JTAG_TMS pull up or pull down? S32K144

[Stanley_Xu]

Hello:

When I'm reading the safety manual of S32K1xx, there's chapter about debug mode. It says JTAG_TMS shall be pulled low to ensure that JTAGC TAP controller is disabled.

How to understand this sentence? Is that mean I shall assert this pin to low by external components during normal operation (non-debug mode) ?

[db16122]

According to RM of Chapter 59 JTAG Controller (JTAGC), TMS status impact entering the dubug mode or not. As the diagram show, when the TMS set to 0, TAP will not enter debug status. So it would be safe to maintain low status while the hardware connection is recommended to pull-up to VDD through resistor 10k-47k according to AN5426

[PavelL · Accepted solution]

Hello @Stanley_Xu ,

The recommendations in the S32K1xx Safety Manual and AN5426 apply to different use cases.

AN5426 recommends a 10 kΩ to 47 kΩ pull-up on the JTAG_TMS/SWD_DIO pin. This is the recommended hardware configuration when JTAG or SWD access is required during development, programming, or debugging.

However, the Safety Manual addresses the final in-field configuration of a safety-relevant application. According to Section 5.6.2.1 and assumption SM_047, debugging must be disabled in the field while the device is used for safety-relevant functions. For this operating condition, JTAG_TMS should be held low so that the JTAGC TAP controller remains disabled and cannot unintentionally interfere with normal application operation.

Therefore, if the application is required to comply with assumption SM_047 of the S32K1xx Safety Manual, the final production hardware shall provide a means of keeping JTAG_TMS low in the field. This can be implemented using an external pull-down resistor or another system-level solution that guarantees the required low level and prevents an external source from asserting this signal.

The TAP state diagram only shows how TMS controls state transitions on the rising edge of TCK. It does not by itself provide permanent debug protection. If an external debugger can actively drive TMS and TCK, the TAP controller can be moved through its state machine. Therefore, the TMS pull-down is a functional-safety measure intended to prevent unintended debug activation, not a security mechanism for permanently locking the debug interface.

If debug access is required during development or manufacturing, separate development and production configurations may be necessary.

Best regards,
Pavel