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74ALVCH32973ZKER EOL: Redesign the Legacy Bus Interface

Confirm the Exact Device and EOL Evidence

Use the complete 74ALVCH32973ZKER order code for the lifecycle conclusion. The supplied TI datasheet and a later TI discontinuance notice answer different questions, so an old Active entry cannot overturn the later exact-part event.

The technical baseline is TI SCES436C, revised September 2004. Its first-page ordering table identifies the Pb-free ZKE option as 74ALVCH32973ZKER, without an added SN prefix, and the supplied March 30, 2005 package addendum lists the 96-ball LFBGA. Family headings use SN74ALVCH32973; that heading does not authorize changing the purchase order's spelling. A public mirror of the TI-authored Rev. C datasheet makes the technical pages available for reference; its later appended material must not be confused with the supplied 2005 appendix.

Table 1: Exact-device identity and historical discontinuance chronology | Source: TI PDN20191218000.3 and TI SCES436C technical document mirror | Compiled by: YG Group

The practical implication is to maintain two records. The engineering record holds the original part identity, document versions and candidate qualification. The sourcing record holds dated offers for the exact part, quantities, packaging and any required traceability. Neither record can substitute for the other: EOL is not proof of zero remaining stock, and a current offer is not proof that a new design should retain the discontinued component.

Before redesigning, also verify that the installed assembly really uses this order. An old schematic may contain only the family name, while the bill of materials, approved-source list and package drawing contain more specific information. Resolve those records together. If they disagree, do not make the new footprint from the shortest label in the archive.

The objective is not to turn a 2020 notice into fresh market news. It is to use that notice as the reason for a present engineering task: maintaining a legacy product without assuming that a similarly named logic device will preserve its behavior. The useful output is a controlled redesign specification that can survive a change of component and a change of engineer.

Decompose the Bus Interface Before Choosing Parts

The original device contains three functional roles with different control dependencies. Preserve those roles explicitly, because replacing one of them does not automatically replace the others.

The A and B buses form a bidirectional transceiver path. Direction control chooses which bus drives the other, while the transceiver output-enable control can isolate that path. Separately, the latch captures information from A onto Q. When latch enable is high, the enabled Q outputs follow A; when latch enable goes low, the captured state is retained. The latch output-enable control changes whether Q drives externally, not whether the internal latch can retain or accept data. These distinctions come from the TI function tables and logic diagrams, pp1–4.

Table 2: Functional decomposition of the original bus-interface device | Source: TI SCES436C, pp1–4, public document mirror | Compiled by: YG Group

A common redesign error is to model the latch as an edge-triggered register. The falling edge of LE closes a transparent interval; it does not make the interval before that edge irrelevant. If the receiving circuitry observes Q while the latch is transparent, changes on A can matter before capture. A register-based redesign might be valid, but only after the system's observation and timing requirements have been changed or shown to be compatible.

Another error is to assume one output-enable signal controls the entire package. The eight D-to-Y buffers are independent of the four named latch/transceiver controls. As a result, disabling the A/B path and Q outputs does not, by itself, establish an all-package high-impedance state. Identify where each Y output goes and whether its receiving circuit requires an additional system-level startup condition.

Build a net-role inventory from the old schematic. For every used signal, record the source, destination, direction, active phase and required inactive state. Mark whether the signal must remain available while the transceiver is isolated. Mark unused data nodes separately from unused control inputs. This inventory catches dependencies that a simple pin-count comparison cannot reveal.

Use changing patterns rather than only all-zero and all-one words when checking the inventory. A single asserted bit moving across the word can reveal crossed channels; alternating patterns exercise adjacent signal relationships; separate patterns on the two eight-bit groups reveal accidental coupling of their controls. For the latch, change A while LE is high, close LE, and then change A again. Observe Q with its output enabled and repeat while it is disabled, verifying the retained state when the output returns. These are proposed functional tests derived from the documented truth table, not measurements of a completed replacement.

Consider a hypothetical 16-bit multiplexed controller interface. During its address phase, the controller presents an address on A, LE allows the address to pass to Q, and the closing event retains it for the later data phase. During a read, another device may drive B toward A; during a write, A may drive B. Eight separate D-to-Y paths carry unrelated control signals. This example describes a possible use of the documented functions, not a recovered customer schematic or a validated replacement circuit.

To qualify a split implementation, walk that sequence with the proposed controls. Ask whether address capture still occurs while the relevant data are stable, whether changing direction exposes contention, and whether independently buffered controls arrive in the intended phase. A two-chip or three-chip solution can match the Boolean roles yet alter the relative arrival times enough to require a new timing budget.

