Optional external MCLK implementation on the Zero?

[GastonP]

Hi,
It would be nice to be able to select an external MCLK input when the on-board crystal stability is not enough or when synchronized operation of several audio board is desired.
This requires (besides some minimal changes in configuration or OS driver) that the on-board crystal be disconnected from the MCLK pin, and that pin to be brought to some point of the board which is accessible. Ideally it should be a couple pins (MCLK-GND pair) but just a pad near the board edge will do too.

As this is a non-trivial use and aimed to "advanced" users (fanatics of low-jitter feel free to include yourselves in the list too), IMO this could be done with a semi-permanent implementation known as solder-bridge selectors: exposed pads (pads without solder mask nor vias that are close enough to be bridged by a solder blob) that can be bridged with a drop of solder to make a connection or left unbridged when that connection is not desired.
An implementation that does not add any additional work (and cost) for users that are comfortable with the on-board crystal is to design the pads that connect the crystal and the MCLK pin on the CODEC shorted by the thinnest possible copper track, and leave the ones that connects the MCLK pin to the external "pin" open.
Thus, out of the box the board uses the crystal and no additional setup or manufacturing cost is incurred. If one wants to use an external MCLK, then cuts the track that shorts the "crystal select" and shorts the ones that connect the MCLK pin to the external world with a blob of solder.
Leaving pads for the crystal means that whenever one wants to get back to the factory settings, the only required tasks are removing the solder blob in the connection to the external pins and shorting the ones that connect the crystal.

Gastón

[flatmax]

I Think your suggestion is a good idea. Lets just double check.
We have a solder jumper which is disconnected for an external MCLK connection point. The people who want to use this would cut the current MCLK track, disconnecting one side of the XTAL and solder the jumper pads to enable external connection. Is that right ?

Matt
p.s. we are talking about this project http://www.audioinjector.net/rpi-zero

[GastonP]

Actually, the best option would be to have two set of pads (solder jumpers). One shorted from factory (no need to do that with solder, a thin track between the two pads will do) that connects the crystal and the other one open, which would be used to connect the external MCLK.
The first solder jumper should be cut to disconnect the crystal, and would eventually allow people to re-connect the crystal in a not extremely blotchy way if desired. But obviously it is not essential.

BTW... thanks for being open to the suggestion!

Gastón

[flatmax]

It is a good idea.

Lets try to expand the topic a little more. In this approach, you would need a common ground as well right ?

I have seen situations where clocks have been connected together to force them to entrain. In other words, we add a pad or through hole pin on the clock line and you can connect them together as well as a common gnd. This should entrain the clocks of multiple cards so they are synchronous !

You could test this idea out by getting two audio injector cards which are available through Amazon or Ebay and connecting their gnd and one of the crystal pins together. You should see that clocks entrain.

What do you think ?
Matt

[GastonP]

Hi Matt,
Sorry for the delay in the answer.
To have a common ground pin available is always good, and it is better better if it connects directly to or, even better, if it is placed near the DGND pin to avoid the introduction of noise to the analog section of the board.
Ground loops are something to avoid at all costs so it would be good to warn potential users that proper grounding practices must be followed when using this feature of the board.

Gastón

[flatmax]

OK - sounds good.
In that case, what if people use the DGND on the GPIO header ?
Also I was thinking of changing the extra through hole on the Zero to an extra SMD pad ... see any problems with that ?

thanks
Matt