C8051F120/1/2/3/4/5/6/7
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Rev. 1.2
15.7. Phase-Locked Loop (PLL)
The C8051F12x Family include a Phase-Locked-Loop (PLL), which is used to multiply the internal oscillator or an
external clock source to achieve higher CPU operating frequencies. The PLL circuitry is designed to produce an out-
put frequency between 25 MHz and 100 MHz, from a divided reference frequency between 5 MHz and 30 MHz. A
block diagram of the PLL is shown in Figure 15.6.
15.7.1. PLL Input Clock and Pre-divider
The PLL circuitry can derive its reference clock from either the internal oscillator or an external clock source. The
PLLSRC bit (PLL0CN.2) controls which clock source is used for the reference clock (see Figure 15.7). If PLLSRC is
set to ‘0’, the internal oscillator source is used. Note that the internal oscillator divide factor (as specified by bits
IFCN1-0 in register OSCICN) will also apply to this clock. When PLLSRC is set to ‘1’, an external oscillator source
will be used. The external oscillator should be active and settled before it is selected as a reference clock for the PLL
circuit. The reference clock is divided down prior to the PLL circuit, according to the contents of the PLLM4-0 bits in
the PLL Pre-divider Register (PLL0DIV), shown in Figure 15.8.
15.7.2. PLL Multiplication and Output Clock
The PLL circuitry will multiply the divided reference clock by the multiplication factor stored in the PLL0MUL reg-
ister shown in Figure 15.9. To accomplish this, it uses a feedback loop consisting of a phase/frequency detector, a
loop filter, and a current-controlled oscillator (ICO). It is important to configure the loop filter and the ICO for the
correct frequency ranges. The PLLLP3-0 bits (PLL0FLT.3-0) should be set according to the divided reference clock
frequency. Likewise, the PLLICO1-0 bits (PLL0FLT.5-4) should be set according to the desired output frequency
range. Figure 15.10 describes the proper settings to use for the PLLLP3-0 and PLLICO1-0 bits. When the PLL is
locked and stable at the desired frequency, the PLLLCK bit (PLL0CN.5) will be set to a ‘1’. The resulting PLL fre-
quency will be set according to the equation:
Where “Reference Frequency” is the selected source clock frequency, PLLN is the PLL Multiplier, and PLLM is the
PLL Pre-divider.
PLL0DIV
PLLM
4
PLLM
3
PLLM
2
PLLM
1
PLLM
0
PLL0MUL
PLLN
7
PLLN
6
PLLN
5
PLLN
4
PLLN
3
PLLN
2
PLLN
1
PLLN
0
PLL0CN
PLLLC
K
PLLSR
C
PLLEN
PLLPW
R
PLL0FLT
PLLIC
O
1
PLLIC
O
0
PLLLP3
PLLLP2
PLLLP1
PLLLP0
0
1
Internal
Oscillator
External
Oscillator
÷
Phase /
Frequency
Detection
÷
Loop Filter
Current
Controlled
Oscillator
PLL Clock
Output
Divided
Reference
Clock
Figure 15.6. PLL Block Diagram
PLL Frequency
Reference Frequency
PLLN
PLLM
----------------
×
=
Summary of Contents for C8051F120
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Page 8: ...C8051F120 1 2 3 4 5 6 7 8 Rev 1 2 26 2 Flash Programming Commands 318 26 3 Debug Support 321 ...
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