From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthijs van Duin Subject: Re: Minimal support for dm814x Date: Wed, 18 Nov 2015 11:01:56 +0100 Message-ID: References: <4999BB3D-4BB5-4F7C-96D7-FB687725CDCC@audioscience.com> <20151109150602.GR3199@atomide.com> <7933071D-84FD-4A85-8CD0-CC504CE1D9D1@audioscience.com> <20151111174028.GF3218@atomide.com> <20151112174155.GM3218@atomide.com> <20151113145216.GC2517@atomide.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Delio Brignoli Cc: Tony Lindgren , "linux-omap@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" List-Id: linux-omap@vger.kernel.org T24gMTggTm92ZW1iZXIgMjAxNSBhdCAwOToyNiwgRGVsaW8gQnJpZ25vbGkgPGRicmlnbm9saUBh dWRpb3NjaWVuY2UuY29tPiB3cm90ZToKPj4gVGhpcyB3b3JrcyBpbiBwcmluY2lwbGUsIGJ1dCBi b3RoIG1pbmltaXppbmcgdGhlIERDTyBhbmQgKG9mdGVuCj4+IG5lZWRsZXNzbHkpIHVzaW5nIHRo ZSBmcmFjdGlvbmFsIG11bHRpcGxpZXIgc2VlbSBsaWtlIHJlY2lwZXMgdG8KPj4gbWF4aW1pemUg dGhlIGNsb2NrIGppdHRlci4gTWluZCB5b3UsIEkgZG9uJ3Qga25vdyBob3cgbXVjaCBqaXR0ZXIK Pj4gd2XigJlyZSB0YWxraW5nIGFib3V0IGhlcmUsIEkgZG9uJ3QgcmVjYWxsIGhhdmluZyBzZWVu IHNwZWNzIGFib3V0IHRoaXMuCj4KPiBXZSBoYXZlbuKAmXQgc2VlbiBhbnkgc3BlY3MgZWl0aGVy IGJ1dCB0ZXN0aW5nIHNob3dzIHRoYXQgY2hhbmdpbmcgRENPIG1vZGUgY2F1c2VzCj4gdGhlIFBM TCB0byBsb3NlIGxvY2sgYXQgbGVhc3QgdGVtcG9yYXJpbHkuCgpMb3NpbmcgbG9jayBvbiByZWNv bmZpZ3VyYXRpb24gaXMgZW50aXJlbHkgYSBzZXBhcmF0ZSBtYXR0ZXIgZnJvbQpjbG9jayBqaXR0 ZXIuIFRoZSBmcmFjdGlvbmFsIG11bHRpcGxpZXIgd29ya3MgYnkgZXNzZW50aWFsbHkgYnkKYWx0 ZXJuYXRpbmcgYmV0d2VlbiB0aGUgbmVhcmVzdCBpbnRlZ2VyIG11bHRpcGxpZXIgdmFsdWVzLiBU aGlzIHdpbGwKYmUgc21vb3RoZWQgb3V0IGJ5IHRoZSBsb29wIGZpbHRlciwgYnV0IGl0J3Mgbm90 IGdvaW5nIHRvIHZhbmlzaC4KClRvIHB1dCB0aGlzIGluIHBlcnNwZWN0aXZlLCBzb21lIGRvY3Mg KEkgY2FuJ3QgaW1tZWRpYXRlbHkgZmluZCB3aGljaApvbmUpIHdhcm5lZCB0aGF0IHdoZW4gdXNp bmcgdGhlIGZyYWN0aW9uYWwgbXVsdGlwbGllciBvZiBEUExMUywgaXRzCnZhbHVlIG5lZWRlZCB0 byBiZSBhdCBsZWFzdCAxMDAgdG8gZW5zdXJlIG1heCAyLjUlIGppdHRlci4gVG8gcHV0IHRoaXMK aW4gcGVyc3BlY3RpdmUsIGlmIEkgZ3JhYiB0aGUgZGF0YXNoZWV0IG9mIGFuIGF1ZGlvIERBQyBJ IGZpbmQ6CiAgIEFsdGhvdWdoIHRoZSBhcmNoaXRlY3R1cmUgb2YgdGhlIFBDTTQxMDQgaXMgdG9s ZXJhbnQgdG8KICAgcGhhc2Ugaml0dGVyIG9uIHRoZSBzeXN0ZW0gY2xvY2ssIGl0IGlzIHJlY29t bWVuZGVkIHRoYXQKICAgdGhlIHVzZXIgcHJvdmlkZSBhIGxvdyBqaXR0ZXIgY2xvY2sgKDEwMCBw aWNvc2Vjb25kcyBvciBsZXNzKQogICBmb3Igb3B0aW1hbCBwZXJmb3JtYW5jZS4KRm9yIGEgdHlw aWNhbCAyNC41NzYgTUh6IGF1ZGlvIHN5c3RlbSBjbG9jayB0aGF0IG1lYW5zIG1heCAwLjI1JQpq