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RISC-V External Debug Support , task_group_vote-4-g4e0bb0f 2019/03/25

8 JTAG Debug Transport Module

This Debug Transport Module is based around a normal JTAG Test Access Port (TAP). The JTAG TAP allows access to arbitrary JTAG registers by first selecting one using the JTAG instruction register (IR), and then accessing it through the JTAG data register (DR).

8.1 JTAG Background

JTAG refers to IEEE Std 1149.1-2013. It is a standard that defines test logic that can be included in an integrated circuit to test the interconnections between integrated circuits, test the integrated circuit itself, and observe or modify circuit activity during the component’s normal operation. This specification uses the latter functionality. The JTAG standard defines a Test Access Port (TAP) that can be used to read and write a few custom registers, which can be used to communicate with debug hardware in a component.

8.2 JTAG DTM Registers

JTAG TAPs used as a DTM must have an IR of at least 5 bits. When the TAP is reset, IR must default to 00001, selecting the IDCODE instruction. A full list of JTAG registers along with their encoding is in Table [table:jtag_registers]. If the IR actually has more than 5 bits, then the encodings in Table [table:jtag_registers] should be extended with 0’s in their most significant bits. The only regular JTAG registers a debugger might use are BYPASS and IDCODE, but this specification leaves IR space for many other standard JTAG instructions. Unimplemented instructions must select the BYPASS register.