R
Functional Description
3.3V Supply
SPI Flash cannot be selected
SPI Flash PROM
minimum voltage
SPI Flash available for
read operations
SPI Flash
PROM CS
SPI Flash PROM must
be ready for FPGA
access, otherwise delay
FPGA configuration
FPGA VCCO_2 minimum
Power On Reset Voltage
(t
)
delay
VSL
(VCCO2T
)
FPGA accesses
SPI Flash PROM
FPGA initializes configuration
(V
, V
CCINT CCAUX
already valid)
memory (TPOR
)
Time
DS312-2_50b_110206
Figure 55: SPI Flash PROM/FPGA Power-On Timing if 3.3V Supply is Last in Power-On Sequence
If the FPGA's V
and V
supplies are already
CCAUX
RISC processor core integrated in the Spartan-3E FPGA.
CCINT
valid, then the FPGA waits for VCCO_2 to reach its mini-
mum threshold voltage before starting configuration. This
See Using the SPI Flash Interface after Configuration.
Table 57: Number of Bits to Program a Spartan-3E
FPGA and Smallest SPI Flash PROM
threshold voltage is labeled as V
in Table 74 of Mod-
CCO2T
ule 3 and ranges from approximately 0.4V to 1.0V, substan-
tially lower than the SPI Flash PROM's minimum voltage.
Once all three FPGA supplies reach their respective Power
On Reset (POR) thresholds, the FPGA starts the configura-
tion process and begins initializing its internal configuration
Number of
Configuration
Bits
Smallest Usable
SPI Flash PROM
Device
XC3S100E
XC3S250E
XC3S500E
XC3S1200E
XC3S1600E
581,344
1,353,728
2,270,208
3,841,184
5,969,696
1 Mbit
2 Mbit
4 Mbit
4 Mbit
8 Mbit
memory. Initialization requires approximately 1 ms (T
,
POR
minimum in Table 111 of Module 3, after which the FPGA
de-asserts INIT_B, selects the SPI Flash PROM, and starts
sending the appropriate read command. The SPI Flash
PROM must be ready for read operations at this time. Spar-
tan-3E FPGAs issue the read command just once. If the SPI
Flash is not ready, then the FPGA does not properly config-
ure.
CCLK Frequency
If the 3.3V supply is last in the sequence and does not ramp
fast enough, or if the SPI Flash PROM cannot be ready
when required by the FPGA, delay the FPGA configuration
process by holding either the FPGA's PROG_B input or
INIT_B input Low, as highlighted in Figure 54. Release the
FPGA when the SPI Flash PROM is ready. For example, a
simple R-C delay circuit attached to the INIT_B pin forces
the FPGA to wait for a preselected amount of time. Alter-
nately, a Power Good signal from the 3.3V supply or a sys-
tem reset signal accomplishes the same purpose. Use an
open-drain or open-collector output when driving PROG_B
or INIT_B.
In SPI Flash mode, the FPGA’s internal oscillator generates
the configuration clock frequency. The FPGA provides this
clock on its CCLK output pin, driving the PROM’s clock input
pin. The FPGA starts configuration at its lowest frequency
and increases its frequency for the remainder of the config-
uration process if so specified in the configuration bitstream.
The maximum frequency is specified using the ConfigRate
bitstream generator option. The maximum frequency sup-
ported by the FPGA configuration logic depends on the tim-
ing for the SPI Flash device. Without examining the timing
for a specific SPI Flash PROM, use ConfigRate = 12 or
lower. SPI Flash PROMs that support the FAST READ com-
mand support higher data rates. Some such PROMs sup-
port up to ConfigRate = 25 and beyond but require careful
data sheet analysis. See Serial Peripheral Interface (SPI)
Configuration Timing for more detailed timing analysis.
SPI Flash PROM Density Requirements
Table 57 shows the smallest usable SPI Flash PROM to
program a single Spartan-3E FPGA. Commercially avail-
able SPI Flash PROMs range in density from 1 Mbit to 128
Mbits. A multiple-FPGA daisy-chained application requires
a SPI Flash PROM large enough to contain the sum of the
FPGA file sizes. An application can also use a larger-den-
sity SPI Flash PROM to hold additional data beyond just
FPGA configuration data. For example, the SPI Flash
PROM can also store application code for a MicroBlaze™
Using the SPI Flash Interface after Configuration
After the FPGA successfully completes configuration, all of
the pins connected to the SPI Flash PROM are available as
user-I/O pins.
82
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DS312-2 (v3.8) August 26, 2009
Product Specification
XILINX [ XILINX, INC ]
