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

Neil Schroeder gigneil at gmail.com
Fri Nov 14 13:54:11 EST 2014

Also, simon the plls can be entirely bypassed. The circuits aren't clearly
exposed but I believe you could get the peripheral clock covered and
definitely the Ethernet clock -  interesting  from a 1588 perspective.

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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