From: Zhang, Michael T ([email protected])
Date: Mon Nov 08 1999 - 16:31:24 PST
When spread spectrum is enabled, basically PLLs are trying to catching up a
slowly modulated host clock. How slow? The modulation frequency is typically
around 30kHz. Comparing that to the 100MHz host bus frequency, you would
have over 3000 bus cycles in one modulation period. The clocks are not
modulated by much (~0.5%). So the peak-to-peak period change is small
(roughly 0.5%*10ns = 50ps), which takes about 1500 cycles to complete. Thus,
the consecutive edge variation is extremely small (50ps/1500 = 33fs).
Most PLLs can track such slowly modulated clocks with small spread amount
and don't require any knowledge of the modulation scheme (or so called
"modulation profile").
-Michael T. Zhang
PMD/PPD, Intel Corp
(503)264-2301
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Monday, November 08, 1999 10:57 AM
To: [email protected]
Subject: RE: [SI-LIST] : Micro Noise
Michael T. Zhang wrote:
One such example is Intel's BX-based motherboards have the optional spread
spectrum clocking feature, which modulates the clocks for processor,
chipset, memory, and PCI, but keeps the USB clock fixed.
Would you elaborate a little bit?
I have this vision of PLL's in different chips chasing
the phase of the clock, but all of them at a different phase
because of different dynamics. Do they all know the modulating
algorithm so they can predict the next phase, or is the rate
of change from one edge to the next held low enough not to be a
problem?
**** To unsubscribe from si-list: send e-mail to
[email protected]. In the BODY of message put: UNSUBSCRIBE
si-list, for more help, put HELP. si-list archives are accessible at
http://www.qsl.net/wb6tpu/si-list ****
**** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
This archive was generated by hypermail 2b29 : Tue Feb 29 2000 - 11:39:43 PST