Difference between revisions of "EJTAG"

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[[Image:Te_jtag_cable.png|thumb|900px|center|Total Embedded buffered cable]]
[[Image:Te_jtag_cable.png|thumb|900px|center|Total Embedded buffered cable]]
[[Image:Wiggler.png|thumb|600px|center|"wiggler" from OpenWRT]]
[[Image:Wiggler.png|thumb|600px|center|"wiggler" clone from OpenWRT]]
[[Image:JTAGunbuffered.png|thumb|700px|center|unbuffered cable from OpenWRT; used by de-brick utility]]
[[Image:JTAGunbuffered.png|thumb|700px|center|unbuffered cable from OpenWRT; used by de-brick utility]]
Return to [[Summer 2007]]
Return to [[Summer 2007]]

Revision as of 16:24, 15 July 2007

EJTAG is a MIPS-specific extension of IEEE 1149.1, the Joint Test Action Group. Allows interfacing with additional logic in SoC

  • direct control of processor for step-by-step debugging
  • access to busses and registers
    • aids in debugging
    • possible usage as additional peripheral data bus
    • direct writing to flash for firmware updates (and de-bricking)


Attempting to use GNU debugger: http://www.gnu.org/software/gdb. GDP uses its own Remote Serial Protocol (RDB) to communicate to remote targets. This protocol could be used to communicate with the XINU backends through the current serial connection. Although, this would require additions]] to XINU: communication with the GDB host; altering of exception handler to allow GDB to take control of target processor.

The use of the EJTAG port on the WRT54-series routers gives the user hardware control of the processor, avoiding the need for strategically placed breakpoints and XINU interrupt subsystem modification. Additionally, requests by the debugger for specicfic data can be aquired directly from registers. The trick to this operation is software that can interpret commands from RDB into EJTAG signals to be sent through the host parallel port, and vice-versa. An implementation of this interpreter can be found at http://www.totalembedded.com/open_source/jtag/mips32_ejtag.php.

Note that the unbuffered cable at the bottom of this page is only proven functional in writing to the Test Access Port. It may not read data back from the target device.

Total Embedded buffered cable
"wiggler" clone from OpenWRT
unbuffered cable from OpenWRT; used by de-brick utility

Return to Summer 2007