FPGA JTAG Diagnostics: DNA, IDCODE and Chain Status Explained
JTAG diagnostics provide a direct view of the FPGA device without using the DMA data path. A useful report separates identity fields, configuration registers and transport information so each result can be checked independently.
Identity fields
The FPGA part, IDCODE, fuse DNA and device DNA answer different questions. The IDCODE identifies the silicon family. Fuse DNA is the permanent device identifier, while device DNA is the value exposed by the configured design. A tool should label them separately instead of presenting one value as the other.
Transport and chain fields
Chain index and instruction-register length describe the device position and JTAG protocol. The backend and live transport identify how the tool reached the cable. These fields are useful when several interfaces or drivers are installed on the same PC.
When a read fails
- Check that the cable is connected to the JTAG port.
- Confirm that the FTDI or CH347 interface is visible locally.
- Close another program that may hold the interface.
- Retry the identity scan before running deeper probes.
Reading identity is non-destructive. Flashing and probing storage are separate operations and should never be started by an automatic diagnostic scan.