The HEF4521B-Q100 consists of a chain of 24 toggle flip-flops with an overriding asynchronous master reset input (MR), and an input circuit that allows three modes of operation. The single inverting stage (A2 to Y2) functions as: a crystal oscillator, an input buffer for an external oscillator or in combination with A1 as an RC oscillator. The crystal oscillator operates in Low-power mode when pins V_SS1 and V_DD1 are supplied via external resistors.

Each flip-flop divides the frequency of the previous flip-flop by two, consequently the HEF4521B-Q100 counts up to 2²⁴ = 16777216. The counting advances on the HIGH-to-LOW transition of the clock (A2). The outputs from each of the last seven stages (2¹⁸ to 2²⁴) are available for additional flexibility.

It operates over a recommended V_DD power supply range of 3 V to 15 V referenced to V_SS (usually ground). Unused inputs must be connected to V_DD, V_SS, or another input.

This product has been qualified to the Automotive Electronics Council (AEC) standard Q100 (Grade 3) and is suitable for use in automotive applications.