上拉和下拉的精辟解释，（e文）

On most devices, all local I/O signals (even unused ones) should have external pull-ups or pull-down resistors. The internal pull-ups/downs are normally relatively weak. Any signals passing close to unused or tri-stated I/O could couple to that I/O and cause unpredictable behaviour. It can also lead to an increase in the power supply current used by the device.
If you are using a CPLD or other such device on your local bus, it may be possible to configure suitable terminators inside it.
The value of a pull-up or pull down is normally between 3K and 10K. However, you should confirm this with the data sheet for the appropriate device.
Signals are normally pulled to their inactive state. That is, active high signals are pulled down, and active low signals are pulled up. Address and data lines are typically pulled up.