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May 2005

AN2149

APPLICATION NOTE

INTEGRATED BUSHOLD CIRCUITRY

Rev. 1

1. ABSTRACT

Unused CMOS inputs which are left floating will experience a gradual charging of the gate input

capacitance. A floating input may see an increase in static current, as both the NMOS and PMOS outputs

voltages between 0.8V and 2.0V applied to the inputs, the outputs will tend to oscillate. Large numbers of

gates left floating, in a 16 bit bus driver for example, will cause large amounts of current to be drawn by

the IC. The floating gate charges up at a rate determined by its leakage current. Intermittent or random

circuit errors may be seen with floating inputs, as outputs switch to a different state for no apparent reason.

down resistor. The disadvantage of this is an extra component needed as well as extra board space, and

the resistor dissipates extra power. Hence, this pull-up resistor method is recommended for ac-powered

systems and not for battery-operated equipment where power consumption is critical. Instead, a special

feature called Bus hold circuit is used.

Bus hold is an improved version of the internal pull-up resistor. It is a weak latch that recalls the last valid

state of a pin when it is three-stated. Bus hold provides a small positive feedback current on device inputs.

When an input changes logic state, the bus-hold circuit will return a small current back to the device input,

effectively adding to the transition of the input. This positive feedback will then hold the final logic level until

an active driver toggles the input voltage to the opposite logic state, where bus-hold will then again hold

the logic state. A number of logic families use an integrated bus hold circuit which eliminates the need for

external resistors and saves board space. The circuit is shown in figure 1:

Figure 1. Generic Bus Hold Circuitry

2. ADVANTAGES OF BUS HOLD

- Reduces part count by eliminating the pull-up resistors

- Saves board space

- Prevents floating busses and thereby avoids device oscillations.

- Reduces bus loading

- Does not toggle the bus when the bus is released

- Reduces power dissipation

Obsolete

Product(s)

- Obsolete

Product(s)