7641 datasheet(84/385 Pages) RENESAS

84 / 385 page

Rev.2.00

Aug 28, 2006

page 67 of 113

7641 Group

REJ09B0336-0200

HARDWARE

FUNCTIONAL DESCRIPTION

FREQUENCY SYNTHESIZER (PLL)

The frequency synthesizer generates the 48 MHz clock required

by fUSB and fSYN, which are multiples of the external input refer-

ence f(XIN). Figure 59 shows the block diagram for the frequency

synthesizer circuit.

The Frequency Synthesizer Input Bit selects either f(XIN) or

f(XCIN) as an input clock fIN for the frequency synthesizer.

The Frequency Synthesizer Multiply Register 2 (FSM2: address

006E16) divides fIN to generate fPIN, where

fPIN = fIN / 2(n + 1), n: value set to FSM2.

When the value of Frequency Synthesizer Multiply Register 2 is

set to 255, the division is not performed and fPIN will equal fIN.

fVCO is generated according to the contents of Frequency Synthe-

sizer Multiply Register 1 (FSM1: address 006D16), where

fVCO = fPIN ✕ {2(n + 1)}, n: value set to FSM1.

Set the value of FSM1 so that the value of fVCO is 48 MHz.

fSYN is generated according to the contents of the Frequency Syn-

thesizer Divide Register (FSD: address 006F16), where

fSYN = fVCO / 2(m + 1), m: value set to FSD.

When the value of the Frequency Synthesizer Divide Register is

set to 255, the division is not performed and fSYN becomes invalid.

[Frequency Synthesizer Control Register] FSC

Setting the Frequency Synthesizer Enable Bit (FSE) to “1” enables

the frequency synthesizer. When the Frequency Synthesizer Lock

Status Bit (LS) is “1” in the frequency synthesizer enabled, this in-

dicates that fSYN and fVCO have correct frequencies.

sNotes

Make sure to connect a low-pulse filter to the LPF pin when using

the frequency synthesizer. In addition, please refer to “Program-

ming Notes: Frequency Synthesizer” when recovering from a

Hardware Reset.

FSM2

Data Bus

FSM1FSC

Prescaler

fIN

fPIN

fVCO

fSYN

fUSB

(address 006E16)

(address 006D16)(address 006C16)(address 006F16)

FSD

Frequency

Multiplier

Frequency Divider

Fig. 59 Frequency synthesizer block diagram

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