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We use Multi-ICE for debugging. We would like to reduce pin count in our system. Is it necessary to have separate connections for nTRST and core reset (nRESET/HRESETn/BnRES)?

Applies to: ARM1020/22E, ARM1026EJ-S, ARM1136, ARM720T, ARM7EJ-S, ARM7TDMI, ARM7TDMI-S, ARM920/922T, ARM926EJ-S, ARM940T, ARM946E-S, ARM966E-S, ARM9TDMI

Answer

The TAP controller and ICE logic is reset independently from the ARM core with nTRST (DBGnTRST on synthesizable cores). For the ARM core to operate correctly, it is essential that both signals are asserted after power-up.

The advantage of having separate connection to the two reset signals is that it allows the person doing software debug to set up breakpoints which are retained by the ICE logic even when the core is reset. (For example, at address 0, to allow the code to be single-stepped as soon as it comes out of reset). This can be particularly useful when first trying to bring up a board with a new ASIC.

You may tie (DBG)nTRST to the core reset, but this removes some of the flexibility and usefulness of the debug tools. What some designers facing similar pin constraints have done is to implement some kind of reset circuit within their device. This typically will assert both nTRST and the core reset for the initial power-on reset, but subsequent 'warm' resets, where the power to the device is maintained, will cause only the core reset to go low.

There is further information about designing an ASIC to work with Multi-ICE, in the Multi-ICE User Guide (2MB PDF).

See also:

Article last edited on: 2008-09-09 15:47:35

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