How to safely disable oscillator on K24

joee — Tue, 18 Apr 2023 21:33:35 GMT

My board is running with an external clock at 50MHz. The MCG and OSC are setup in the bootloader with the following code (from Keil Uvision startup file):

#elif (CLOCK_SETUP == 2) /* SIM_CLKDIV1: OUTDIV1=0,OUTDIV2=0,OUTDIV3=1,OUTDIV4=1,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0,??=0 */ SIM->CLKDIV1 = (uint32_t)0x00110000u; /* Update system prescalers */ /* Switch to FBE Mode */ /* OSC->CR: ERCLKEN=0,??=0,EREFSTEN=0,??=0,SC2P=0,SC4P=0,SC8P=0,SC16P=0 */ OSC->CR = (uint8_t)0x00u; /* MCG->C7: OSCSEL=0 */ MCG->C7 = (uint8_t)0x00u; /* MCG->C2: ??=0,??=0,RANGE0=2,HGO=0,EREFS=1,LP=0,IRCS=0 */ MCG->C2 = (uint8_t)0x24u; /* MCG->C1: CLKS=2,FRDIV=5,IREFS=0,IRCLKEN=0,IREFSTEN=0 */ MCG->C1 = (uint8_t)0xA8u; /* MCG->C4: DMX32=0,DRST_DRS=0 */ MCG->C4 &= (uint8_t)~(uint8_t)0xE0u; /* MCG->C5: ??=0,PLLCLKEN=0,PLLSTEN=0,PRDIV0=0 */ MCG->C5 = (uint8_t)0x00u; /* MCG->C6: LOLIE=0,PLLS=0,CME=0,VDIV0=0 */ MCG->C6 = (uint8_t)0x00u; while((MCG->S & MCG_S_OSCINIT0_MASK) == 0u) { /* Check that the oscillator is running */ } while((MCG->S & MCG_S_IREFST_MASK) != 0u) { /* Check that the source of the FLL reference clock is the external reference clock. */ } while((MCG->S & 0x0Cu) != 0x08u) { /* Wait until external reference clock is selected as MCG output */ } /* Switch to BLPE Mode */ /* MCG->C2: ??=0,??=0,RANGE0=2,HGO=0,EREFS=1,LP=0,IRCS=0 */ MCG->C2 = (uint8_t)0x24u; #endif

I need to disable the oscillator in the main code so that I can use the XTAL pin as GPIO. I am trying to do this by setting the EREFS bit of MCG_C2 as seen in the following code, but occasionally I get a bus fault afterwards.

MCG->C2 &= ~MCG_C2_EREFS0_MASK; //set for external clock while((MCG->S & MCG_S_OSCINIT0_MASK ) != 0x00u); /* Wait until external reference clock is selected as MCG output */

Is there some other sequence I need to perform to do this or some other status to wait for?

thanks

Re: How to safely disable oscillator on K24

vicentegomez — Mon, 24 Apr 2023 16:06:23 GMT

maybe this post can help you

https://community.nxp.com/t5/Kinetis-Microcontrollers/Kinetis-System-Clocks/ta-p/1149854

Regards

Re: How to safely disable oscillator on K24

joee — Mon, 01 May 2023 18:25:56 GMT

Thanks for your response. The post does make things a bit clearer, but I don't see that it addresses switching from external crystal to external clock.

For now I am switching back to the FEI state and then switching to FBE, then BLPE with the EREFS bit of MCG_C2 set to 0. So far I have not had any issues with this method.

topic Re: How to safely disable oscillator on K24 in Kinetis Microcontrollers

How to safely disable oscillator on K24

Re: How to safely disable oscillator on K24

Re: How to safely disable oscillator on K24