9.3.1. ICs containing multiple devices

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


There is no support in RealView ICE 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. 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 (as shown in Figure 9.7).

Figure 9.7. TAP Controllers serially chained within an ASIC

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.

Copyright © 2002, 2004-2008 ARM Limited. All rights reserved.ARM DUI 0155J