Texas Instruments SN74ACT3651-15PQ

Overview

SN74ACT3651-15PQ from Texas Instruments is a 2048 × 36-bit clocked FIFO memory IC with dual asynchronous ports (A/B), 67 MHz max clock frequency, 11 ns read access time, and synchronous retransmit capability. It implements mailbox registers, programmable almost-full/almost-empty flags, and microprocessor interface logic for high-throughput data buffering in telecom backplanes and DSP interconnects.

For engineers reviewing the SN74ACT3651-15PQ datasheet, SN74ACT3651-15PQ pinout, SN74ACT3651-15PQ application, or SN74ACT3651-15PQ equivalent, key selection criteria include dual-port SRAM depth/width, CLKA/CLKB synchronization behavior, flag offset programming methods (serial/parallel/preset), retransmit mode timing constraints, and PQ-package thermal and routing implications for high-speed PCB layout.

Technical Context

The SN74ACT3651-15PQ uses a dual-clock architecture where CLKA governs port-A write/read and flag synchronization (IR, AF), while CLKB governs port-B operations and flag synchronization (OR, AE). Its 2048 × 36-bit SRAM core supports independent read/write pointers and shadow pointer logic during retransmit mode.

Flag offset registers (X for AE, Y for AF) are programmable via three methods-preset (8 or 64), parallel load from port A (A10–A0), or serial load via FS0/SD and FS1/SEN-and directly determine FIFO fill-level thresholds used by synchronized output-ready and input-ready signaling.

Key Specifications

Pinout & Package

SN74ACT3651-15PQ is housed in a 132-pin Plastic Quad Flat (PQ) package with exposed thermal pad, optimized for high-density PCB layouts and thermal dissipation in multi-FIFO configurations.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar clocked FIFO applications.

Compared with SN74ACT3631-15PQ and SN74ACT3641-15PQ, the SN74ACT3651-15PQ uniquely combines mailbox registers, full retransmit capability, and parallel flag-offset programming-making it the only option among the three that supports dynamic, software-controlled buffer management in real-time systems.

Availability

SN74ACT3651-15PQ is available at Aetrix Electronics and suitable for telecom infrastructure, DSP subsystems, industrial PLCs, and PCIe bridge interfaces requiring stable component supply across extended production lifecycles.

Supply support for SN74ACT3651-15PQ 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 global semiconductor leader specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74ACT3651-15PQ belongs to TI's high-speed clocked FIFO product line, designed specifically for deterministic, low-latency data buffering across asynchronous clock domains in mission-critical digital systems.

FAQ

What is the maximum supported clock frequency for SN74ACT3651-15PQ?

The SN74ACT3651-15PQ supports clock frequencies up to 67 MHz on both CLKA and CLKB inputs. This specification is guaranteed across the full commercial temperature range (0°C to 70°C) and defines the upper limit for reliable synchronous data transfer through either port. Exceeding this frequency may result in setup/hold violations, flag metastability, or data corruption in the 2048 × 36-bit SRAM core.

How does the retransmit mode function in SN74ACT3651-15PQ?

The SN74ACT3651-15PQ enters retransmit mode when RTM is high and OR is high, marking the current FIFO output register content as the first retransmit word. Subsequent reads-triggered by RFM high on CLKB-cycle back to this marked location. The feature uses dual read pointers (current and shadow) and maintains full write capability during retransmit, enabling repeatable data delivery without CPU overhead.

Can SN74ACT3651-15PQ be used with asynchronous CLKA and CLKB clocks?

Yes, SN74ACT3651-15PQ explicitly supports asynchronous CLKA and CLKB operation. Its IR and AF flags are synchronized to CLKA using two-stage flip-flops, while OR and AE are synchronized to CLKB-minimizing metastability risk. The functional block diagram and timing diagrams confirm robust cross-domain handshaking, making it suitable for bridging unrelated clock domains such as Ethernet PHY and MAC layers.

What are the three methods to program the almost-empty and almost-full offset registers in SN74ACT3651-15PQ?

The SN74ACT3651-15PQ supports three offset programming methods selected via FS1/FS0 during RST assertion: (1) preset values (8 or 64), (2) parallel load from port A (A10–A0), and (3) serial load via FS0/SD with FS1/SEN as enable. Each method writes to separate X (AE) and Y (AF) registers, allowing runtime adaptation of buffer occupancy thresholds without hardware changes.

Is SN74ACT3651-15PQ pin-compatible with other devices in the same family?

Yes, SN74ACT3651-15PQ is pin-to-pin compatible with SN74ACT3631-15PQ and SN74ACT3641-15PQ in the 132-pin PQ package. This allows direct substitution in existing PCB layouts. However, functional differences exist: SN74ACT3651-15PQ adds mailbox registers and full retransmit capability absent in SN74ACT3631-15PQ, and supports parallel flag-offset programming unlike SN74ACT3641-15PQ.