HEF4521B

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 (I₂/O₂) will function as a crystal oscillator, or in combination with I₁ as an RC oscillator, or as an input buffer for an external oscillator. Low-power operation as a crystal oscillator is enabled by connecting external resistors to pins 3 (V_SS') and 5 (V_DD').

Each flip-flop divides the frequency of the previous flip-flop by two, consequently the HEF4521B will count up to 224 = 16777216. The counting advances on the HIGH to LOW transition of the clock (I₂). The outputs of the last seven stages are available for additional flexibility.