K64 FRDM via cJTAG?

[cjtag_on_frdm]

We have a K64 FRDM board and would like to use cJTAG instead of SWD for evaluation purposes with a Segger J-Link probe. SWD is working fine.

According to the schematic PTA0/JTAG_TCLK and PTA3/JTAG_TMS are on Jumper 9 - so, the connection for cJTAG is given.

From the reference manual 9.2 The Debug Port (Document Number: K64P144M120SF5RM, Rev. 5, 01/2021):

"The debug port comes out of reset in standard JTAG mode and is switched into either
cJTAG or SWD mode by the following sequences."

It looks like the K64 is automatically in SWD (always?). Can the interface changed by code (fuses or similar)? Since JTAG pins TDI/TDO are not connected, we cannot send any change sequence (see 9.2.2 JTAG-to-cJTAG change sequence).

Is it possible at all to make cJTAG working on the K64 FRDM board?

[RaRo]

Hello @cjtag_on_frdm,

In K64P144M120SF5RM mentions: "The debug port comes out of reset in standard JTAG mode and is switched into either cJTAG or SWD mode by the following sequences. Once the mode has been changed, unused debug pins can be reassigned to any of their alternative muxed functions."

Nonetheless, talking to the FDRM-K64, even though, the K64 chip can support the JTAG, the FRDM-K64 board can't support it directly. The board as you mentioned it is normally using the SWD interface.

We still have not validated that cJTAG could be used with the FRDM-K64F. However here is a workaround that could be done to test it:

Hardware Connection

• JTAG_TSRT could be connected through jumper wire to pin 7 of RN1, with a 10k resistor in Pull-Up.
• JTAG_TDI might be connected through jumper wire to pin 8 of J1 connector, with a 10k resistor in Pull-Up.
• Use the J9 connector to get the rest of the JTAG signals.

Further Steps

• Once you have the hardware connection for JTAG you could use JLINK Commander to see if the connection it is working.
• Finally, as you mentioned you could follow the JTAG-to-cJTAG change sequence in K64P144M120SF5RM. Section 9.2.2. to Debug with cJTAG.

Best regards, Raul.

[cjtag_on_frdm]

Thanks for the quick and helpful response!

I'm able to communicate via JTAG :). However, switching to cJTAG seems to fail.

This is the sequence SEGGER is using:

https://wiki.segger.com/J-Link_cJTAG_specifics

More or less the sequence mentioned in the datasheet (9.2.2 JTAG-to-cJTAG change sequence) - the datasheet is not too detailed here.

Any hints appreciated?

Can you connect me to anyone with profound cJTAG knowledge?