6–4
Chapter 6: I/O Features in the Cyclone III Device Family
I/O Element Features
Slew Rate Control
The output buffer for each Cyclone III device family I/O pin provides optional
programmable output slew-rate control. The Quartus II software allows three settings
for programmable slew rate control—0, 1, and 2—where 0 is the slow slew rate and 2
is the fast slew rate. The default setting is 2. A faster slew rate provides high-speed
transitions for high-performance systems. However, these fast transitions may
introduce noise transients in the system. A slower slew rate reduces system noise, but
adds a nominal delay to rising and falling edges. Because each I/O pin has an
individual slew-rate control, you can specify the slew rate on a pin-by-pin basis. The
slew-rate control affects both the rising and falling edges. Slew rate control is available
for single-ended I/O standards with current strength of 8 mA or higher.
1
You cannot use the programmable slew rate feature when using OCT with or without
calibration.
You cannot use the programmable slew rate feature when using the
3.0-V PCI,
3.0-V PCI-X, 3.3-V LVTTL,
and
3.3-V LVCMOS
I/O standards. Only fast slew rate
(default) setting is available.
1
Open-Drain Output
The Cyclone III device family provides an optional open-drain (equivalent to an
open-collector) output for each I/O pin. This open-drain output enables the device to
provide system-level control signals (for example, interrupt and write enable signals)
that are asserted by multiple devices in your system.
Bus Hold
Each Cyclone III device family user I/O pin provides an optional bus-hold feature.
The bus-hold circuitry holds the signal on an I/O pin at its last-driven state. Because
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 in which 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
V
CCIO
to prevent overdriving signals.
1
If you enable the bus-hold feature, 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 dedicated clock pins.
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.
f
For the specific sustaining current for each V
CCIO
voltage level driven through the
resistor and for the overdrive current used to identify the next driven input level, refer
to the
and
chapters.
Cyclone III Device Handbook
Volume 1
July 2012 Altera Corporation
ALTERA [ ALTERA CORPORATION ]
