linux-kernel - Re: Discussion: quick_pit

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Message-ID: <20150610162738.24160.qmail@ns.horizon.com>
Date:	10 Jun 2015 12:27:38 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	arjanvandeven@...il.com, linux@...izon.com
Cc:	a.p.zijlstra@...llo.nl, adrian.hunter@...el.com,
	ak@...ux.intel.com, akpm@...ux-foundation.org, arjan@...radead.org,
	bp@...en8.de, hpa@...or.com, linux-kernel@...r.kernel.org,
	luto@...capital.net, mingo@...nel.org, penberg@....fi,
	tglx@...utronix.de, torvalds@...ux-foundation.org
Subject: Re: Discussion: quick_pit_calibrate is slow

Arjan van de Ven <arjanvandeven@...il.com> wrote:
> btw one thing to think about; if you calibrate VERY quickly, you need
> to consider spread spectrum clocking.

Excellent point!  But spread spectrum clock modulation rates are typically
about 30 kHz, so 1 ms will average over 30 cycles, which should be enough.

The faster the modulation rate, the more the EMI peak is flattened.
But too fast produces unaceptable cycle-to-cycle timing variations,
also known as clock jitter.
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