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logic state

In some applications, inputs are required to have a default logic state at power-up. This requirement often occurs when buffers are used to interface both control and data signals to a bus. As the power-up state of the bus-hold circuit is not guaranteed, an external resistor is required to set the default state. The value of the resistor should be such that it overdrives the bus-hold input to the desired state during power-up. Equations (1) and (2) can be used to estimate the maximum value of pull-down resistor RPD or pull-up resistor RPU to overdrive the bus-hold circuit. VTH is the input switching threshold voltage; this can be estimated as (VIH + VIL)/2.

PTM Published on: 2014-09-25