Renesas 70121S55J

Overview

70121S55J from Integrated Device Technology is a high-speed 2K × 9 dual-port static RAM configured as a MASTER device with integrated BUSY arbitration logic and interrupt flag support. It delivers 55 ns read/write cycle time, operates on 5.0 V ±10% supply, and features push-pull BUSY/INT outputs, 9-bit bidirectional I/O per port, and battery-backed data retention down to 2 V. It is used in real-time inter-processor communication systems requiring concurrent, conflict-free memory access between two independent buses.

For engineers reviewing the 70121S55J datasheet, 70121S55J pinout, 70121S55J application, or 70121S55J equivalent, this page provides verified functional identity (MASTER dual-port SRAM), confirmed package (52-pin PLCC), validated timing (tRC = 55 ns, tBAA ≤ 30 ns), true power behavior (ISB3 = 1.0 mA typ. full standby), and exact alternative part mapping - all extracted from IDT's official DSC 2654/15 datasheet (September 2019).

Technical Context

The 70121S55J implements fully asynchronous dual-port operation with independent left/right address, control, and 9-bit I/O buses. Its on-chip arbitration logic resolves port contention by asserting BUSY on the losing port when both ports access the same address simultaneously - based solely on CE and address signal timing, not R/W state.

As a MASTER device, it drives BUSY as a non-tri-stated push-pull output and supports interrupt signaling via dedicated INTL/INTR flags tied to fixed mailbox addresses (7FEh and 7FFh). It enables width expansion with IDT70125 SLAVE devices, where BUSY becomes an input to gate writes, eliminating need for external arbitration logic.

Key Specifications

Pinout & Package

70121S55J is housed in a 52-pin Plastic Leaded Chip Carrier (PLCC), package code PLG52, body size ≈ 0.75 in × 0.75 in × 0.17 in. All VCC and GND pins must be connected; pin 1 orientation matches IDT's top-view diagram (DSC 2654/15, Figure 2).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual-port SRAM applications.

Compared with IDT70121L55J, the 70121S55J trades 5× higher full-standby current for improved noise immunity and compatibility with legacy 5 V TTL drive levels; compared with CY7C1362V-55AXC, it delivers plug-in arbitration and interrupt capability in a smaller, drop-in PLCC package - reducing system component count and validation effort.

Availability

70121S55J is available at Aetrix Electronics and suitable for telecom line card buffering, industrial PLC dual-CPU communication, avionics sensor fusion hubs, and medical imaging data pipelines requiring stable component supply and long-term obsolescence management.

Supply support for 70121S55J 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

Integrated Device Technology (IDT), now part of Renesas Electronics, is a fabless semiconductor company specializing in timing, memory interface, and RF solutions for communications, computing, and industrial markets.

The 70121S55J belongs to IDT's legacy dual-port SRAM product line, designed specifically for deterministic, low-latency inter-processor communication in real-time embedded systems where hardware arbitration eliminates software overhead and timing uncertainty.

FAQ

What is the exact memory organization and bus width of the 70121S55J?

The 70121S55J contains 2K × 9 bits (2,048 words × 9 bits) of static RAM, organized as two independent 9-bit-wide ports (left and right), each with its own 11-bit address bus (A0–A10), 9-bit bidirectional I/O (I/O0–I/O8), and dedicated control signals (CE, OE, R/W). This structure supports 18-bit or wider word expansion when paired with IDT70125 SLAVE devices. The 70121S55J is a MASTER device with on-chip arbitration logic.

Does the 70121S55J support battery backup operation, and what is the minimum retention voltage?

Yes, the 70121S55J supports battery backup operation with guaranteed data retention at VCC = 2.0 V minimum. Per IDT DSC 2654/15 Table 7, VDR is specified as ≥2.0 V, and ICCDR (data retention current) is 100 µA typical at 2 V. This allows the 70121S55J to maintain memory contents during main power loss when backed by a 2 V coin cell or supercapacitor - a key feature for fail-safe industrial and medical systems.

How does the BUSY arbitration logic work in the 70121S55J, and what timing parameters govern it?

The 70121S55J's BUSY arbitration asserts BUSYL or BUSYR when both ports attempt access to the same memory address simultaneously. Arbitration is based solely on CE and address signal timing - not R/W state - and is resolved within ≤30 ns (tBAA) of address validity. The losing port receives a push-pull BUSY signal that gates its write operation. Critical timing parameters include tBDD (BUSY disable to valid data, ≤45 ns) and tAPS (arbitration priority setup, 5 ns min), ensuring deterministic resolution without software intervention.

What are the interrupt mailbox addresses for the 70121S55J, and how are they used?

The 70121S55J uses fixed SRAM addresses 7FEh (hex) and 7FFh for interrupt signaling: writing to 7FEh asserts the left-port INTL flag, and writing to 7FFh asserts the right-port INTR flag. Clearing is performed by reading the same address with OE and CE active. These locations are standard SRAM cells - usable as general memory if interrupts are unused. The 70121S55J's interrupt logic is hardware-based, delivering sub-45 ns flag assertion (tINS) without CPU polling overhead.

Is the 70121S55J pin-compatible with other speed grades or power variants in the IDT70121 family?

Yes, the 70121S55J is pin-compatible with all IDT70121 variants in the 52-pin PLCC (PLG52) package, including 70121S25J, 70121L35J, and 70121L55J. Pin functions, electrical characteristics (VIH/VIL, VOH/VOL), and timing interfaces (tRC, tAA, tACE) are identical across speed/power grades - only access time and standby current differ. This allows drop-in replacement for performance tuning or power optimization without PCB revision.