ARM DS-5 Using the Debug Hardware Configuration Utilities

Configuring CoreSight systems6.17 Configuring CoreSight systems with multiple devices per JTAG-AP multiplexorportThe debug hardware unit does not support auto-detection of devices behind a JTAG-AP.Therefore, you must manually specify the JTAG scan chain attributes so that the unit can put theother devices into BYPASS.To debug CoreSight systems that have processors connected to the Debug Access Port (DAP)through JTAG-AP, debug hardware must know the pre-bits and post-bits for JTAG operations.The following figure shows a hypothetical scan chain that could be connected to a JTAG-AP.PostScan-IR=5 (Sum of IR Lengths)PostScan-DR=2 (Device Count)PreScan-IR=23 (Sum of IR Lengths)PreScan-DR=3 (Device Count)DebugPortDebug Access Port (DAP)DAPInternalBusTCK0RTCK0PostScanCSTDICSTDOIR=2 IR=3 CorePreScanIR=5 IR=7 IR=11JTAG-APMUXCSTDITCKnRTCKnCSTDOFigure 6-15 Scan chain connected to a JTAG-APMultiple devices on the scan chain are connected in series, with data flowing serially from TDIto TDO. This means that debugging a given target in the chain requires that certain pre-scan andpost-scan bits are used to ensure that the other devices are not affected by the data directed atthe target device, and that the data is positioned correctly in the serial scan for the target device.To debug this system, you must set the following four configuration items:• Pre-scan IR bits for Devices after the core on the JTAG-AP scanchain(JTAG_AP_IR_PRE_BITS)This is the total length of the JTAG instruction registers (IRs) for devices appearingbetween the processor being configured and the CSTDO input on the JTAG-AP port. Inthe figure above, the three devices that appear between the target processor and theCSTDO input on the JTAG-AP port have IR lengths 5, 7 and 11, respectively. Therefore,you must set this value to 23.• Post-scan IR bits for Devices before the core on the JTAG-AP scanchain(JTAG_AP_IR_POST_BITS)This is the total length of the JTAG IRs for devices appearing between the CSTDI outputon the JTAG-AP port and the processor being configured. In the figure above, the twodevices that appear between the CSTDI output on the JTAG-AP port and the processorbeing configured have IR lengths 2 and 3, respectively. Therefore, you must set this valueto 5.• Pre-scan DR bits for Devices after the core on the JTAG-AP scanchain(JTAG_AP_DR_PRE_BITS)This is the total number of devices appearing between the processor being configured andthe CSTDO input on the JTAG-AP port. In the figure above, there are three devices thatappear between the processor being configured and the CSTDO input on the JTAG-APport. Therefore, you must set this value to 3.• Post-scan DR bits for Devices before the core on the JTAG-AP scanchain(JTAG_AP_DR_POST_BITS)ARM DUI 0498F Copyright © 2010-2012 ARM. All rights reserved. 6-26ID021112Non-Confidential