Chapter 2: MAX II Architecture
I/O Structure
2–31
Bus Hold
Each MAX II device 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. The designer
can select this feature individually for each I/O pin. The bus-hold output will drive
no higher than V
CCIO
to prevent overdriving signals. If the bus-hold feature is enabled,
the device cannot use the programmable pull-up option.
The bus-hold circuitry uses a resistor to pull the signal level to the last driven state.
The
chapter in the
MAX II Device Handbook
gives the
specific sustaining current for each V
CCIO
voltage level driven through this resistor and
overdrive current used to identify the next-driven input level.
The bus-hold circuitry is only active after the device has fully initialized. The bus-hold
circuit captures the value on the pin present at the moment user mode is entered.
Programmable Pull-Up Resistor
Each MAX II device I/O pin provides an optional programmable pull-up resistor
during user mode. If the designer enables this feature for an I/O pin, the pull-up
resistor holds the output to the V
CCIO
level of the output pin’s bank.
1
The programmable pull-up resistor feature should not be used at the same time as the
bus-hold feature on a given I/O pin.
Programmable Input Delay
The MAX II IOE includes a programmable input delay that is activated to ensure zero
hold times. A path where a pin directly drives a register, with minimal routing
between the two, may require the delay to ensure zero hold time. However, a path
where a pin drives a register through long routing or through combinational logic
may not require the delay to achieve a zero hold time. The Quartus II software uses
this delay to ensure zero hold times when needed.
MultiVolt I/O Interface
The MAX II architecture supports the MultiVolt I/O interface feature, which allows
MAX II devices in all packages to interface with systems of different supply voltages.
The devices have one set of
VCC
pins for internal operation (V
CCINT
), and up to four
sets for input buffers and I/O output driver buffers (V
CCIO
), depending on the number
of I/O banks available in the devices where each set of
VCC
pins powers one I/O
bank. The EPM240 and EPM570 devices have two I/O banks respectively while the
EPM1270 and EPM2210 devices have four I/O banks respectively.
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