General description
The HEF4521B 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 VSS1 and VDD1 are supplied via external resistors.
Each flip-flop divides the frequency of the previous flip-flop by two, consequently the HEF4521B
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 VDD power supply range of 3 V to 15 V referenced to VSS (usually
ground). Unused inputs must be connected to VDD, VSS, or another input.

Features and benefits
• Wide supply voltage range from 3.0 V to 15.0 V
• CMOS low power dissipation
• High noise immunity
• Low power crystal oscillator operation
• Fully static operation
• 5 V, 10 V, and 15 V parametric ratings
• Standardized symmetrical output characteristics
• Complies with JEDEC standard JESD 13-B
• ESD protection:
• HBM JESD22-A114F exceeds 2000 V
• MM JESD22-A115-A exceeds 200 V
• Specified from -40 °C to +85 °C