IMX8M Mini processor - JTAG_TCK pull-down recommendation

[hugo_letourneau]

Hi,

Concerning the IMX8M Mini and its JTAG-TCK signal:

- document IMX8MMHDG recommends a 10k external pull-down on JTAG-TCK (page 11/49)

- The reference design "8MMINILPD4-EVK-DESIGNFILES" provided by NXP also contains a 10k pull-down (R18)

- The reference design "8MMINID4-EVK-DESIGNFILES"  provided by NXP also contains a 10k pull-down (R18)

- We have reproduce the pull-down in our design

However:

- the datasheet document "IMX8MMIEC" states on page 78 that JTAG_TCK is an input with an internal pull-up

- probing the JTAG_TCK signal when a 10k pull-down is present on the line shows an intermediate voltage that is consistent with a potential divider caused by the internal pull-up and the external pull-down - is this wanted?

Questions:

1- What is the value of the internal pull-up for JTAG_TCK signal?

2- What is the reason to recommend the use of an external 10k pull-down on JTAG_TCK?

3- Currently the power domain of the JTAG is 1V8, the 10k resistor and the internal pullup is causing a signal that is aproximately 0-600mV on the JTAG_TCK line where I would expect to have 0-1V8 instead.  Is my expectation correct?

Please let me know.

Thanks and have a great day.

Hugo

[Yuri]

@hugo_letourneau
Hello,
 
  Please look at my comments below.

 1. According to table “GPIO DC parameters” in the Datasheet, internal pull-up resistors are
about 20 KOhm.

2. JTAG TCK pulling resistor is recommended in order to avoid any possible pulses on this JTAG
signal, say - during power up. Unpredictable entering JTAG mode can take place.

3. I think it makes sense to use external pulling up for the JTAG TCK, or just to remove
the pull down resistor on the board.

Regards,
Yuri.