LPC845 running at 30 MHz

[lubomirbogdanov]

Hi there!

I'm trying to run a LPC845 at 30 MHz with an external crystal resonator of value 12 MHz. I try this:

CLOCK_EnableClock(kCLOCK_Iocon);
CLOCK_EnableClock(kCLOCK_Swm);
GPIO_PortInit(GPIO, 0);
GPIO_PortInit(GPIO, 1);

//---------------------------------------------------------------

clock_sys_pll_t pll_config;

pll_config.src = kCLOCK_SysPllSrcExtClk;
pll_config.targetFreq = 30000000;

POWER_DisablePD(kPDRUNCFG_PD_SYSOSC);
CLOCK_InitSysOsc(12000000);
CLOCK_InitSystemPll(&pll_config);
CLOCK_SetCoreSysClkDiv(1);
CLOCK_SetMainClkSrc(kCLOCK_MainClkSrcSysPll);

SystemCoreClock = CLOCK_GetMainClkFreq();

but SystemCoreClock returns 24000000 Hz. If I connect an oscilloscope probe at the XTAL I can see a 12-MHz oscillation, my PLL locks (I can tell because CLOCK_InitSystemPll( ) returns and does not block) but I suspect I still might be running off of the FRO.

I also tried a target frequency of 60 MHz and a division factor of 2 but no luck - then SystemCoreClock is 60000000.

Thank you in advance,

Lubo

[Alice_Yang · Accepted solution]

Hello Lubomir Bogdanov ,

From the User Manual of LPC845, we can know:

Fclkout = M * Fclkin

In you case, Fclkin = 12MHZ,

M = MSEL +1,

MSEL:

and FCLKOUT must be lower than 30 MHz.

So in your case, the the max of Fclkout is 24MHZ, and the system clock is 24MHZ, can't 30MHZ.

Have a great day,
TIC

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