USB-CYBERCLOCK - For low-noise VCOs&PLLs （1）

Published:2011/8/10 1:01:00 Author:Phyllis From:SeekIC

By Burkhard Kainka

Quartz oscillators are the way to go when you need a precise, stable frequency source but what do you do when an application calls for a non-standard frequency? Custom-made crystals are an expensive option but now Cypress has come up with a cheaper alternative...

The buzzword is CyberClock and describes a relatively new technology in the field of clock generators for use in complex systems like PC motherboards. If you take a look at an old motherboard it is easy to spot the crystal generators dotted around the board providing all the different clock sources. More modern systems are increasingly employing just a single clock generator and deriving all the other signals from this one source.

PLL Technology





The clock generator chips contain a Phase Locked Loop (PLL) with the fastest versions having an output frequency range from 100 MHz to 400 MHz. The PLL layout is shown in Figure 1. It differs from a standard PLL by having programmable dividers at the input and output and in the feedback path. The input clock F^p (typically a crystal) is connected to a programmable divider Q. This clock signal is now compared in the Phase Detector (PD) with the output frequency from the Voltage Controlled Oscillator (VCO) divided by P and fed to the Charge Pump (CP). The CP output is a function of the phase difference and controls the VCO. The VCO output is passed through the output divider N to produce FQUT- Altogether there are three dividers (Q, P and N) all of which can be independently programmed.





We shall take a closer look at two of the Cypress clock chips. Firstly the CY22393 (’Three-PLL Serial Programmable, Flash Programmable Clock Generator’) has six configurable outputs (Figure 2). The device can be flash-programmed (non volatile) from a special external programmer. The programming algorithm is not in the public domain presumably for license protection and to guard against over-clocking or similar abuse. While the oscillator is running it is possible for the contents of internal registers can be overwritten via the I^C-Bus but this storage is volatile and they will be lost on power down.

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