User guide

Spartan-6 FPGA Configuration User Guide www.xilinx.com 111
UG380 (v2.7) October 29, 2014
Spartan-6 FPGA Unique Device Identifier (Device DNA)
Identifier Memory Specifications
The unique FPGA identifier value is retained for a minimum of ten years of continuous
usage under worst-case recommended operating conditions. The identifier can be read,
using the READ operation defined in Table 5-51, a minimum of 30 million cycles, which
roughly correlates to one read operation every 11 seconds for the operating lifetime of the
Spartan-6 FPGA.
Extending Identifier Length
As shown in Figure 5-15, most applications that use the DNA_PORT primitive tie the DIN
data input to a static value.
As shown in Figure 5-16, the length of the identifier can be extended by feeding the DOUT
serial output port back into the DIN serial input port. This way, the identifier can be
extended to any possible length. However, there are still only 55 unique bits, with a 57-bit
repeating pattern. A buffer is included in Figure 5-16 to demonstrate a user inserting logic
for the user’s DNA logic extension or delay for the loopback to meet hold time
requirements.
X-Ref Target - Figure 5-15
Figure 5-15: Shift in Constant
X-Ref Target - Figure 5-16
Figure 5-16: Circular Shift
DIN DOUT
DNA_PORT
READ
S HIFT
CLK
UG380_c5_15_121112
DIN DOUT
DNA_PORT
READ
S HIFT
CLK
UG380_c5_16_021010