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ARM microcontrollers can be clocked im several different ways. One of the ways is to use external clock with duty cycle 50-50 and a frequency range from 1MHz to 50MHz (LPC21xx series) connected to XTAL1 pin. Other way is to connect External crystal oscillator, but its range is lower (1MHz to 30MHz for LPC21xx series). And last but not least is on-chip PLL oscillator, then external clock source frequency should not exceed range from 10MHz to 25MHz. Lets analyse more deeply each clocking mechanism. 
In the picture above there is fosc selection diagram shown. External clock source This mode is also called slave mode, because it clocking is done by master clocking circuit – ARM doesn't act as clock generator. In this mode clock source is connected to ARM through XTAL1 pin and should be coupled with capacitor of 100pF. Amplitude of clock signal should be at least 200mVRMS. XTAL2 pin can be left unconnected in this case.: 
Signal frequency can be in range from 1MHz to 50MHz with duty cycle 50 – 50. Oscillation mode:External crystal only In this mode ARM clocks itself with internal circuit, just external crystal and capacitors are needed . Using this oscillation mode crystal clock frequency can be chosen in range between 1MHz and 30MHz (LPC21xx series): 
For capacitor values refer to datasheet. Oscillation mode:External crystal with PLL PLL (Phase Locked Loop) is used to generate system clock from between 10MHz to 25MHz. PLL may multiply frequency to range from 10MHz to 60MHz (LPC21xx series) and 48MHz for USB if used. PLL uses frequency multiplier which can be in range from 1 to 32 (practically this value cannot be higher than 6 due to upper frequency limit). PLL generator allows running ARM at high speed with low speed oscillator connected. Also this minimises EMC emission as frequency is multiplied inside ARM chip. PLL also allows changing frequency dynamically. It may rise calculation power when it needed and lower frequency when power saving is needed. PLL unit itself uses CCO(Current Controlled Oscillator) which operates in the range of 156MHz to 320MHz, so there is additional divider which keeps CCO within it's range, while PLL provides desired frequency. Output clock is generated by dividing CCO frequency by 2, 4, 8 or 16. Minimum divider is 2 so output of PLL will always have duty cycle 50% for sure. PLL Activation and control is done via PLLCON register. PLL multiplier and divider values are controlled by register PLLCFG. These registers are protected from accidental alteration of PLL parameters or deactivation of PLL. Lets see how this is done in practice. Say we have 12MHz crystal connected to LPC2148. Then we can say Osc=12MHz. Wee want core frequency to be 60MHz then we have to multiply crystal frequency by five: Cclk=M*Osc=5*12=60MHz; Second thing is that we have to keep OCC oscillator frequency (Fcco)within its range [156Mhz-320MHz], so wee have to control another constant P: Fcco=Cclk*2*P; Fcco=60MHz*2*2=240MHz; So we can have P=2 ant this meets CCO requirements (156MHz<240MHz<320MHz). Programming PLL has to be done in some sequence to make new PLL settings effective. First of all we have to write Multiplier M and divider P values to PLLCFG register. Since M and P values can have only specific values then there is a look up table used to determine PLLCFG bits: 
So for our calculations M=5 and P=2, then PLLCFG=0b00100100=0x24; Further there has to be some sequence of activating PLL. After PLLCFG register is updated, then update PLLCON register and then you have to write 0x000000AA and then 0x00000055 to PLLFEDD register these values have to be written on consecutive cycles. For instance using C PLL initialisation would look like:
void PLL_Init(void) { PLLCFG=0x24; //Multipler and divider setup PLLCON=0x01; //Enable PLL PLLFEED=0xAA; //Feed sequence PLLFEED=0x55; while(!(PLLSTAT & 0x0400)) ; //is locked? PLLCON=0x03; //Connect PLL after PLL is locked PLLFEED=0xAA; //Feed sequence PLLFEED=0x55; VPBDIV=0x2; //peripheral bus runs 2 times slower }
Note that power-down mode disconnects and turns off PLL. Waking up doesn't restore PLL – you have to do this in software. Simply you can call PLL activation routine at the beginning of any interrupt service routine which is called due to wakeup.
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