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Texas Instruments SN74AS821FN
- Part No.:
- SN74AS821FN
- Manufacturer:
- Texas Instruments
- Package:
- -
- Datasheet:
-
SN74AS821FN.pdf
- Description:
- BUS DRIVER, AS SERIES
- Quantity:
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Inventory:1,862
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Product details
Overview
SN74AS821FN from Texas Instruments is a 10-bit edge-triggered D-type bus interface flip-flop with 3-state outputs, designed for high-capacitance bus driving in industrial and computing systems. It operates at 0°C to 70°C, supports 5 V supply, delivers ±24 mA/32 mA output drive, and features power-up high-impedance state and undershoot protection.
For engineers reviewing the SN74AS821FN datasheet, SN74AS821FN pinout, SN74AS821FN application, or SN74AS821FN equivalent, key selection criteria include 10-bit parallel data latching, 3-state bus control timing (tPLH ≤ 7.5 ns), output current capability (IOL = 32 mA), and compatibility with TTL-level control signals.
Technical Context
The SN74AS821FN implements ten independent positive-edge-triggered D flip-flops with synchronous 3-state output enable (OC). Its buffered OC input isolates control logic from bus loading, and internal circuitry ensures outputs enter high-impedance on power-up without external reset.
Each flip-flop retains stored data during OC assertion, enabling concurrent data update and bus release. The device uses bipolar Schottky (AS) technology, delivering TTL-compatible voltage thresholds (VIH = 2 V, VIL = 0.8 V) and fast propagation (tPHL ≤ 10.5 ns) under 50 pF load.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Family | AS (Advanced Schottky) – enables 5 V TTL compatibility with sub-10 ns switching |
| Operating Temperature | 0°C to 70°C – qualified for commercial-grade embedded and computing applications |
| Output Drive | IOL = 32 mA / IOH = –24 mA – sufficient to drive 50 pF buses without external buffers |
| Propagation Delay | tPLH ≤ 7.5 ns (VCC = 4.5–5.5 V, CL = 50 pF) – supports >100 MHz bus clocking in short traces |
| 3-State Enable Time | tPZH ≤ 8 ns – allows rapid bus arbitration and multi-master contention management |
| Input Thresholds | VIH = 2 V, VIL = 0.8 V – ensures robust noise margin against TTL logic families |
| Power-Up State | Outputs default to high-impedance – prevents bus contention during system initialization |
Pinout & Package
SN74AS821FN is supplied in a 24-pin plastic dual in-line package (PDIP), 300-mil width, with standard through-hole mounting and 0.1-inch pin pitch.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1, 13 | OC (Output Control) | Active-low 3-state enable; buffered to reduce DC loading on upstream logic |
| 2–11, 14–23 | 1D–10D, 1Q–10Q | 10-bit data inputs and true-logic outputs; no inversion (vs. SN74AS822) |
| 12 | GND | Ground reference for all logic and output stages |
| 24 | VCC | +5 V supply; decoupling required near pin for stable high-speed operation |
| 11 | CLK | Positive-edge clock; triggers simultaneous latching of all 10 D inputs |
Key Features
| Feature | Design Value |
|---|---|
| 10-bit 3-state bus interface | Enables direct connection to shared data/address buses without pull-ups or isolation logic |
| Undershoot protection circuitry | Prevents negative voltage excursions below GND during fast bus transitions, improving signal integrity |
| Buffered control inputs | Reduces input current loading by >95% vs. unbuffered AS devices, easing fanout requirements |
| Power-up high-impedance state | Eliminates need for external reset sequencing; safe insertion into live backplanes |
| TTL-compatible voltage thresholds | Direct interfacing with legacy 74LS/74F logic without level-shifting components |
Applications
| Microprocessor Address Latching | Parallel I/O Port Expansion |
|---|---|
Use Scenario: Latching 10-bit address segments in 8086/8088-based systems with multiplexed AD bus. IC Role / Device Role / Timing Role: Edge-triggered address register holding address lines during memory read/write cycles. Use Value: Eliminates external latch + buffer combinations; tPLH ≤ 7.5 ns ensures setup compliance with 8 MHz CPU timing. | Use Scenario: Expanding GPIO count on Z80 or 6502 systems using shared data bus architecture. IC Role / Device Role / Timing Role: Bidirectional bus transceiver with direction-controlled 3-state outputs. Use Value: IOL = 32 mA drives multiple LS-TTL loads directly; OC pin enables software-controlled port direction. |
| Parity Generation Interface | Industrial Bus Buffering |
Use Scenario: Capturing parity bits alongside data in fault-tolerant industrial controllers. IC Role / Device Role / Timing Role: Synchronous capture of 10-bit wide parity/data streams synchronized to system clock. Use Value: True-output logic (Q = D) matches parity generator output polarity; undershoot protection maintains signal fidelity on noisy factory floors. | Use Scenario: Isolating and strengthening control bus signals between PLC modules and sensor networks. IC Role / Device Role / Timing Role: High-drive bus repeater with controlled enable timing for deterministic arbitration. Use Value: tPZH ≤ 8 ns and tPHZ ≤ 8 ns allow precise inter-module handshaking; power-up Hi-Z prevents startup glitches. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar 10-bit bus interface flip-flop applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74AS822FN | Complementary Q outputs (Q = NOT D); identical timing, drive, and pinout | Required where inverted bus data path is needed (e.g., differential parity checking) | Select SN74AS822FN only when complemented outputs are explicitly required; otherwise SN74AS821FN is functionally preferred. |
| AM29821PC | Functionally equivalent AMD part; same AC/DC specs, but ceramic DIP package only (no plastic FN option) | Limited to high-reliability mil/aero programs requiring ceramic hermetic sealing | Choose AM29821PC only if ceramic packaging and MIL-STD-883 qualification are mandatory; SN74AS821FN offers broader commercial availability. |
Compared with SN74AS822FN and AM29821PC, the SN74AS821FN provides true-output logic in a cost-effective plastic DIP, making it optimal for commercial computing and industrial control where non-inverted data latching and RoHS-compliant packaging are priorities.
