Spartan-3E FPGA Family: Functional Description
The FPGA configuration sequence can also be initiated by
asserting PROG_B. Once released, the FPGA begins
clearing its internal configuration memory, and progresses
through the remainder of the configuration process.
Maximum Bitstream Size for Daisy-Chains
The maximum bitstream length supported by Spartan-3E
FPGAs in serial daisy-chains is 4,294,967,264 bits
(4 Gbits), roughly equivalent to a daisy-chain with 720
XC3S1600E FPGAs. This is a limit only for serial
daisy-chains where configuration data is passed via the
FPGA’s DOUT pin. There is no such limit for JTAG chains.
Configuration Sequence
For additional information including I/O behavior before and
during configuration, refer to the “Sequence of Events”
chapter in UG332.
The Spartan-3E configuration process is three-stage
process that begins after the FPGA powers on (a POR
event) or after the PROG_B input is asserted. Power-On
Reset (POR) occurs after the V
, V
, and the
CCINT
CCAUX
V
Bank 2 supplies reach their respective input threshold
CCO
levels. After either a POR or PROG_B event, the
three-stage configuration process begins.
1. The FPGA clears (initializes) the internal configuration
memory.
2. Configuration data is loaded into the internal memory.
3. The user-application is activated by a start-up process.
Figure 66 is a generalized block diagram of the Spartan-3E
configuration logic, showing the interaction of different
device inputs and Bitstream Generator (BitGen) options. A
flow diagram for the configuration sequence of the Serial
and Parallel modes appears in Figure 66. Figure 67 shows
the Boundary-Scan or JTAG configuration sequence.
Initialization
Configuration automatically begins after power-on or after
asserting the FPGA PROG_B pin, unless delayed using the
FPGA’s INIT_B pin. The FPGA holds the open-drain INIT_B
signal Low while it clears its internal configuration memory.
Externally holding the INIT_B pin Low forces the
configuration sequencer to wait until INIT_B again goes
High.
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
After initialization, configuration data is written to the
FPGA’s internal memory. The FPGA holds the Global
Set/Reset (GSR) signal active throughout configuration,
holding all FPGA flip-flops in a reset state. The FPGA
signals when the entire configuration process completes by
releasing the DONE pin, allowing it to go High.
DS312 (v4.2) December 14, 2018
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Product Specification
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XILINX [ XILINX, INC ]
