maintain ≥ 20–30% timing margin after accounting for ppm drift and jitter. Include evidence of interlock overlap and no-chatter operation during ramps, ripple, and thermal extremes to support production readiness.

How do I size pull-ups to avoid back-power while meeting rise-time on OD resets?

Choose R_pullup so t_rise ≈ 2.2·R_pullup·C_bus still meets the sampled pulse width with margin. Verify domain leakage remains < 1 µA when powered down, adding series resistors or diodes if needed. Check the receiving threshold across temperature and voltage so the effective high level and timing are guaranteed in worst conditions.

What second-source pitfalls exist when swapping WDT/supervisor families?

Pitfalls include different window definitions, reset polarity, t_RST requirements, OD/PP output behavior, hidden common-cause timebases, and AEC-Q variants. Mitigate with A/B samples, I/O semantics review, and timing histograms under stop/derate faults and ramps. Confirm sourcing windows and keep a validated backup option to avoid late-cycle surprises.

Clock Independence for Watchdogs & Resets

What It Solves

Symptoms

Mechanism

Cost

Common Pitfalls

Decision Checks

Architectures for Clock Independence

A · RC-WDT (On-Chip Independent Timebase)

B · External µP Supervisor + Windowed WDT

C · Dual Timebases (RC ⇄ XTAL) with Interlock/Voting

D · RTC-Derived 1 Hz / CLKOUT Interlock

Selection Hints

Anti-Pitfalls

Timing Budget & Drift Mapping

Collect Parameters

Design the Window

Handle Slow Ramps

Reset I/O & Level Domains (OD vs PP, Pull-ups, Back-Power)

Open-Drain (OD) + Pull-up

Push-Pull (PP)

Injection & Fault Modeling (Stopped/Slowed/Jittered Clock)

Clock Injections

Power Conditions

Temperature Sweep

Common Pitfalls

Remediation Hooks

Interlocks & Voting (PG/FAULT Semantics, N-of-M)

PG/FAULT Semantics

Window Overlap

Voting

Selection Guide (RC vs XTAL vs Hybrid)

Cross-Brand Shortlist (Seven Vendors)

TI TPS3430-Q1

ST STWD100

NXP VR5510

Renesas ISL88015

onsemi NCV97400

Microchip MCP1316/1321

Melexis MLX80051

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Accepted Formats

Attachment

BOM & Procurement Notes

Verification Plan & Acceptance (DOE)

Stat rules

I/O integrity

Overlap

FAQs — Clock Independence & Common-Cause Isolation

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