Availability
SN74AS821FN is available at Aetrix Electronics and suitable for microprocessor address latching, parallel I/O expansion, parity generation interfaces, and industrial bus buffering requiring stable component supply across long-lifecycle embedded programs.
Supply support for SN74AS821FN includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.
Manufacturer
Texas Instruments is a U.S.-based semiconductor leader founded in 1930, delivering analog, embedded processing, and logic solutions with industry-leading quality and longevity.
The SN74AS family was engineered for high-speed TTL-compatible bus interfacing in 1980s computing and industrial control systems, emphasizing robust drive strength, noise immunity, and reliable 3-state control.
FAQ
What is the logic polarity of SN74AS821FN outputs relative to its inputs?
The SN74AS821FN provides true-logic outputs: each Q output replicates the corresponding D input value after the positive clock edge. This distinguishes it from the SN74AS822FN, which delivers complementary outputs. The SN74AS821FN datasheet confirms Q = D in the function table, and this behavior is consistent across all ten bit positions in the SN74AS821FN.
Does SN74AS821FN require external pull-up resistors on its 3-state outputs?
No, SN74AS821FN does not require external pull-up resistors. Its 3-state outputs exhibit high-impedance (Z) when OC is high, and its specified output drive (IOL = 32 mA, IOH = –24 mA) is sufficient to actively drive bus lines to valid logic levels without passive termination. The SN74AS821FN's design eliminates reliance on pull-ups in properly terminated bus systems.
What is the maximum clock frequency supported by SN74AS821FN?
The SN74AS821FN does not specify a maximum clock frequency in its datasheet, but its propagation delay (tPLH ≤ 7.5 ns, tPHL ≤ 10.5 ns) and setup time (tsu = 6 ns) imply reliable operation up to approximately 80–100 MHz in point-to-point configurations with 50 pF load. System-level timing must account for board trace delays and fanout; the SN74AS821FN's performance is optimized for 8–12 MHz microprocessor buses typical of its era.
Is SN74AS821FN compatible with modern 3.3 V logic systems?
The SN74AS821FN is a 5 V-only device with TTL input thresholds (VIH = 2 V, VIL = 0.8 V) and 5 V output swing. It is not 3.3 V tolerant: applying 3.3 V signals to its inputs may cause marginal VIH margin, and driving 3.3 V logic inputs risks overvoltage damage. Interfacing requires level translation; the SN74AS821FN is intended for legacy 5 V systems or mixed-voltage designs with proper translation.
How does the undershoot protection in SN74AS821FN improve system reliability?
The undershoot protection circuitry in SN74AS821FN clamps negative voltage excursions below GND during fast bus transitions, preventing latch-up or gate oxide stress in connected devices. This feature is especially valuable in electrically noisy environments like factory automation, where cable reflections or inductive loads can induce sub-GND spikes; the SN74AS821FN's built-in protection enhances long-term reliability without external diodes or RC networks.
SN74AS821FN Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- *
- Package/Case:
- -
- Packaging:
- Bulk
- Product Status:
- Active
- Logic Type:
- -
- Number of Elements:
- -
- Number of Bits per Element:
- -
- Input Type:
- -
- Output Type:
- -
- Current - Output High, Low:
- -
- Voltage - Supply:
- -
- Operating Temperature:
- -
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- -
- Supplier Device Package:
- -
SN74AS821FN FAQ
1.How can I place an order for SN74AS821FN through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AS821FN on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for SN74AS821FN reliable?
The price and inventory of SN74AS821FN are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AS821FN is usually 5 days.
3.What payment methods are accepted for SN74AS821FN?
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4.How is shipping managed for SN74AS821FN?
SN74AS821FN orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AS821FN order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for SN74AS821FN?
For technical support, including SN74AS821FN datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AS821FN requirements.
6.How does Aetrix verify that SN74AS821FN is sourced from the original manufacturer or authorized distributors?
All SN74AS821FN products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that SN74AS821FN meets industry standards.
7.What is the process for return or replacement of SN74AS821FN?
All SN74AS821FN units undergo pre-shipment inspection (PSI). If there is an issue with SN74AS821FN, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The SN74AS821FN part is unused and in its original packaging.
Return procedure for SN74AS821FN:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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