Virtex-5 FPGA User Guide www.xilinx.com 159
UG190 (v5.0) June 19, 2009
Built-in Error Correction
ECC Modes Overview
In the standard ECC mode (EN_ECC_READ = TRUE and EN_ECC_WRITE = TRUE), both
encoder and decoder are enabled. During write, 64-bit data and 8-bit ECC generated parity
are stored in the array. The external parity bits are ignored. During read, the 72-bit decoded
data and parity are read out.
The encoder and decoder can be accessed separately for external use in RAMB36SDP. To
use the encoder by itself, send the data in through the DI port and sample the ECCPARITY
output port. To use the decoder by itself, disable the encoder, write the data into the block
RAM and read the corrected data and status bits out of the block RAM. See “Block RAM
(RAMB36SDP) Attributes.”
To use the decoder in ECC decode-only mode, set EN_ECC_WRITE = FALSE and
EN_ECC_READ = TRUE.
The encoder can be used in two ways:
• To use the encoder in standard ECC mode, set (EN_ECC_WRITE = TRUE and
EN_ECC_READ = TRUE). In this mode, the DI setup time is smaller but the clock-to-
out for ECCPARITY is larger.
• To use the encoder-only mode, set (EN_ECC_WRITE = TRUE and
EN_ECC_READ = FALSE). In this mode, the DI setup time is larger but the clock-to-
out for ECCPARITY is smaller.
The functionality of the block RAM when using the ECC mode is described as follows:
• The block RAM ports still have independent address, clocks, and enable inputs, but
one port is a dedicated write port, and the other is a dedicated read port (simple dual-
port).
• DO represents the read data after correction.
• DO stays valid until the next active read operation.
• Simultaneous decoding and encoding, even with asynchronous clocks, is allowed, but
requires careful clock timing if read and write addresses are identical.
• The NO_CHANGE or WRITE_FIRST modes of the normal block RAM operation are
not applicable to the ECC configuration.
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