XC3S250E Xilinx, Inc., XC3S250E Datasheet - Page 19

• Manufacturer Part Number: XC3S250E
• Description: Spartan-3E FPGA Family
• Manufacturer: Xilinx, Inc.

Functional Description
levels (see Table 2 of
are in a high-impedance state. V
V
cuit (POR).
A Low level applied to the HSWAP input enables pull-up
resistors on User I/Os from power-on throughout configura-
tion. A High level on HSWAP disables the pull-up resistors,
allowing the I/Os to float. HSWAP contains a weak pull-up
and defaults to High if left floating. As soon as power is
applied, the FPGA begins initializing its configuration mem-
ory. At the same time, the FPGA internally asserts the Glo-
bal Set-Reset (GSR), which asynchronously resets all IOB
storage elements to a default Low state.
Upon the completion of initialization and the beginning of
configuration, INIT_B goes High, sampling the M0, M1, and
M2 inputs to determine the configuration mode. At this point
in time, the configuration data is loaded into the FPGA. The
I/O drivers remain in a high-impedance state (with or with-
out pull-up resistors, as determined by the HSWAP input)
throughout configuration.
At the end of configuration, the GSR net is released, placing
the IOB registers in a Low state by default, unless the
12
CCAUX
serve as inputs to the internal Power-On Reset cir-
Module
3). At this time, all I/O drivers
CCO
Bank 2, V
CCINT
www.xilinx.com
, and
loaded design reverses the polarity of their respective SR
inputs.
The Global Three State (GTS) net is released during
Start-Up, marking the end of configuration and the begin-
ning of design operation in the User mode. After the GTS
net is released, all user I/Os go active while all unused I/Os
are weakly pulled down (PULLDOWN). The designer can
control how the unused I/Os are terminated after GTS is
released by setting the Bitstream Generator (BitGen) option
UnusedPin to PULLUP, PULLDOWN, or FLOAT.
One clock cycle later (default), the Global Write Enable
(GWE) net is released allowing the RAM and registers to
change states. Once in User mode, any pull-up resistors
enabled by HSWAP revert to the user settings and HSWAP
is available as a general-purpose I/O. For more information
on PULLUP and PULLDOWN, see
Resistors.
JTAG Boundary-Scan Capability
All Spartan-3E IOBs support boundary-scan testing com-
patible with IEEE 1149.1/1532 standards. See
page 86
for more information on programming via JTAG.
Advance Product Specification
DS312-2 (v1.1) March 21, 2005
Pull-Up and Pull-Down
JTAG Mode,
R