Embedded Freaks..

September 9, 2010

Reason for Tying Unused Input Buffers to Either the Power Rail or the Ground Rail through a Resistor

Filed under: embedded-tips — Tags: — kunilkuda @ 10:08 am

A simple CMOS inverter-type input buffer has a narrow voltage range typically located a little below the Vdd/2 input voltage level at which the P-channel device and the N-channel device are turned-on to some degree simultaneously.  Current is flowing directly from the Power-supply rail to the ground rail.  This input voltage level typically occurs when the input buffer is left floating and not connected to either one of the rails.  When there are many input pins left floating the net result is that a large amount of current can be flowing from the power-supply to ground through these floating input buffers.
CMOS InverterIn NXP chips, we use a more-complex Schmitt-Trigger type input buffer in which the input trip-point is determined by the state of the output voltage level.  Because of the added feedback circuits required to achieve this functionality, there is even a greater amount of current flowing between the power-supply and ground when the input is left floating, and in addition, the central voltage-range where this occurs is much broader.

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