UM1250
Appendix A JTAG control
A.1
JTAG control
Introduction to JTAG
JTAG is an acronym for the Joint Test Access Group which specified the IEEE 1149.1 Test
Access Port and Boundary-Scan Architecture. The ST40 User Debug Interface (UDI) conforms to the IEEE 1149.1 standard and uses all five signals defined in the standard
(TCK, TMS, TDI, TDO, and notTRST) plus notASEBRK/BRKACK which is an additional signal specific to the ST40 UDI debug emulation function.
On some ST40 system-on-chip (SoC) devices, the TCK signal for the ST40 UDI is provided separately to the TCK signal used by the Test Access Port (TAP) for the SoC. In these instances, the TCK signal for the ST40 UDI is normally referred to as DCK however, in this appendix, the signal name TCK is used to refer to the ST40 UDI signal.
A.2 The command
The jtag command is enabled by issuing the GDB user command enable_jtag (defined in the jtag.cmd GDB command script file).
jtag commands
This command enables access to the ST40 UDI for manual control of the standard IEEE
1149.1 TAP and notASEBRK signals plus the notRESET and TRIG_OUT signals of the target platform. Also, when used with a target which is connected through an
ST MultiCore/MUX device, the jtag command provides control of the signal to switch between the TAP of the target and the TAP of the ST MultiCore/MUX.
The subcommands supported by the jtag command are listed in
Table 103
.
Table 103.
jtag subcommands
Command
asebrk=signal help mode=mode nrst=signal ntrst=signal stmmx=signal tck=signal tdi=variable
Description
Specify the notASEBRK signal sequence.
Display help for the jtag command.
Set the mode of the TAP, where mode is one of the modes listed in
Table 104: JTAG modes
.
Specify the notRESET signal sequence.
Specify the notTRST signal sequence.
Specify the signal sequence to switch between the TAP of the
ST MultiCore/MUX and the TAP of the target.
Specify the TCK signal sequence. Only valid when mode is set to manual otherwise the signal is ignored.
Return a numerical representation of the TDI signal sequence into a GDB convenience variable. This is the signal received by the STMC’s TDI signal from the TDO of the target TAP.
See
Section A.2.3: TDI signal capture on page 76
for details on the representation of the TDI signal in variable.
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