16.7.3 CPU Reset Extension

“CPU reset extension” refers to the extension of the reset phase of the device’s CPU core(s) when a debugger connects with the cold-plugging procedure. This ensures that when a debugger connects:

  • The CPU is not executing code at startup
  • The device is in a safe and in a known state (reset)

For security reasons, the hot-plugging method is not allowed when CPU0.DAL=0. Cold-plugging must be used in this case.

For each CPU held in the reset extension phase, the CPU Reset Extension bits of the Status A register (STATUSA.CRSTEXTx) reads one. To release CPUx from reset, write a '1' in STATUSA.CRSTEXTx. STATUSA.CRSTEXTx will then be cleared. More than one CPU can be released at a time. (Writing a '0' to STATUSA.CRSTEXTx has no effect.) Releasing the "CPU reset extension" is possible for all DAL.CPUx levels. Each CPU then executes its Boot ROM. The host CPU (CPU 0) Boot ROM implements security checks at startup. It is not possible to access the bus system until the Boot ROM has performed these security checks.

Figure 16-4. Typical CPU Reset Extension Set and Clear Timing Diagram