NXP Semiconductors PCF7936AS/3851,122

Overview

PCF7936AS/3851,122 from NXP Semiconductors (formerly Philips Semiconductors) is a contactless security transponder IC implementing the HITAG 2 protocol for automotive immobiliser systems. It performs 64/32-bit mutual authentication in 39 ms, contains a factory-programmed 32-bit unique ID, 48-bit secret key, and 128-bit user EEPROM with cipher/plain access modes.

For engineers reviewing the PCF7936AS/3851,122 datasheet, PCF7936AS/3851,122 pinout, PCF7936AS/3851,122 application, or PCF7936AS/3851,122 equivalent, key selection criteria include RF field sensitivity (35 µT), EEPROM data retention (20 years), read/write protection flags, Read Only mode compatibility, and leadless stick-package mechanical fit for embedded transponder coil integration.

Technical Context

The PCF7936AS/3851,122 integrates a contactless interface with internal clock recovery, rectifier, voltage limiter, and power-on reset to operate solely from the magnetic component of a 125 kHz base station RF field. Its calculation unit executes the HITAG 2 mutual authentication algorithm using a challenge-response sequence with random number, secret key, and password inputs.

Memory architecture separates 128 bits of user EEPROM from configuration and control bits, supporting both plain and cipher-mode access with configurable read/write protection flags. Two dedicated Read Only modes ensure interoperability with legacy 125 kHz read-only transponder systems without requiring protocol modification.

Key Specifications

Pinout & Package

PCF7936AS/3851,122 is housed in a leadless stick-shaped plastic package (dimensions: 11.9–12.1 mm × 4.4–4.6 mm × 1.9–2.1 mm) designed for direct embedding into antenna coils. No external pins or terminals - RF coupling occurs via integrated coil connection pads on the underside of the package.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar contactless security transponder applications.

Compared with PCF7936AS/3851,122, PCF7931A/3851,122 sacrifices all security features for lowest cost and widest reader compatibility, while PCF7961/3851,122 extends memory and key size at the expense of authentication latency and legacy system support - making PCF7936AS/3851,122 the balanced choice for production-grade HITAG 2 deployments.

Availability

PCF7936AS/3851,122 is available at Aetrix Electronics and suitable for automotive immobiliser modules, key fob assemblies, and aftermarket security systems requiring stable component supply across multi-year production cycles.

Supply support for PCF7936AS/3851,122 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

NXP Semiconductors is a global semiconductor company specialising in secure connectivity solutions for automotive, industrial, and IoT applications.

The PCF7936AS/3851,122 belongs to NXP's HITAG transponder product line, engineered specifically for contactless vehicle access and immobiliser systems requiring robust, battery-free cryptographic authentication.

FAQ

What communication protocol does the PCF7936AS/3851,122 implement?

The PCF7936AS/3851,122 implements the HITAG 2 contactless protocol operating at 125 kHz. It uses load modulation for backscatter communication and supports both mutual authentication and legacy Read Only modes. The protocol is defined in NXP's HITAG 2 specification and requires compatible base station firmware - the PCF7936AS/3851,122 itself contains no programmable microcontroller or protocol stack beyond the hardened HITAG 2 logic.

Does the PCF7936AS/3851,122 require an external power source?

No, the PCF7936AS/3851,122 is a fully passive transponder. It derives all operating power from the magnetic field of a 125 kHz base station via its integrated coil interface. Internal rectifier, voltage limiter, and power-on reset circuits condition this harvested energy to run the authentication engine and EEPROM - eliminating batteries and enabling maintenance-free operation for 20+ years.

How is memory protected on the PCF7936AS/3851,122?

Memory protection on the PCF7936AS/3851,122 is implemented via dedicated EEPROM access flags stored in configuration registers. These flags enable per-segment read lock, write lock, or cipher-only access modes. Once set, protection bits are irreversible - preventing runtime modification of secured data. The PCF7936AS/3851,122 enforces these restrictions at hardware level during every memory transaction.

What is the purpose of the two Read Only modes on the PCF7936AS/3851,122?

The two Read Only modes on the PCF7936AS/3851,122 provide backward compatibility with legacy 125 kHz base stations that only support simple ID broadcast. Mode 1 outputs the 32-bit unique ID in plain format; Mode 2 outputs it in a format compatible with earlier Philips transponders. Neither mode engages authentication or accesses EEPROM - ensuring plug-and-play integration into existing infrastructure without base station upgrades.

Can the PCF7936AS/3851,122 be reprogrammed after initial personalisation?

Yes, the PCF7936AS/3851,122 supports field reprogramming of its 128-bit user EEPROM via cipher-mode commands, provided the correct 48-bit secret key is presented during authentication. However, memory protection flags and the factory-programmed 32-bit UID are permanently locked and cannot be altered. Reprogramming requires a compliant HITAG 2 base station and valid session keys - not possible through casual RF exposure.