2.3.3.3 Using the Weak Pull-Up and Pull-Down Circuits

Each Axcelerator I/O comes with a weak pull-up/down circuit (on the order of 10 kΩ). These are weak transistors with the gates tied on, so the resistance of the transistor emulates a resistor. The weak pull-up and pull-down is active only when the device is powered up, and they must be biased to be on. When the rails are coming up, they are not biased fully, so they do not behave as resistors until the voltage is at sufficient levels to bias the transistors. The key is they really are transistors, they are not traces of poly silicon, which is another way to do an on-chip resistor (those take much more room). I/O macros are provided for combinations of pull up/down for LVTTL, LVCMOS (2.5V, 1.8V, and 1.5V) standards. These macros can be instantiated if a keeper circuit for any input buffer is required.