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Each IOB also provides for bus-hold circuitry (also called “keeper”)that is active during valid user operation. The
bus-hold feature eliminates the need to tie unused pins
either high or low by holding the last known state of the
input until the next input signal is present. The bus-hold cir-cuit
drives back the same state via a nominal resistance
(R BH ) of 50k ohms. See Figure 13. Note the bus-hold out-put
will drive no higher than V CCIO to prevent overdriving
signals when interfacing to 2.5V components.
When the device is not in valid user operation, the bus-hold
circuit defaults to an equivalent 50k ohm pull-up resistor order to provide a known repeatable device state. This
occurs when the device is in the erased state, in program-ming
mode, in JTAG INTEST mode, or during initial
power-up. A pull-down resistor (1k ohm) may be externally
added to any pin to override the default R BH resistance force a low state during power-up or any of these other
modes.
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