[time-nuts] How to clock a Beaglebone Black from an external referenceL

[Simon Marsh]

I'd certainly be interested, and any suggestions for doing a time-nut 
suitable 10 to 24mhz step up gratefully received.

Cheers


Simon


On 14/11/2014 18:50, Neil Schroeder wrote:
> This is fantastic.  If anyone is interested i would be happy  to share my
> design for a couple of PLL/oscillator boards in cape form factor that would
> feed this perfectly.  I've been waffling on my choice of microcontroller
> but in the spirit of this application I'll likely go  with an msp430.
>
>
> On Friday, November 14, 2014, Simon Marsh <subscriptions at burble.com> wrote:
>
>> Most processors can be clocked from a variety of sources and we know that
>> with a bit of hacking it can be possible to connect them up to a
>> time-nut-standard reference (either directly for simple microprocessors or
>> with a synthesizer/pll).
>>
>> The Beaglebone Black is my weapon of choice when it comes to embedded
>> boards and being able to lock it to an external reference should give some
>> obvious benefits, such as being a great NTP server (e.g. a more modern
>> equivalent of the Soekris boards) to having access to a large number of
>> timers & peripherals synchronised to the reference. The BBB requires a
>> 24mhz clock to operate, so the end goal here will be to get it running from
>> a 10mhz reference multiplied up by a PLL.
>>
>> The TL;DR summary is that despite the scary amount of tiny surface mount
>> components on the board, the modifications actually turned out to be quite
>> simple and, on first look, the result is great performance.
>>
>> So here's how to do it.
>>
>> Modification Details
>>
>> The BBB contains a TI Sitara AM3358 SoC and section 6.2 of the relevant
>> datasheet (http://www.ti.com/lit/ds/symlink/am3358.pdf) details the
>> various clocking options for the core. The key bit of information is that
>> the core will automatically use an external crystal or an LVCMOS clock
>> source and does not require any specific configuration to be made either
>> way.
>>
>> The schematic for the BBB is readily available (
>> https://github.com/CircuitCo/BeagleBone-Black/blob/master/BBB_SCH.pdf)
>> and the upper left corner of page 3 details how the crystal on the board is
>> connected.
>>
>> Together, the datasheet and schematic suggest that hooking up the BBB to
>> an external LVCMOS source should be as easy as simply removing the existing
>> crystal and attaching the source to OSC0_IN (pad 2 of the crystal). The
>> crystal is marked as Y2, has a couple of supporting capacitors (C25 & C26),
>> and an associated resistor (R17).
>>
>> The crystal is nicely marked up on the board itself and is easy to spot.
>> It's on the underside and attached are a couple of photos for reference.
>> The photo is of a Rev C. Element 14 BBB; earlier revisions of the board
>> have a different, large, black crystal but the board layout is the same.
>>
>> The main risk with removing the crystal is the proximity of all the tiny
>> surface mount parts, but it turned out to be very simple with a basic hot
>> air gun and some tweezers. I also removed R17 (the spec of dust sat between
>> C25 & C26), as the SoC datasheet stated OSC0_OUT should be left
>> unconnected. The whole process was suprisingly easy, took less than a
>> minute and I didn't need to resort to any magnifying aids.
>>
>> The location of C25 & C26 help understand the orientation of the crystal,
>> the external source needs to be attached to the pad nearest C25. This is
>> the left hand pad in the photos. After the crystal has been removed, the
>> remaining pads are nice and big making soldering of a coax cable
>> straightforward.
>>
>> A final photo shows the crystal and R17 removed, and with coax attached.
>>
>> Test & Performance
>>
>> In order to check the change was working, I clocked the BBB using a
>> MicroCrystal OCXO connected to a cheap PLL-on-a-chip. The PLL I used has
>> woefully few specs with regards to jitter etc, but had the virtue of being
>> to hand, operated at 3.3v and directly provided a 2.4 multiplier to get
>> 24mhz needed for the BBB. The BBB was connected to an adafruit GPS breakout
>> and the lot was left out overnight on an open desk running NTP and using
>> the gps as a PPS source.
>>
>> I'd intended to provide some nice graphs from NTP, but in practice the NTP
>> jitter flatlined at 4us and the offset all night was practically flat as
>> well, showing only occasional variation with maximums of +- 2us. This was
>> great from a performance view, suggesting performance is better than NTP
>> can report, but does make for some dull graphs.
>>
>> The frequency plot was barely more interesting but is attached; the scale
>> is ppm and shows a drift of less than 0.1 ppm over 12 hours; this I think
>> is consistent with the spec of the OCXO. Note the room is not air
>> conditioned and my heating comes on between 6am and 7am; there is a nice
>> lack of impact, as you would hope. For comparison, my RasPI NTP server
>> varies about 1ppm, with offsets of +- 50us corresponding to temperature
>> variations.
>>
>> Overall, this was quite a trivial test but nicely succesful.
>>
>> Internally the BBB has quite a few different clock domains so, longer
>> term, it will be interesting to see if the impact of the SoC internal PLLs
>> can be measured. Whilst not an issue for something as high level as NTP,
>> the PLLs will determine the detail of how the reference stability transfers
>> to peripherals like the BBB timers and PRU.
>>
>> Cheers
>>
>>
>>
>> Simon
>>
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