aXR0ZXIuIFdob29wcyA6UAoKTm93IERQTExMSiB3aWxsIHByZXN1bWFibHkgZG8gYmV0dGVyLCBi dXQgd2l0aG91dCBhY3R1YWwgc3BlY3MgdGhlCnNhZmUgb3B0aW9uIGlzIHRvIGF2b2lkIHRoZSBm cmFjdGlvbmFsIG11bHRpcGxpZXIgYWx0b2dldGhlci4KCk1hdHRoaWpzCgpfX19fX19fX19fX19f X19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fXwpsaW51eC1hcm0ta2VybmVsIG1haWxp bmcgbGlzdApsaW51eC1hcm0ta2VybmVsQGxpc3RzLmluZnJhZGVhZC5vcmcKaHR0cDovL2xpc3Rz LmluZnJhZGVhZC5vcmcvbWFpbG1hbi9saXN0aW5mby9saW51eC1hcm0ta2VybmVsCg== From mboxrd@z Thu Jan 1 00:00:00 1970 From: matthijsvanduin@gmail.com (Matthijs van Duin) Date: Wed, 18 Nov 2015 11:01:56 +0100 Subject: Minimal support for dm814x In-Reply-To: References: <4999BB3D-4BB5-4F7C-96D7-FB687725CDCC@audioscience.com> <20151109150602.GR3199@atomide.com> <7933071D-84FD-4A85-8CD0-CC504CE1D9D1@audioscience.com> <20151111174028.GF3218@atomide.com> <20151112174155.GM3218@atomide.com> <20151113145216.GC2517@atomide.com> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 18 November 2015 at 09:26, Delio Brignoli wrote: >> This works in principle, but both minimizing the DCO and (often >> needlessly) using the fractional multiplier seem like recipes to >> maximize the clock jitter. Mind you, I don't know how much jitter >> we?re talking about here, I don't recall having seen specs about this. > > We haven?t seen any specs either but testing shows that changing DCO mode causes > the PLL to lose lock at least temporarily. Losing lock on reconfiguration is entirely a separate matter from clock jitter. The fractional multiplier works by essentially by alternating between the nearest integer multiplier values. This will be smoothed out by the loop filter, but it's not going to vanish. To put this in perspective, some docs (I can't immediately find which one) warned that when using the fractional multiplier of DPLLS, its value needed to be at least 100 to ensure max 2.5% jitter. To put this in perspective, if I grab the datasheet of an audio DAC I find: Although the architecture of the PCM4104 is tolerant to phase jitter on the system clock, it is recommended that the user provide a low jitter clock (100 picoseconds or less) for optimal performance. For a typical 24.576 MHz audio system clock that means max 0.25% jitter. Whoops :P Now DPLLLJ will presumably do better, but without actual specs the safe option is to avoid the fractional multiplier altogether. Matthijs