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Is the DAP which enable to read CPU registers during JTAG debug session ?

Question asked by fmajeric on Apr 13, 2016
Latest reply on Apr 19, 2016 by Artur Petukhov

Hi all,


I need some precision about JTAG scan chain.

I'm misunderstanding what is the device which halt/run the CPU during a JTAG debug session.

For example when I connect a JTAG debug tool to the JTAG pins, which device on the JTAG daisy-chain enable to read the CPU registers or halt it ?


In the document IMX6SDLRM, Rev. 2, 04/2014:


It is explaned (p. 364/5829):

"The SJC module is the bridge between external development and test instrumentation and

the internal JTAG-accessible debug and test resources."


An overview of the JTAG architecture  is given(p. 4914/5829)



On the JTAG daisy-chain there are in particular 2 devices :

- The OnCE (On chip Emulator) of the SDMA

- The DAP (Debug Acces Port)


It is explained (p.366/5829) :

"The SDMA debug features are primarily defined by the OnCE portion of its design."


Please, I need some precision:

[Q1] Does the OnCE portion is used for debugging only the SDMA 32-bit RISC processor ?

[Q2] If I want to halt Cortex-A9 core, read its registers this is the DAP module which work on the JTAG chain ?


Thank you for any help.