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ICs containing multiple devices

If your ASIC contains multiple devices that have a JTAG Test Access Port (TAP) controller, you must serially chain them so that can communicate with all of them simultaneously. The chaining can either be within the ASIC, or externally.


There is no support in DSTREAM for multiplexing TCK, TMS, TDI, TDO, and RTCK between a number of different processors.

TAP controllers serially chained within the ASIC

The JTAG standard originally described serially chaining multiple devices on a PCB. This concept can be extended to serially chaining multiple TAP controllers within an ASIC, as shown in the following figure:

Figure 6. TAP Controllers serially chained within an ASIC

Figure 6. TAP Controllers serially chained
within an ASIC

This configuration does not increase the package pin count. It does increase JTAG propagation delays, but this impact can be small if you put unaddressed TAP controllers into bypass mode.

TAP controllers serially chained externally

You can use separate pins on the ASIC for each JTAG port, and serially chain them externally (for example on the PCB). This configuration can simplify device testing, and gives the greatest flexibility on the PCB. However, this is at the cost of many pins on the device package.