9.2.2. Reset signals

This section describes the reset signals that are available on ARM devices and how RealView ICE expects them to be wired. It is presented in the following sections:

ARM reset signals

All ARM cores have a main processor reset that might be called nRESET, BnRES, or HRESET. This is asserted by one or more of these conditions:

  • power on

  • manual push button

  • remote reset from the debugger (using RealView ICE)

  • watchdog circuit (if appropriate to the application).

Any ARM processor core including the JTAG interface has a second reset input called nTRST (TAP Reset). This resets the EmbeddedICE logic, the TAP controller, and the boundary scan cells. It is activated by remote JTAG reset (from RealView ICE).

It is strongly recommended that both signals are separately available on the JTAG connector. If the nRESET and nTRST signals are linked together, resetting the system also resets the TAP controller. This means that:

  • it is not possible to debug a system from reset, because any breakpoints previously set are lost

  • you might have to start the debug session from the beginning, because RealView ICE does not recover when the TAP controller state is changed.

RealView ICE reset signals

The RealView ICE run control unit has two reset signals connected to the debug target hardware:

  • nTRST drives the JTAG nTRST signal on the ARM processor core. It is an output that is activated whenever the RealView ICE software has to re-initialize the debug interface in the target system.

  • nSRST is a bidirectional signal that both drives and senses the system reset signal on the target. The output is driven LOW by the debugger to re-initialize the target system.

The target hardware must include a pull-up resistor on both reset signals. In the RealView ICE run control unit, the strong pull-up/pull-down resistance is approximately 100Ω, and the weak pull-up/pull-down resistance is approximately 4.7kΩ. Because you can select the drive strength for nTRST and nSRST (see Advanced configuration), target assemblies with a variey of different reset configurations can be supported.

Example reset circuits

The circuits shown in Figure 9.5 and Figure 9.6 illustrate how the behavior described in Reset signals can be achieved. The MAX823 used in Figure 9.6 is a typical power supply supervisor. It has a current limited nRESET output that can be overdriven by the RealView ICE run control unit.

Figure 9.5. Example reset circuit logic


Note

The symbol for nReset1 signifies a sink driver, with an open collector output.

Figure 9.6. Example reset circuit using power supply monitor ICs


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