??? 08/01/06 17:12 Read: times |
#121505 - Some encoders "see" pretty high impedances Responding to: ???'s previous message |
The "classic" (74C922/923) keypad encoders (not necessarily a "good buy," BTW) recognize a 50K-ohm impedance as a switch closure. They don't use any external resistors and simply drive the rows and sense the columns. This kind of encoding can be done with a "johnson counter" e.g. CD4017, and a 74xx148 priority encoder. That's essentially what the firmware attempts to do when one's using microcontroller.
There's a penalty for using low-impedance push-pull configuration of the port in question. In the simplest case, it's that one has to provide current-limiting in the form of series resistors. That requires choices, e.g. "do they go on the high side or the low side?" That requires study of the port behavior. When using other families of MCU's, one can configure a port to be either input or output. That sometimes solves the problem. In this family, one has to know whether that particular MCU at that particular port, is capable of open-drain or weak-pullup operation. That's why one has to READ the specifications before proceeding. RE |