User guide

142 www.xilinx.com Spartan-6 FPGA Configuration User Guide
UG380 (v2.7) October 29, 2014
Chapter 8: Readback CRC
Because JTAG has the highest priority in the configuration mode, it takes over the
configuration bus whenever it needs to. The JTAG Instruction Register must not be parked
at the CFG_IN, CFG_OUT, or ISC_ENABLE instructions.
Post_CRC Constraints
There are several Spartan-6 FPGA constraints used for signaling SEU events. All
constraints have the same propagation rule. They are placed as an attribute on the
CONFIG block, then propagated to the physical design object.
POST_CRC
POST_CRC enables the readback CRC feature in the FPGA. It uses the
POST_CRC_INTERNAL primitive's CRCERROR pin for signaling SEU events. By default,
INIT is reserved as an SEU CRC error indicator but can be disabled by setting the
POST_CRC_INIT_FLAG constraint.
The POST_CRC constraint is the best way to convey this information. It attaches to the
CONFIG constraint. POST_CRC can be used by PAR and BitGen to reserve the INIT pin by
not programming the IOB to drive the INIT pin.
POST_CRC can take two values:
•ENABLE
SEU detection is enabled.
• DISABLE
SEU detection is disabled.
POST_CRC_INIT_FLAG
POST_CRC_INIT_FLAG determines whether the Spartan-6 FPGA INIT_B pin is a source
of the SEU error signal. Whether or not the INIT_B pin is used as the error signal, it cannot
be used as user I/O when POST_CRC is enabled.
During configuration, the INIT pin operates normally. After configuration, if SEU analysis
is enabled and INIT is reserved, the INIT pin (default) serves as an SEU status pin. An SEU
is detected when a comparison of the real-time computed CRC differs from the pre-
computed CRC, the CRCERROR pin is driven High, and the INIT pin is driven Low.
POST_CRC_INIT_FLAG is used to disable the INIT_B pin from acting as the readback
CRC error status output pin. The error condition is still available from the
POST_CRC_INTERNAL site.
POST_CRC_INIT_FLAG can take two values:
•ENABLE
The INIT_B pin is used as an indicator of the SEU error signal (default).
• DISABLE
INIT_B is not used as an indicator of the SEU error signal. The error condition is only
available via POST_CRC_INTERNAL primitive.
POST_CRC_SOURCE
POST_CRC_SOURCE determines the source of the golden CRC value.
POST_CRC_SOURCE can take two values: