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Scholar ronnywebers
Scholar
385 Views
Registered: ‎10-10-2014

SmartLynq PGND use case and hardware interfacing

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I'm designing a Zynq UltraScale+ board, and want to put a JTAG connector on my board.

UG1258 says about PGND:

  • direction : out
  • JTAG Pseudo Ground. Use of this pin is optional. PGND is LOW during JTAG operation.

I didn't find an explanation/use case of PGND, and secondly, it's unclear what I should do with this pin on my board : leave it unconnected, connect it to GND, ...? 

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Scholar ronnywebers
Scholar
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Registered: ‎10-10-2014

回复: SmartLynq PGND use case and hardware interfacing

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thanks @iguo for that AR#. However it still doesn't answer if there would be a use case for PGND in combination with Zynq (UltraScale+) devices. But I think I found at least one (that I was actually looking for):

 I collected this info:

in DS5893 (v1.5.1, august 6, 2018), I found a clear use case with 'regular FPGAs' (non-Zynq) : to have muxes automatically switch between the primary config device and a JTAG Plaform Cable USB II:

pgnd.png

the explanation can be found here:

PGND vivado.png

So ... looks like the doc was updated with the info from AR# 62446, and thus indeed Vivado supports control of the PGND pin of a JTAG Plaform Cable USB II

Now back to the SmartLynq and PGND

the only thing I found in UG1258 (v1.2) of SmartLynq is :

PGND SmartLynq.png

So SmartLynq / Vivado do handle the PGND pin too. 

Q1: But .. it looks like this PGND is of little value in Zynq (UltraScale+) designs, as the device is not configured over these JTAG pins (but from QSPI, eMMC, ...), so no dynamic switching is needed as with 'regular FPGA's ?

Q2: the only use case with Zynq (UltraScale+) that  I can see would be an on-board microcontroller supporting Xilinx Virtual Cable (which is something we actually are implementing on our board in a microcontroller ...). In that case the microcontroller could be 'aware' of a SmartLynq cable being plugged in and actively driven by Vivado, by checking the status of PGND, is that correct? Any other use case? 

** kudo if the answer was helpful. Accept as solution if your question is answered **
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Xilinx Employee
Xilinx Employee
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Registered: ‎08-10-2008

回复: SmartLynq PGND use case and hardware interfacing

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https://www.xilinx.com/support/answers/62446.html some more info about this pin.

Connect it to ground on your board.

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Scholar ronnywebers
Scholar
348 Views
Registered: ‎10-10-2014

回复: SmartLynq PGND use case and hardware interfacing

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thanks @iguo for that AR#. However it still doesn't answer if there would be a use case for PGND in combination with Zynq (UltraScale+) devices. But I think I found at least one (that I was actually looking for):

 I collected this info:

in DS5893 (v1.5.1, august 6, 2018), I found a clear use case with 'regular FPGAs' (non-Zynq) : to have muxes automatically switch between the primary config device and a JTAG Plaform Cable USB II:

pgnd.png

the explanation can be found here:

PGND vivado.png

So ... looks like the doc was updated with the info from AR# 62446, and thus indeed Vivado supports control of the PGND pin of a JTAG Plaform Cable USB II

Now back to the SmartLynq and PGND

the only thing I found in UG1258 (v1.2) of SmartLynq is :

PGND SmartLynq.png

So SmartLynq / Vivado do handle the PGND pin too. 

Q1: But .. it looks like this PGND is of little value in Zynq (UltraScale+) designs, as the device is not configured over these JTAG pins (but from QSPI, eMMC, ...), so no dynamic switching is needed as with 'regular FPGA's ?

Q2: the only use case with Zynq (UltraScale+) that  I can see would be an on-board microcontroller supporting Xilinx Virtual Cable (which is something we actually are implementing on our board in a microcontroller ...). In that case the microcontroller could be 'aware' of a SmartLynq cable being plugged in and actively driven by Vivado, by checking the status of PGND, is that correct? Any other use case? 

** kudo if the answer was helpful. Accept as solution if your question is answered **
Tags (2)
Xilinx Employee
Xilinx Employee
335 Views
Registered: ‎08-10-2008

回复: SmartLynq PGND use case and hardware interfacing

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Good finding. I usually take it as a useless, or not that useful pin. 

Just remember it's behavior and you can make use of it. My opinion, not necessary to dig into this, and do not design heavily rely on this. As you see, it drives by the tool, not sure if it's very stable as few people uses this feature before.

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284 Views
Registered: ‎01-08-2012

Re: SmartLynq PGND use case and hardware interfacing

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On my Zynq-7 boards, I tie this pin (via some circuitry) into the boot mode selection.

For most boards, I can select three boot sources without needing to change any jumpers.  (As an aside, all my boards are completely free of jumpers or dipswitches, and have been for many years (after I had a bad experience with shipping boards with jumpers in wrong location)).

JTAG cable plugged in and PGND low => JTAG boot.  This is useful for debugging the FSBL, etc. (which is a rare occurrence).

JTAG cable not plugged in (or JTAG cable plugged in and PGND open) => boot from SDCard if SDCard plugged in, else boot from Flash.

 

I did it that way so that I could leave the JTAG connected and still have it boot normally.  I actually posted the schematic somewhere ...

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Registered: ‎01-08-2012

Re: SmartLynq PGND use case and hardware interfacing

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