Programmable Switch

18 026 GENERAL INTEREST The programmable switch may be used, for instance, to simulate a data stream or, as 1 KBM 6 er may be used as the basis of a programmable oscillator. The circuit is based on Na- tional Semiconductor keyboard decoder Type MM74C922 (IC1). This device is intended to read a 4x4 matrix keyboard in a sim- ple and fast manner. Apart from a 4-bit output, the lc also has a DATA AVAILABLE output, DA, which is high as long as one of the keys is pressed. The data associated with the last pressed key will remain on the output, even after the key has been released. The speed with which the keyboard is scanned is determined by Cl. When this capacitor has a value of 100 nF, as shown in the diagram, the scanning rate is 600 Hz. The anti-bounce period of the key- board is determined by C2: with a value as shown, the peri- od is about 10 ms (rule of thumb: C2.---10C1 ). 7 x4 * see text PROGRAMMABLE SWITCH shown, to control an analogue 10 X2 Osc multiplexer, IC5. The multiplex...