Review the 16-bit A↔B path, the 16-bit A→Q transparent-latch path and eight D→Y buffers as distinct functions. The two control groups must retain their separate behavior; a single global enable cannot describe the original interface. Matching this functional structure is necessary, but it does not by itself establish a complete replacement pinout or schematic.

An interface migration can require board and behavior changes even when the new part has a nominally similar role. This concern applies across interface types, but another interface's specifications cannot qualify the parallel-bus logic here. Identify each required function, its control dependencies and the surrounding circuit before judging a candidate.

Preserve Bus Hold and Startup High-Impedance Behavior

Bus hold and safe control states solve different problems. The original device provides bus-hold circuitry on the A and B I/Os and D inputs, while unused control inputs and startup output enables require their own defined treatment.

TI advises against adding pullup or pulldown resistors to the bus-held data inputs as a default remedy. It separately directs the designer to pull LOE and TOE toward VCC to establish the stated high-impedance behavior during power-up or power-down, with resistor selection constrained by the controlling drivers' sinking capability. Those statements are compatible: they concern different nodes and different purposes. SCES436C, p1 and p5 note 1.

Table 3: Electrical and startup conditions that survive a package change | Source: TI SCES436C, pp1, 5–6, public document mirror | Compiled by: YG Group

A useful startup review follows the power domains rather than the nominal logic diagram. Identify which device drives each control while reset is asserted, while one rail is present and another is absent, and while the controller changes a pin from its reset function to its operating function. Do not assume a replacement's powered-off behavior matches the original simply because its operating supply range overlaps.

For the resistor decision, begin with the required inactive logic level, the candidate's leakage and the controller's guaranteed sink capability. Then check the transition time and the current when the control is asserted. This article does not select a universal pullup value because the driver, capacitance and startup timing have not been supplied. A resistor value copied from an unrelated board would create apparent specificity without establishing a valid design.

Observe startup at the pins that matter to the receiving circuit. Measuring a clean control signal at the controller is not the same as proving the enabled output remains harmless at the load. Check the intended inactive state, the first driven transition and any overlap between opposing bus drivers. Include power-down and reset recovery, not only a normal cold start.

Logic defaults need explicit review whenever a component changes. Behavior documented for an isolated interface does not establish the default state of this parallel-bus device. Make the required startup and inactive states part of the replacement specification, then verify them against the candidate and board instead of assuming they follow from a replacement label.

Rebuild the Timing Budget at the Actual Load

Use the timing parameter for the event being analyzed. Propagation through an already enabled path, enabling a previously isolated output and closing a transparent latch are different events, even when they occur in the same bus cycle.

TI's 3.3 V ±0.3 V column gives the values below over the recommended operating free-air temperature range, unless otherwise noted. The associated measurement network uses 50 pF and 500 Ω as specified on p9. That laboratory load definition is part of the specification; it is not a model of every PCB or receiving device.

Table 4: Distinct timing requirements under the stated TI conditions | Source: TI SCES436C, pp7 and 9, public document mirror | Compiled by: YG Group

Work the path that actually exists

Consider an illustrative enabled data path with 8 ns available from a source transition to a receiver's capture event. Assume the original logic's applicable maximum delay is 3 ns, routing and other board delay are budgeted at 1 ns, and the receiving device requires 1 ns setup. The arithmetic margin is 8 − 3 − 1 − 1 = 3 ns. Only the 3 ns logic term comes from Table 4; the other quantities are explicit example assumptions requiring real design evidence.

If the event instead begins by enabling the Q outputs, substituting the relevant 4.7 ns LOE-to-Q term into that illustrative budget leaves 1.3 ns. Do not add 3 ns and 4.7 ns automatically: whether separate delays accumulate depends on the actual causal path and timing definition. Conversely, do not use the smaller propagation number for an enable event merely to obtain a positive margin.

Hold-time analysis needs its own minimum-delay path. A design can satisfy the maximum-delay setup budget and still change data too soon after capture. For a split implementation, compare the earliest possible new data arrival with the latest relevant capture/control event, including skew. The minimum hold requirement is not an extra maximum propagation delay to add to a setup equation.

Bus turnaround is another distinct check. Establish when the old driver stops driving and when the new driver can begin. Compare the appropriate disable and enable limits, controller sequencing and route differences. A nominal software delay or a typical waveform does not prove absence of contention across the specified conditions. Likewise, an excessively long high-impedance gap can affect the receiving logic's interpretation even if contention is avoided.

