AT89C5131-RDTIL datasheet(20/176 Pages) ATMEL

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AT89C5131

4136B–USB–09/03

Figure 6. Crystal Connection

PLL

PLL Description

The AT89C5131 PLL is used to generate internal high frequency clock (the USB Clock)

synchronized with an external low-frequency (the Peripheral Clock). The PLL clock is

used to generate the USB interface clock. Figure 7 shows the internal structure of the

PLL.

The PFLD block is the Phase Frequency Comparator and Lock Detector. This block

makes the comparison between the reference clock coming from the N divider and the

reverse clock coming from the R divider and generates some pulses on the Up or Down

signal depending on the edge position of the reverse clock. The PLLEN bit in PLLCON

in PLLCON register (see Figure 7) is set.

The CHP block is the Charge Pump that generates the voltage reference for the VCO by

injecting or extracting charges from the external filter connected on PLLF pin (see

Fi gure 8 ) . Value of the filter components ar e detailed in the Section “ DC

Characteristics”.

The VCO block is the Voltage Controlled Oscillator controlled by the voltage V

REF pro-

duced by the charge pump. It generates a square wave signal: the PLL clock.

Figure 7. PLL Block Diagram and Symbol

Figure 8. PLL Filter Connection

The typical values are: R = 100

Ω, C1 = 10 nf, C2 = 2.2 nF.

VSS

PLLEN

PLLCON.1

N3:0

N divider

R divider

VCO

USB Clock

USBclk

OSCclk

()

-----------------------------------------------

OSC

CLOCK

PFLD

PLOCK

PLLCON.0

PFILT

CHP

Vref

Down

R3:0

USB

CLOCK

USB Clock Symbol

VSS

PFILT

VSS

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