The ’HC221 and CD74HCT221 are dual monostable multivibrators with reset. An external resistor (R_X) and an external capacitor (C_X) control the timing and the accuracy for the circuit. Adjustment of R_X and C_X provides a wide range of output pulse widths from the Q and Q\ terminals. Pulse triggering on the B input occurs at a particular voltage level and is not related to the rise and fall time of the trigger pulse.

Once triggered, the outputs are independent of further trigger inputs on A\ and B. The output pulse can be terminated by a LOW level on the Reset (R)\ pin. Trailing Edge triggering (A)\ and leading-edge-triggering (B) inputs are provided for triggering from either edge of the input pulse. On power up, the IC is reset. If either Mono is not used each input (on the unused device) must be terminated either high or low.

The minimum value of external resistance, R_X, is typically 500. The minimum value of external capacitance, C_X, is 0pF. The calculation for the pulse width is t_W = 0.7 R_XC_X at V_CC = 4.5V.