Axcelerator Family FPGAs
2- 14
v2.8
I/O Clusters
Each I/O cluster incorporates two I/O modules, four RX
modules and two TX modules, and a buffer module. In
turn, each I/O module contains one Input Register
(InReg), one Output Register (OutReg), and one Enable
Using an I/O Register
To access the I/O registers, registers must be instantiated
in the netlist and then connected to the I/Os. Usage of
each I/O register (register combining) is individually
controlled and can be selected/deselected using the
PinEditor tool in Actel's Designer software. I/O register
combining can also be controlled at the device level,
affecting all I/Os. Please note, the I/O register option is
deselected by default in any given design.4
In addition, Designer software provides a global option to
enable/disable the usage of registers in the I/Os. This option
is design-specific. The setting for each individual I/O
overrides this global option. Furthermore, the global set
fuse option in the Designer software, when checked, causes
all I/O registers to output logic High at device power-up.
Using the Weak Pull-Up and Pull-Down
Circuits
Each Axcelerator I/O comes with a weak pull-up/down
circuit (on the order of 10 k
Ω). I/O macros are provided
for combinations of pull up/down for LVTTL, LVCMOS
(2.5V, 1.8V, and 1.5V) standards. These macros can be
instantiated if a keeper circuit for any input buffer is
required.
Customizing the I/O
A five-bit programmable input delay element is
associated with each I/O. The value of this delay is
It is optional for each input buffer within the bank
(i.e. the user can enable or disable the delay
element for the I/O). When the input buffer drives a
register within the I/O, the delay element is
Figure 2-5 I/O Cluster Interface
EnReg
DIN YOUT
Y
DCIN
OutREg
DIN YOUT
InReg
I/O CLUSTER
FPGA
LOGIC
CORE
OEP
UOP
UIP
programmable delay
slew rate
I/O
OEN
UON
UIN
drive strength
P PAD
N PAD
routed input track
output track
routed input track
output track
routed input track
output track
EnReg
DIN YOUT
Y
DCIN
OutREg
DIN YOUT
InReg
routed input track
output track
programmable delay
slew rate
I/O
drive strength
VREF
BSR
4. Please note that register combining for multi fanout nets is not supported.