IMX8M Mini processor - JTAG_TCK pull-down recommendation

hugo_letourneau — Fri, 11 Dec 2020 15:46:22 GMT

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

Re: IMX8M Mini processor - JTAG_TCK pull-down recommendation

Yuri — Mon, 14 Dec 2020 06:23:19 GMT

@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.

i.MX Processors中的主题 IMX8M Mini processor - JTAG_TCK pull-down recommendation

IMX8M Mini processor - JTAG_TCK pull-down recommendation

Re: IMX8M Mini processor - JTAG_TCK pull-down recommendation