As systems grow smaller, it becomes increasingly attractive to replace mechanical potentiometers with electronic potentiometers, which are smaller and less expensive silicon equivalents. A common interface for such devices comprises a Chip-Select, Increment and, Up/ line. CS activates the device and, on a rising edge of steps the wiper in a direction that the U/ pin indicates. The simple circuit of Figure 1 uses two pushbuttons—one for up and one for down—and a few tiny silicon devices to implement a debounced, ESD-protected electronic-potentiometer system. The normally open pushbutton switches feed into the MAX6817, an ESD-protected switch debouncer in an SOT-23 package. It has internal pullup resistors on the inputs and buffered, noninverting CMOS outputs. In the absence of a switch closure, the normally open switches hold the MAX6817 outputs high. In turn, that condition ensures a low state for the active-low, push-pull output of the MAX6308, an SC70 reset device with two reset inputs that are independent of the V_CC pin.

The reset device must have extra reset inputs rather than a manual-reset input, because the glitch-immunity protection of manual-reset inputs is not adequate to guarantee proper operation. The MAX5161 is a 32-tap, linear-taper electronic potentiometer in an SOT-23 package with the standard -U/ interface. (The electronic potentiometer pulls the CS input high internally.) Its setup requirement is 50 nsec, meaning that the U/ signal must be stable for 50 nsec preceding a rising edge at the  pin. The transient-filtering circuitry internal to the MAX6308 introduces a delay that satisfies the setup requirement. The delay, t_f, is typically 10 to 30 µsec (Figure 2). goes high again only after the reset timeout interval expires. For the MAX6308, that interval (t_RESET) is factory-preset with a value as short as 1 msec.

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