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Debug Transport Module (DTM) (non-ISA extension)

Debug Transport Modules provide access to the DM over one or more transports (e.g. JTAG or USB).

There may be multiple DTMs in a single hardware platform. Ideally every component that communicates with the outside world includes a DTM, allowing a hardware platform to be debugged through every transport it supports. For instance a USB component could include a DTM. This would trivially allow any hardware platform to be debugged over USB. All that is required is that the USB module already in use also has access to the Debug Module Interface.

Using multiple DTMs at the same time is not supported. It is left to the user to ensure this does not happen.

This specification defines a JTAG DTM in JTAG Debug Transport Module. Additional DTMs may be added in future versions of this specification.

An implementation can be compatible with this specification without implementing any of this section. In that case it must be advertised as conforming to "RISC-V Debug Specification, with custom DTM." If the JTAG DTM described here is implemented, it must be advertised as conforming to the "RISC-V Debug Specification, with JTAG DTM.""

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).

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.

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 [tab:jtag_registers]. If the IR actually has more than 5 bits, then the encodings in [tab:jtag_registers] should be extended with 0’s in their most significant bits, except for the 0x1f encoding of BYPASS, which must be extended with 1’s in the 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.

JTAG DTM TAP Registers

build/jtag_registers.adoc

JTAG Connector

To make it easy to acquire debug hardware, this spec recommends a connector that is compatible with the MIPI-10 .05 inch connector specification, as described in MIPI Debug & Trace Connector Recommendations, Version 1.20, 2 July 2021.

The connector has .05 inch spacing, gold-plated male header with .016 inch thick hardened copper or beryllium bronze square posts (SAMTEC FTSH or equivalent). Female connectors are compatible \(20\mu m\) gold connectors.

Viewing the male header from above (the pins pointing at your eye), a target’s connector looks as it does in MIPI 10-pin JTAG + nRESET Connector Diagram. The function of each pin is described in JTAG Connector Pin Functions.

Table 1. MIPI 10-pin JTAG + nRESET Connector Diagram

VREF DEBUG

1

2

TMS

GND

3

4

TCK

GND

5

6

TDO

GND or KEY

7

8

TDI

GND

9

10

nRESET

If a hardware platform requires nTRST then it is permissible to reuse the nRESET pin as the nTRST signal, resulting in a MIPI 10-pin JTAG
nTRST connector.

Alternate JTAG Connector

The MIPI-10 connector should provide plenty of signals for all modern hardware. If a design does need legacy JTAG signals, then the MIPI-20 connector should be used. Pins whose functionality isn’t needed may be left unconnected.

Its physical connector is virtually identical to MIPI-10, except that it’s twice as long, supporting twice as many pins. Its pinout is shown in MIPI 20-pin JTAG Connector Diagram. The function of each pin is described in JTAG Connector Pin Functions.

Table 2. JTAG Connector Pin Functions

Essential

GND

Connected to ground.

TCK

JTAG TCK signal, driven by the debug adapter.

TDI

JTAG TDI signal, driven by the debug adapter.

TDO

JTAG TDO signal, driven by the target.

TMS

JTAG TMS signal, driven by the debug adapter.

VREF DEBUG

Reference voltage for logic high.

Recommended

nRESET

Open drain active low reset signal, usually driven by the debug adapter. The signal may be used bi-directional to drive or sense the target reset signal.
Asserting reset should reset any RISC-V cores as well as any other peripherals on the PCB. It should not reset the debug logic. This pin is optional but strongly encouraged.
nRESET should never be connected to the TAP reset, otherwise the debugger might not be able to debug through a reset to discover the cause of a crash or to maintain execution control after the reset.

KEY

This pin may be cut on the male and plugged on the female header to ensure the header is always plugged in correctly. It is, however, recommended to use this pin as an additional ground, to allow for fastest TCK speeds. A shrouded connector should be used to prevent the cable from being plugged in incorrectly.

Advanced

EXT

Reserved for custom use. Could be an input or an output.

TRIGIN

Not used by this specification, to be driven by debug adapter. (Can be used for extended functions like UART or boot mode selection by some debug adapters).

TRIGOUT

Not used by this specification, driven by the target.

Specialized

nTRST

Test reset, driven by the debug adapter. Asserting nTRST initializes the JTAG DTM asynchronously. It is used in systems where the JTAG DTM is not ready to be used after a normal power up. This signal is sometimes called TRST*.

Legacy

RTCK

Return test clock, driven by the target. A target may relay the TCK signal here once it has processed it, allowing a debugger to adjust its TCK frequency in response.
This signal should only be used to support legacy components that rely on this functionality.

nTRST_PD

Test reset pull-down, driven by the debug adapter. Same function as nTRST, but with pull-down resistor on target.
This signal should only be used to support legacy components that rely on this functionality.

Table 3. MIPI 20-pin JTAG Connector Diagram

VREF DEBUG

1

2

TMS

GND

3

4

TCK

GND

5

6

TDO

GND or KEY

7

8

TDI

GND

9

10

nRESET

GND

11

12

GND or RTCK

GND

13

14

NC or nTRST_PD

GND

15

16

nTRST or NC

GND

17

18

TRIGIN or NC

GND

19

20

TRIGOUT or GND

cJTAG

This spec does not have specific recommendations on how to use the cJTAG protocol.

When implementing cJTAG access to a JTAG DTM, the MIPI 10-pin Narrow JTAG connector should be used. Pins whose functionality isn’t needed may be left unconnected.

Viewing the male header from above (the pins pointing at your eye), a target’s connector looks as it does in MIPI 10-pin Narrow JTAG Connector Diagram.

Table 4. MIPI 10-pin Narrow JTAG Connector Diagram

VREF DEBUG

1

2

TMSC

GND

3

4

TCKC

GND

5

6

EXT or NC

GND or KEY

7

8

NC or nTRST_PD

GND

9

10

nRESET