Under the stated common supply, load network and temperature conditions, the maximum delays are 3.0 ns for propagation, 4.7 ns for LOE-to-Q enable and 4.4 ns for TOE-to-bus enable. These values describe different initiating events. Choose the term that matches the actual transition being budgeted; adding all three would not produce a meaningful universal delay for the device.

Protection and loading outside the replacement IC remain board-level questions. Interface components, traces and receiving loads can affect the behavior observed at the system boundary. Include their effects in the actual design instead of expecting a logic timing table to qualify the complete interface; use the relevant specifications for each external element.

Evaluate Functional Candidates and Board Changes

The TI notice identifies SN74ALVCH16973DGGR as a functional candidate, but one candidate device does not reproduce the original channel count. The candidate combines an 8-bit transceiver/latch with four buffers; two devices are needed to cover the original 16-bit and eight-buffer counts before considering package, control and timing details.

TI's logic support discussion of this migration also describes using two SN74ALVCH16973 devices for the functional roles. That support answer is useful architectural guidance, not a released schematic or proof of pin compatibility. Candidate electrical limits must still come from each candidate's own documentation.

Table 5: Function-related candidates, not an approved drop-in list | Source: TI SN74ALVCH16973, SN74ALVCH16245, SN74ALVCH16373 and SN74LVC244A documentation | Compiled by: YG Group

A split design changes more than placement area. It can add control fanout, route length, relative skew and separate decoupling locations. Review whether shared control signals still reach both halves with acceptable timing and whether their inactive states remain defined. Keep power-domain behavior, footprint and assembly constraints in the same change record as the logic comparison.

Compare two implementation strategies on the same requirements sheet. A pair of multifunction candidates may retain a recognizable grouping of controls, while separate transceiver, latch and buffer devices may offer more placement flexibility. Neither approach is automatically smaller, faster or easier to qualify. Count the actual required functions, control connections and power pins, then estimate board impact from the selected packages. If only part of the original device was used, document that reduced requirement rather than designing every candidate around unused channels.

Before approving samples, produce a pin-role mapping, a candidate-by-candidate limits comparison and a test plan covering startup, normal transfers, latch closure and bus turnaround. Define the acceptance limits before looking at measurements. Otherwise, a waveform that looks plausible can become the standard simply because it is the first result obtained.

Keep the evidence associated with each board revision. A timing result from an early prototype does not automatically qualify a later reroute, a different pullup or a changed receiving load. Record the supply settings, observation points and instrument loading with the test, then repeat the affected checks when those conditions change. This makes the qualification reusable without pretending that one successful capture establishes every future configuration.

Separate controller logic from the electrical interface when planning a redesign. A system protocol, the logic that controls data flow and the circuitry that drives the physical connection each need their own evidence. Devices intended for a different interface are not substitutes for this bus component simply because they serve a broad communication role.

Availability and Procurement

For an existing-product maintenance decision, check current availability of the exact 74ALVCH32973ZKER order or a separately qualified candidate. Include the complete manufacturer/model identity, quantity and required packaging in a quotation inquiry. Historical EOL does not prove that no stock remains, and a present offer does not qualify a replacement's electrical or functional behavior.

Conclusion

The defensible redesign is the one that preserves the required functions and timing, not the one with the closest part number. Keep the historical TI notice, map the transceiver, latch and buffer roles, define startup controls, and rebuild each timing path with the actual candidate and load. That process turns an EOL response into an engineering release decision with explicit evidence and limits.

References

• Texas Instruments SN74ALVCH32973 Datasheet, SCES436C, September 2004 — public document mirror: technical pp1–9; the source article's separate ordering appendix is dated March 30, 2005.
• Texas Instruments PDN20191218000.3, January 7, 2020 — archived manufacturer notice: exact order, functional candidate and historical ordering/shipment deadlines.
• Texas Instruments SN74ALVCH16973DGGR product documentation: candidate transceiver/latch and independent-buffer functions.
• Texas Instruments E2E Support: SN74ALVCH32973 Drop in Alternative: functional migration guidance, not complete board interchangeability.
• Texas Instruments SN74ALVCH16245DGGR product documentation: comparison transceiver block.
• Texas Instruments SN74ALVCH16373DGGR product documentation: comparison transparent-latch block.
• Texas Instruments SN74LVC244APWR product documentation: comparison eight-buffer block.