What is bushold of the general-purpose logic ICs?

A bushold circuit is added to the data input of an IC. Consisting of two inverters in a feedback loop, the bushold circuit holds the state at an input pin at its last known state whenever it is left open (i.e., floating).

Fig. 1 Bushold circuit for general-purpose Logic IC
Fig. 1 Bushold circuit for general-purpose Logic IC

The following Fig. shows an equivalent circuit for a bushold circuit.

In the case of general-purpose logic circuits, unused input pins assume the High-Z state if they are left open or floating. Typically, this situation is prevented by connecting external pull-up or pull-down resistors because the High-Z state causes input capacitance to be gradually charged by leakage current, eventually turning on the input P-channel and N-channel MOSFETs simultaneously and thereby causing undesired current or abnormal oscillation.
In contrast, the bushold circuit employs a weak feedback gate tied back to the input to hold the last input state until it changes state next time.

Therefore, ICs with a bushold circuit eliminate the need for external pull resistors.

There are two electrical characteristics concerning the bushold circuit: 1) bushold input minimum drive hold current (II (HOLD) ) that specifies the minimum current that the bushold circuit can supply to a device or a bus, and 2) bushold input overdrive current to change state (II (OD) ) that specifies the minimum overdrive current necessary to change the state held in the bushold circuit. An example of bushold characteristics shown in the datasheet (Table-1) is given below.

Table-1 Bus transceiver with bushold (TC74VCXH16245FT) Guaranteed bushold input currents (in μA)
Table-1 Bus transceiver with bushold (TC74VCXH16245FT) Guaranteed bushold input currents (in μA)

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