I/O Structure
PAD
The I/O pad is the one that connects the I/O to the outside world. Note that not all I/Os
have pads: the ones without pads are called Unbonded I/Os. The number of unbonded
I/Os varies with the device size and package. These unbonded I/Os are used to perform
a variety of bus turns at the edge of the array.
PULL-UP/PULL-DOWN
Each pad has a programmable pull-up and pull-down attached to it. This supplies a
weak “1” or “0” level to the pad pin. When all other drivers are off, this control will dictate
the signal level of the pad pin.
The input stage of each I/O cell has a number of parameters that can be programmed
either as properties in schematic entry or in the I/O Pad Attributes editor in IDS.
TTL/CMOS
SCHMITT
The threshold level can be set to either TTL/CMOS-compatible levels.
A Schmitt trigger circuit can be enabled on the inputs. The Schmitt trigger is a regenera-
tive comparator circuit that adds 1V hysteresis to the input. This effectively improves the
rise and fall times (leading and trailing edges) of the incoming signal and can be useful
for filtering out noise.
DELAYS
DRIVE
The input buffer can be programmed to include four different intrinsic delays as specified
in the AC timing characteristics. This feature is useful for meeting data hold require-
ments for the input signal.
The output drive capabilities of each I/O are programmable. They can be set to FAST,
MEDIUM or SLOW (using IDS tool). The FAST setting has the highest drive capability
(20 mA at 5V) buffer and the fastest slew rate. MEDIUM produces a medium drive
(14 mA at 5V) buffer, while SLOW yields a standard (6 mA at 5V) buffer.
TRI-STATE
The output of each I/O can be made tri-state (0, 1 or Z), open source (1 or Z) or open
drain (0 or Z) by programming an I/O’s Source Selection mux. Of course, the output can
be normal (0 or 1), as well.
SOURCE SELECTION MUX
The Source Selection mux selects the source for the output signal of an I/O, see
Figure 12 on page 20.
18
AT40K/AT40KLV Series FPGA
0896C–FPGA–04/02
ATMEL [ ATMEL ]
