Reduce power dissipation through bus hold circuitry

Article By : STMicroelectronics

Bus hold replaces pull-up resistors for power dissipation reduction, and also to save on space and costs.

Unused CMOS inputs that are left floating experience a gradual charging of the gate input capacitance. A common solution for this issue is to connect the floating inputs to VCC or to ground through a pull-up or pull-down resistor. The disadvantage of this is that an extra component and extra board space are needed 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-state. This feature eliminates the need to use pull-up resistors to prevent the device inputs from floating and causing potential system errors.

Since it is weak, it requires only a relatively small current to keep the input in the required state. The bus hold also does not need much current to overcome the clamp once an active driver toggles the input. In this way, bus hold circuitry effectively replaces pull-up resistors, reducing power dissipation and increasing the savings on space and component costs.

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