3 Multiple Devices in JTAG Chain

The JTAG standard allows for a single JTAG debug probe to make a daisy-chain connection with multiple devices. In practice, various constraints will limit the number of target devices that can be connected in a chain. The XDS-class debuggers all have a limited number of instruction register (IR) bits that they can circulate before they expect to see the bits returned. Many older C2000 devices require 38 IR bits to be circulated per device, while all newer devices only require six IR bits to be circulated because of their ICEPick JTAG route controller. At the time of writing this application note, the following devices have ICEPicks: F2807x, F28M3x, F2837xD, F2837xS, F28004x, F2838x, F28002x, F28003x, F280013x. If you would like to check if your device has an ICEPick, you can use Code Composer Studio to look at the advanced tab of your target configuration, as shown in Figure 3-1.

For devices without an ICEPick, this means that the XDS100, XDS110, and XDS200 debug probes can only reliably connect up to two devices in a chain. However, you could connect to up to 12 devices with an ICEPick in a JTAG chain. The XDS560 can reliably connect to three devices without an ICEPick, and up to 18 devices with an ICEPick. To learn more about the ICEPick, see the TI ICEPick Module Type C Reference Guide.