Samsung S3C2440A - Manual (Page 211)

CLOCK & POWER MANAGEMENT S3C2440A RISC MICROPROCESSOR
7 -4
PHASE LOCKED LOOP (PLL)
The MPLL within the clock generator, as a circuit, synchronizes an output signal with a reference input signal in
frequency and phase. In this application, it includes the following basic blocks as shown in Figure 7-2: the Voltage
Controlled Oscillator (VCO) to generate the output frequency proportional to input DC voltage, the divider P to
divide the input frequency (Fin) by p, the divider M to divide the VCO output frequency by m which is input to
Phase Frequency Detector (PFD), the divider S to divide the VCO output frequency by “s” which is Mpll (the output
frequency from MPLL block), the phase difference detector, the charge pump, and the loop filter. The output clock
frequency Mpll is related to the reference input clock frequency Fin by the following equation:
Mpll = (2*m * Fin) / (p * 2 s )
m = M (the value for divider M)+ 8, p = P (the value for divider P) + 2
The UPLL within the clock generator is similar to the MPLL in every aspect.
The following sections describes the operation of the PLL, including the phase difference detector, the charge
pump, the Voltage controlled oscillator (VCO), and the loop filter.
Phase Frequency Detector (PFD)
The PFD monitors the phase difference between Fref and Fvco, and generates a control signal (tracking signal)
when the difference is detected. The Fref means the reference frequency as shown in the Figure 7-2.
Charge Pump (PUMP)
The charge pump converts PFD control signals into a proportional change in voltage across the external filter that
drives the VCO.
Loop Filter
The control signal, which the PFD generates for the charge pump, may generate large excursions (ripples) each
time the Fvco is compared to the Fref. To avoid overloading the VCO, a low pass filter samples and filters the
high-frequency components out of the control signal. The filter is typically a single-pole RC filter with a resistor and
a capacitor.
Voltage Controlled Oscillator (VCO)
The output voltage from the loop filter drives the VCO, causing its oscillation frequency to increase or decrease
linearly as a function of variations in average voltage. When the Fvco matches Fref in terms of frequency as well
as phase, the PFD stops sending control signals to the charge pump, which in turn stabilizes the input voltage to
the loop filter. The VCO frequency then remains constant, and the PLL remains fixed onto the system clock.
Usual Conditions for PLL & Clock Generator
PLL & Clock Generator generally uses the following conditions.
MPLLCAP: 1.3 nF ± 5%
Loop filter capacitance C
LF
UPLLCAP: 700 pF ± 5%
External X-tal frequency - 12 – 20 MHz (note)
External capacitance used for X-tal C
EXT
15 – 22 pF
NOTES:
1. The value could be changed.
2. FCLK
OUT
must be bigger than 200MHz(It does not mean that the ARM core has to run more than 200Mhz).