OR4E2 Datasheet, PDF (34/124 Pages) Agere Systems – Field-Programmable Gate Arrays

ORCA Series 4 FPGAs
Preliminary Data Sheet
December 2000
Programmable Input/Output Cells
(continued)
Inputs
There are many major options on the PIO inputs that
can be selected in the ORCA Foundry tools listed in
Table 15. Inputs may have a pull-up or pull-down resis-
tor selected on an input for signal stabilization and
power management. A weak keeper circuit is also
available on inputs. Input signals in a PIO are passed to
CIB routing and/or a fast route into the clock routing
system.
There is also a programmable delay available on the
input. When enabled, this delay affects the INFF and
INDD signals of each PIO, but not the clock input. The
delay allows any signal to have a guaranteed zero hold
time when input. This feature is discussed subse-
quently.
Inputs should have transition times of less than 500 ns
and should not be left floating. If any pin is not used, it
is 3-stated with an internal pull-up resistor enabled
automatically after configuration.
Floating inputs increase power consumption, produce
oscillations, and increase system noise. The inputs
have a typical hysteresis of approximately 250 mV to
reduce sensitivity to input noise. The PIC contains
input circuitry that provides protection against latch-up
and electrostatic discharge.
The other features of the PIO inputs relate to the latch/
FF structure in the input path. In latch mode, the input
signal is fed to a latch that is clocked by either the pri-
mary, secondary, or edge clock signal. The clock may
be inverted or noninverted. There is also a local set/
reset signal to the latch. The senses of these signals
are also programmable and have the capability to
enable or disable the global set/reset signal and select
the set/reset priority. The same control signals may
also be used to control the input latch/FF when it is
configured as a FF instead of a latch, with the addition
of another control signal used as a clock enable. The
PIOs are paired together and have independent CE,
set/reset, and GSRN control signals for the pair. Note
that these control signals are paired to the same pair of
pins used for differential signaling.
The input path is also capable of accepting data from
any pad using a fast capture feature. This feature can
be programmed as a latch or FF referenced to any
clock. There are two options for zero-hold input capture
in the PIO. If input delay mode is selected to delay the
signal from the input pin, data can be either registered
or latched with guaranteed zero-hold time in the PIO
using a system clock. To further improve setup time,
the fast zero-hold mode of the PIO input takes advan-
tage of the latch/FF combination and sources the input
FF data from a dedicated latch that is clocked by a fast
edge clock from the dedicated clock pads or any local
pad. The input FF is then driven by a primary clock
sourced from a dedicated input pin designed for fast,
low-skew operation at the I/Os. These dedicated pads
are located in pairs in the center of each side of the
array and if not utilized by the clock spine can be used
as general user I/O. The clock inputs to both the dedi-
cated fast capture latch and the input FF can also be
driven by the on-chip PLLs.
The combination of input register capability provides for
input signal demultiplexing without any additional
resources such as for address and data arriving on the
same pins. On the positive edge of the clock, the data
would come from the pad to latch. The PIO input signal
is sent to both the input latch and directly to INDD. The
signal is latched on the falling edge of the clock and
output to routing at INFF. The address and data are
then both available at the rising edge of the clock.
These signals may be registered or otherwise pro-
cessed in the PLCs.
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Lucent Technologies Inc.