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Describe how the UDI and UDS are actually stored in the FPGA, how they are accessed, and how they are initialled by the patch_uds_udi.py script. Co-authored-by: Joachim Strömbergson <joachim@assured.se> |
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uds
Unique Device Secret core
Introduction
This core store and protect the Unique Device Secret (UDS) asset. The UDS can be accessed as eight separate 32-bit words. The words can only be accessed as long as the fw_app_mode input is low, implying that the CPU is executing the FW.
The UDS words can be accessed in any order, but a given word can only be accessed once between reset cycles. This read once functionality is implemented with a companion read bit for each word. The read bit is set when the word is first accessed. The read bit controls if the real UDS word is returned or not.
This means that the even if the chip select (cs) control input is forced high, the content will become all zero when the read bit has been set after one cycle.
API
There are eight addresses in the API. These are defined by the two values ADDR_UDS_FIRST and ADDR_UDS_LAST:
ADDR_UDS_FIRST: 0x10
ADDR_UDS_LAST: 0x17
These addresses are read only and read once between reset.
Any access to another address will be ignored by the core.
Implementation
These read-only registers provide read access to the 256-bit UDS.
The eight UDS words are stored using 32 named SB_LUT4 FPGA multiplexer (MUX) instances, identified in the source code as "uds_rom_idx". One instance for each bit in the core read_data output bus.
During build of the FPGA design, the UDS is set to a known bit pattern, which means that the SB_LUT4 instantiations are initialized to a fixed bit pattern.
The tool 'patch_uds_udi.py' is used to replace the fixed bit pattern with a unique bit pattern before generating the per device unique FPGA bitstream. This allows us to generate these device unique FPGA bitstreams without haveing to do a full FPGA build.
Each SB_LUT4 MUX is able to store 16 bits of data, in total 512 bits. But since the UDS is 256 bits, we only use the eight LSBs in each MUX.
The eighth MSBs in each MUX will be initialized to zero. The read access bit (se description above) for a given word is used as the highest address bit to the MUXes. This forces any subsequent accesses to a UDS word to read from the MUX MSBs, not the LSBs where the UDS is stored.