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5.1. About clocks and resets

The processor has one functional clock input, HCLK, and one reset signal, SYSRESETn.

If debug is implemented there is also a SWJ-DP clock, SWCLKTCK, a debug reset signal, DBGRESETn, and a JTAG reset signal, nTRST. SWCLKTCK and nTRST relate to the Debug Access Port (DAP) logic and the debug reset signal DBGRESETn relates to the debug logic clocked by HCLK.

The SYSRESETn signal resets the entire processor system with the exception of debug. The DBGRESETn signal resets all the debug logic in the processor, when present.

The following are not reset:

  • the TCMs, when present

  • the register file.

SWCLKTCK is the clock for the debug interface domain of the SWJ-DP. In JTAG mode this is equivalent to TCK. In Serial Wire Mode this is the Serial Wire clock. It can be asynchronous to the system clock HCLK.

Figure 5.1 shows the reset signals for the processor.

Figure 5.1. Reset signals

Figure 5.1. Reset signals


Both DBGRESETn and SYSRESETn must be asserted at power on reset.

Depending on your requirements, you might want to reset the system outside the processor independent of the state of SYSRESETREQ. If this is the case, ensure that:

  • Any logic required for debug is not reset.

  • SYSRESETREQ is not connected combinatorially to SYSRESETn. SYSRESETREQ must be registered to ensure that SYSRESETn is driven for the minimum reset time of your FPGA. SYSRESETREQ is cleared by SYSRESETn.

  • DBGRESETn is driven at power on reset and not by SYSRESETREQ otherwise the debugger cannot maintain a connection when the processor is reset.

  • If DBGRESETn is driven SYSRESETn must also be driven.


If you do not reset the system at the same time as the processor, you must also ensure accesses that might be in progress as reset occurs do not disrupt the system.

You must ensure resets are:

  • held LOW for a minimum of two cycles

  • deasserted synchronously to HCLK.

You can stop all of the processor clocks indefinitely without loss of state.


  • When the External AHB system and the processor are held in reset by SYSRESETn, the debugger can only access the PPB space of the processor and the TCMs. The debugger cannot access external memory space.

  • If the external system is reset by SYSRESETn and is reset during a DAP access, the results of the access cannot be guaranteed. For example, a read transaction might receive corrupt data and a faulting transaction might not be recognized by the DAP.

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