Posts: 4
Joined: Wed Dec 26, 2018 4:53 pm

About the jtag

Wed Dec 26, 2018 5:00 pm

Is there a difference between the JTAG pins on the Compute Module (labeled VC_TRST_N/TDI/TDO/TMS/TCK) and the alternative GPIO functions on the data sheet for the Compute Module? I know this is vague, but the CM3 datasheet has a table of alternative functions for the GPIO pins, they have the naming conventions ARM_TDI/TDO/RTCK/TMS/TCK. Basically, what is the difference between JTAG pins labeled VC_ and ARM_

John Westlake
Posts: 107
Joined: Thu Nov 09, 2017 4:34 am

Re: About the jtag

Fri Dec 28, 2018 8:39 am

OK a wild guess here, but the way I understand the CM3 SoC design is that basically 90% of the chip die is the GPU (the Video core - VC_JTAG) with quad ARM processors "hung off the side" - ARM_ JTAG.

Presuming this genesis is correct then I can imagine that the JTAG's have been kept separated between the ARM and Video core sections of the chip design... I can imagine that the Soc came about via "organic development" - in the early days a VideoCore processor existed in the deep dark bowels of the Broadcom Corporation then someone came up with the bright idea of adding ARM cores to the VC to make a more useful standalone Soc.

Only a guess...

Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 1581
Joined: Sat Sep 10, 2011 11:43 am

Re: About the jtag

Fri Dec 28, 2018 9:37 am

That is correct... The JTAG on the PCB is the videocore one, the arm JTAG can be enabled on the gpio connector

Turns out Eben talked our management at the time to add the arms to the device just for future interesting opportunities... Which of course turned out to be raspberry pi

Gordon Hollingworth PhD
Raspberry Pi - Director of Software Engineering

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