Set the PLL to non-standard settings and can no longer communicate with the MCU

[lpcware]

Content originally posted in LPCWare by BasicPoke on Tue May 10 15:13:14 MST 2016
I set the PLL to non-standard settings and can no longer communicate with the MCU (LPC3131).  I was trying to see if the PLL could run at lower than 24 MHz, so I changed the 24 MHz settings, changing Pdec from 10 to 20, as follows:

  HP1_MDEC = 1023;    //M divider
  HP1_NDEC = 770;     //N divider
  HP1_PDEC = 20;      //P divider
  HP1_SELR = 0;
  HP1_SELI = 16;
  HP1_SELP = 7;

Now when I try to "Download and Debug" (using IAR embedded workbench IDE), I get a dialog "Failed to get CPU status after 4 retries.  Retry?".  I am using IAR System J-Link as a debugger.  I have tried restarting the IDE and unplugging and replugging the J-Link.  Here is some gibberish in the Debug Log:

Tue May 10, 2016 16:56:37: Loaded macro file: \\...\FlashLoader\FlashEmbArtLPC313x_SPINOR.mac 
Tue May 10, 2016 16:56:37: JLINK command: ProjectFile = \\...\settings\WGProj_Debug_SPINOR_boot_iRAM.jlink, return = 0 
Tue May 10, 2016 16:56:37: Device "LPC3131" selected (0 KB flash, 192 KB RAM). 
Tue May 10, 2016 16:56:37: DLL version: V4.78m, compiled Dec 18 2013 20:40:58 
Tue May 10, 2016 16:56:37: Firmware: J-Link ARM V6 compiled Jun 20 2012 19:43:26 
Tue May 10, 2016 16:56:37: JTAG speed is fixed to: 1000 kHz 
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..FFFFFFFF8442780003997707 
Tue May 10, 2016 16:56:37: Hardware reset with strategy 0 was performed 
Tue May 10, 2016 16:56:37: Initial reset was performed 
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..FFFFFFFFFFFFFFE072D20001 
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..FFFFFFFFFFFFFFF062120001 
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..FFFFFFFFFFFFFFE072920001 
Tue May 10, 2016 16:56:37: Prepare hardware for Flashloader  
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..000000000000000000000000 
Tue May 10, 2016 16:56:37: Hardware reset with strategy 0 was performed 
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..FFFFFFFFFFFFFFF062130001 
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..000000000000000000000000 
Tue May 10, 2016 16:56:37: TotalIRLen = ?, IRPrint = 0x..FFFFFFFFFFFFFFE072120001 
...

From the CLOCK_OUT pin running at 2.5 MHz, it appears the PLL is running at 5 MHz.  My app is still running normally.  How can I get out of this state?  It is very unusual for this to happen and I am confident it was caused by my PLL settings.
Thanks
Ron

[lpcware]

bump

[lpcware]

Content originally posted in LPCWare by BasicPoke on Wed May 11 06:42:06 MST 2016
wmues, thank you so much.  See attachment.  I set the JTAG/SWD speed to Auto instead of Fixed and was able to use the debugger again.  At "Fixed" 1 MHz, it would not be less than 1/6 of the CPU clock, which appeared to be 5 MHz.  Now I am back in business.
Ron

[lpcware]

Content originally posted in LPCWare by wmues on Wed May 11 02:44:31 MST 2016
Lower the speed of the JTAG adaptor to 1/6 of the CPU clock, or less.