DESCRIPTION
There are two trigger inputs, A INPUT (negative edge) and B INPUT (positive edge). Triggering on the B input occurs at a particular voltage threshold and is not related to the rise and fall time of the applied pulse. The device may also be triggered by using the CLR input (positive-edge) because of the Schmitt-trigger input ; after triggering the output maintains the MONOSTABLEstate for the time period determined by the external resistor Rx and capacitor Cx.Taking CLR low breaks this MONOSTABLE STATE. If the next trigger pulse occurs during the MONOSTABLE period it makes the MONOSTABLE period longer. Limit for values of Cx and Rx : Cx : NO LIMIT Rx : VCC < 3.0 V 5 K Ω to 1 M Ω VCC ≥ 3.0 V 1 K Ω to 1 M Ω Two different pulse width constants are available: K ≅ 0.7 for HC221 K ≅ 1 for HC221A All inputs are equipped with protection circuits against static discharge and transient excess voltage. October 1993 PIN CONNECTIONS (top view) NC = No Internal Connection 1/14 M54/M74HC221/221A SYSTEM DIAGRAM TIMING CHART 2/14 M54/M74HC221/221A BLOCK DIAGRAM Note : (1) Cx, Rx, Dx are external components. (2) Dx is a clamping diode.
STMicroelectronics
