Cyclone II Architecture
Slew Rate Control
Slew rate control is performed by using programmable output drive
strength.
Bus Hold
Each Cyclone II device user I/O pin provides an optional bus-hold
feature. The bus-hold circuitry can hold the signal on an I/O pin at its
last-driven state. Since the bus-hold feature holds the last-driven state of
the pin until the next input signal is present, an external pull-up or
pull-down resistor is not necessary to hold a signal level when the bus is
tri-stated.
The bus-hold circuitry also pulls undriven pins away from the input
threshold voltage where noise can cause unintended high-frequency
switching. You can select this feature individually for each I/O pin. The
bus-hold output drives no higher than VCCIO to prevent overdriving
signals.
1
If the bus-hold feature is enabled, the device cannot use the
programmable pull-up option. Disable the bus-hold feature
when the I/O pin is configured for differential signals. Bus hold
circuitry is not available on the dedicated clock pins.
The bus-hold circuitry is only active after configuration. When going into
user mode, the bus-hold circuit captures the value on the pin present at
the end of configuration.
The bus-hold circuitry uses a resistor with a nominal resistance (RBH) of
approximately 7 kΩto pull the signal level to the last-driven state. Refer
to the DC Characteristics & Timing Specifications chapter in Volume 1 of the
Cyclone II Device Handbook for the specific sustaining current for each
VCCIO voltage level driven through the resistor and overdrive current
used to identify the next driven input level.
Programmable Pull-Up Resistor
Each Cyclone II device I/O pin provides an optional programmable
pull-up resistor during user mode. If you enable this feature for an I/O
pin, the pull-up resistor (typically 25 kΩ) holds the output to the VCCIO
level of the output pin’s bank.
1
If the programmable pull-up is enabled, the device cannot use
the bus-hold feature. The programmable pull-up resistors are
not supported on the dedicated configuration, JTAG, and
dedicated clock pins.
Altera Corporation
February 2007
2–51
Cyclone II Device Handbook, Volume 1
ALTERA [ ALTERA CORPORATION ]
