Obsolete

Product(s)

- Obsolete

Product(s)

5/7

AN2149 - APPLICATION NOTE

Figure 5. Typical Bus Hold Characteristics of 74VCXH1632245 and the Effect of Various Pull-up

Resistor Values

6. CONCLUSION

To summarize, an optimum pull-up resistor should be high enough to keep the power dissipation low, but

at the same time low enough to switch logic states within the time required. More importantly, it should

guarantee that the voltage level on the bus is a valid logic state. Due to such complex influencing factors,

it is strongly recommended to avoid using pull-up resistors on bus-hold inputs unless absolutely

necessary.

Components with bus-hold offer several advantages over the use of external pull-up or pull-down

resistors. The bus-hold circuit holds the last known valid state of the input when the bus starts to float. It

acts as a weak clamp at the output and maintains the last known state. 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.

-0.0004

-0.0002

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

5K pull-up resistor

8K pull-up resistor

10K pull-up resistor

Typical Bus hold

3.6K pull-up resistor