[[sec:jtagdtm]]
== 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
Table #dtmTable:jtagregisters[[dtmTable:jtagregisters]]. If the IR
actually has more than 5 bits, then the encodings in
Table #dtmTable:jtagregisters[[dtmTable:jtagregisters]] 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.

include::build/jtag_registers.adoc[]

=== JTAG Connector

==== Recommended 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 latexmath:[$20\mu m$]
gold connectors.

Viewing the male header from above (the pins pointing at your eye), a
target’s connector looks as it does in Table #tab:mipiten[1]. The
function of each pin is described in Table #tab:pinout[[tab:pinout]].

[[tab:mipiten]]
.MIPI 10-pin JTAG + nRESET Connector Diagram
[cols=">,^,^,<",options="header",]
|===
|VREF DEBUG |1 |2 |TMS
|GND |3 |4 |TCK
|GND |5 |6 |TDO
|GND or KEY |7 |8 |TDI
|GND |9 |10 |nRESET
|===

|l|c|L| 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. +

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 Table #tab:mipitwenty[2]. The function of each pin is described in
Table #tab:pinout[[tab:pinout]].

[[tab:mipitwenty]]
.MIPI 20-pin JTAG Connector Diagram
[cols=">,^,^,<",options="header",]
|===
|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 Table #tab:mipicjtag[3].

[[tab:mipicjtag]]
.MIPI 10-pin Narrow JTAG Connector Diagram
[cols=">,^,^,<",options="header",]
|===
|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
|===
