AN2149 Datasheet, PDF (1/7 Pages) STMicroelectronics – INTEGRATED BUSHOLD CIRCUITRY

AN2149
APPLICATION NOTE
INTEGRATED BUSHOLD CIRCUITRY
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
will turn on and conduct current simultaneously. Or, if the gate voltage reaches the threshold level with
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.
A common solution for this is to connect the floating inputs to V_CC or to ground through a pull-up or pull-
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
t(s ^{systems and not for battery-operated equipment where power consumption is critical. Instead, a special}
feature called Bus hold circuit is used.
uc ^{Bus hold is an improved version of the internal pull-up resistor. It is a weak latch that recalls the last valid}
d ^{state of a pin when it is three-stated. Bus hold provides a small positive feedback current on device inputs.}
ro ^{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
P _{an active driver toggles the input voltage to the opposite logic state, where bus-hold will then again hold}
te ^{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:
sole Figure 1. Generic Bus Hold Circuitry
Obsolete ^{Product(s) - Ob 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
May 2005
Rev. 1
1/7