AM2431: What to do with unused TRSTn

Fangjia Zhu

Part Number: AM2431

Hello,

We plan to use the device JTAG functions but not this reset. The datasheet mentions the TRSTn pin has pull down internally but the name implies it's an actively low signal. Do I need to pull the pin on PCB to *not* reset the JTAG? If so what should the PU value be?

Thanks,

Fang

over 3 years ago

0 Fleenor over 3 years ago

TI__Mastermind 25133 points

Hello Fang,

I found this previous E2E thread with similar details:

(+) AM2432: JTAG Reset TRSTn - Arm-based microcontrollers forum - Arm-based microcontrollers - TI E2E support forums

Details included below for quick reference.

The TRSTn is an active low reset signal. This means that as long as the signal is held low, the device TAP controller will be in reset. When the signal is pulled high by an external debugger, the TAP controller can then be used.

http://www.interfacebus.com/Design_Connector_JTAG_Bus.html

Since the TRSTn is an active low signal, the signal should be pulled low when a debugger isn't required. Pulling down the TRSTn signal is not an abnormal case, having an external pull down is actually suggested to prevent possible noise from falsely bouncing high and enabling the TAP controller.

An external debugger is in charge of pulling the TRSTn signal high to enable an active debug state.

Best Regards,

Zackary Fleenor

0 Fangjia Zhu over 3 years ago in reply to Fleenor

Prodigy 80 points

Thanks! In our use case, we don't have an external debugger but rather something custom made on the PCB that does not have a reset output - i.e. will not be able to pull the signal high. So we would need to just permanently pull the signal high on the PCB using a PU. What should the PU value be?

0 Fleenor over 3 years ago

TI__Mastermind 25133 points

Hey Fang,

A 4.7k ohm resistor should work. You can find additional details and suggestions in the link below.

https://software-dl.ti.com/ccs/esd/documents/xdsdebugprobes/emu_xds_target_connection_guide.html#jtag-and-ntrst-special-considerations-includes-tms-and-tdi-

Best Regards,

Zackary Fleenor