请问有关于ADSP21489 IO管脚内部描述有相应文档介绍吗 ？ 对下面画红线部分不太理解 ，谢谢 ！
The following symbols appear in the Type column of this table: A = asynchronous, I = input, O = output, S = synchronous, A/D = active drive,
O/D = open drain, and T = three-state, ipd = internal pull-down resistor, ipu = internal pull-up resistor.
The internal pull-up (ipu) and internal pull-down (ipd) resistors are designed to hold the internal path from the pins at the expected logic
levels. To pull-up or pull-down the external pads to the expected logic levels, use external resistors. Internal pull-up/pull-down resistors cannot
be enabled/disabled and the value of these resistors cannot be programmed. The range of an ipu resistor can be between 26 kΩ–63 kΩ. The
range of an ipd resistor can be between 31 kΩ–85kΩ. The three-state voltage of ipu pads will not reach to the full VDD_EXT level; at typical
conditions the voltage is in the range of 2.3 V to 2.7 V.
