JTAG Chains

The JTAG chain is the way in which XJTAG moves data to and from the circuit being tested. It is imperative to get the best performance from the chain and also to have good signal integrity in order to avoid bit-errors in the JTAG chain data.

A JTAG chain consists of 4 or 5 signals. The data for the chain starts at a TDI pin on the JTAG controller (XJLink) which is connected to the TDI pin of the first device in the JTAG chain.

The data path emerges from each device in the chain at the TDO pin, which is then connected to the TDI pin of the following device. The TDO pin of the final device is connected back to the TDO pin of the JTAG controller to complete the data path.

The other JTAG signals (TCK, TMS and optionally nTRST) are connected in parallel to all of the devices in the JTAG chain.

A PCB may have multiple separate JTAG chains (TDI/TDO pairs). These chains may each have their own TCK, TMS and nTRST pins or they may share them with other chains.

More information about how XJTAG represents and supports JTAG chains is found here.