CM4: Can processor halt itself by writing DHCSR

Hello,

As part of my diagnostic regime I wanted the diag to halt when completed.  It doesn't seem like it can.  It seems to keep running when I 

  CoreDebug->DHCSR = (0xA05FUL << CoreDebug_DHCSR_DBGKEY_Pos) |
                     CoreDebug_DHCSR_C_HALT_Msk |
                     CoreDebug_DHCSR_C_DEBUGEN_Msk;

Thanks, Tom