XC3S100E Datasheet, PDF (98/193 Pages) Xilinx, Inc – DC and Switching Characteristics

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Functional Description
The FPGA signals when the memory-clearing phase is
complete by releasing the open-drain INIT_B pin, allowing
the pin to go High via the external pull-up resistor to
VCCO_2.
Loading Configuration Data
Configuration data is then written to the FPGA’s internal
memory. The FPGA holds the Global Set/Reset (GSR) sig-
nal active throughout configuration, holding all FPGA
flip-flops in a reset state. The FPGA signals when the entire
configuration process completes be releasing the DONE
pin, allowing it to go High.
The FPGA configuration sequence can also be initiated by
asserting the PROG_B. Once release, the FPGA begins
clearing its internal configuration memory, and progresses
through the remainder of the configuration process.
Start-Up
At the end of configuration, the Global Set/Reset (GSR) sig-
nal is pulsed, placing all flip-flops in a known state. After
configuration completes, the FPGA switches over to the
user application loaded into the FPGA. The sequence and
timing of how the FPGA switches over is programmable as
is the clock source controlling the sequence.
The default start-up sequence appears in Figure 66, where
the Global Three-State signal (GTS) is released one clock
cycle after DONE goes High. This sequence allows the
DONE signal to enable or disable any external logic used
during configuration before the user application in the FPGA
starts driving output signals. One clock cycle later, the Glo-
bal Write Enable (GWE) signal is released. This allows sig-
nals to propagate within the FPGA before any clocked
storage elements such as flip-flops and block ROM are
enabled.
Start-Up Clock
Phase
Default Cycles
0 1 2 3 4 5 67
DONE
GTS
GWE
Start-Up Clock
Phase
Sync-to-DONE
0 1 2 3 4 5 67
DONE
GTS
DONE High
GWE
DS312-2_60_022305
Figure 66: Default Start-Up Sequence
DS312-2 (v1.1) March 21, 2005
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Advance Product Specification