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Problem setting CCLKDIV register in LPC1778

Question asked by Carsten Groen on Nov 4, 2016
Latest reply on Nov 6, 2016 by Carsten Groen

I have a system running on a LPC1778 (using SDRAM, USB etc etc). All is working fine so far, stable etc.

However, I'm trying to do "clock throttling" of the CPU and thats where things go downhill


The system starts up running 120 MHz (EMCCLK for the SDRAM runs at 1/2 speed so 60 MHz, PBOOST set to 0x03). I then have a function where I can set the CCLKDIV register to divide the CPU clock with either 1, 2 or 4 (giving me 120, 60 or 30 MHz). 

This is needed to avoid one of the errata where IAP will fail on 5% of devices if running at 120 MHz and to conserve power at certain times. PCLKDIV is always set to divide by 4.


When changing the divider, I get very often, around 50% of the time, a hard fault. It does not depend on what divider I select, if I go to divide by 2 and then back to 1 or to 4, it can hardfault, there is no consistency in this (and sometimes I can change divider 1 or 2 times successfully and at the 3rd time it faults).

The cause and information of the hardfault differs between different runs, so not very consistent. If I select the same divider as currently selected (basically just rewriting the CCLKDIV register with the same value, no faults are triggered). The PC points to a SVC handler (the change is done via a call thru SVC). None of the other information makes much sense to me..


Is there any thing I must consider when writing the CCLKDIV register ?



One example of hardfault:

161104 09:01:17.627::E:[diagnose.c:224] R0=0xA01C8470/-1608743824
161104 09:01:17.709::E:[diagnose.c:225] R1=0x01000000/16777216
161104 09:01:18.197::E:[diagnose.c:226] R2=0xFFFFFFE0/-32
161104 09:01:18.278::E:[diagnose.c:227] R3=0xA01C8528/-1608743640
161104 09:01:18.400::E:[diagnose.c:229] R12=0x0004DBAD
161104 09:01:18.891::E:[diagnose.c:230] LR [R14]=0xFFFFFFFD
161104 09:01:18.972::E:[diagnose.c:231] PC [R15]=0x0002031A
161104 09:01:19.075::E:[diagnose.c:233] PSR=0x0100000B
161104 09:01:19.135::E:[diagnose.c:236] CFSR (Config. Fault SR)=0x00008200
161104 09:01:19.645::E:[diagnose.c:245] :BusFault->Bus Fault Address Register (BFAR) holds a valid address=0x00FFFFFE
161104 09:01:19.726::E:[diagnose.c:254] HFSR (Hard Fault SR)=0x40000000
161104 09:01:19.809::E:[diagnose.c:258] :FORCED, Bus fault/Memory management fault/usage fault
161104 09:01:19.891::E:[diagnose.c:263] DFSR (Debug Fault SR)=0x00000008
161104 09:01:20.382::E:[diagnose.c:265] AFSR (Auxiliary Fault SR)=0x00000000
161104 09:01:20.484::E:[diagnose.c:266] SCB->SHCSR (System Handler Control and State R (exception))=0x00000080


and another:

161104 09:11:46.120::E:[diagnose.c:224] R0=0x00000001/1
161104 09:11:46.181::E:[diagnose.c:225] R1=0x00000000/0
161104 09:11:46.711::E:[diagnose.c:226] R2=0x00000000/0
161104 09:11:46.773::E:[diagnose.c:227] R3=0x0004D74D/317261
161104 09:11:46.874::E:[diagnose.c:229] R12=0x00024E01
161104 09:11:46.936::E:[diagnose.c:230] LR [R14]=0x00000000
161104 09:11:47.424::E:[diagnose.c:231] PC [R15]=0x40000000
161104 09:11:47.525::E:[diagnose.c:233] PSR=0x4000C000
161104 09:11:47.606::E:[diagnose.c:236] CFSR (Config. Fault SR)=0x00020000
161104 09:11:47.790::E:[diagnose.c:254] HFSR (Hard Fault SR)=0x40000000
161104 09:11:48.320::E:[diagnose.c:258] :FORCED, Bus fault/Memory management fault/usage fault
161104 09:11:48.381::E:[diagnose.c:263] DFSR (Debug Fault SR)=0x00000008
161104 09:11:48.461::E:[diagnose.c:265] AFSR (Auxiliary Fault SR)=0x00000000
161104 09:11:48.563::E:[diagnose.c:266] SCB->SHCSR (System Handler Control and State R (exception))=0x00000000