Saab mic

[Karlis]

Great work! Yeah, the whole magic behind Mic_bias and lack of info in the datasheet was confusing us a lot.

If you can't get a dev board from Microchip, consider getting this RN52 breakout board from SparkFun. This is the one I used in early stages of BlueSaab development. For access, you will need a 3.3V FTDI cable. Make usual UART connections and make sure PWREN pin is tied to 3.3V. Then you will have access to ASCII interface of RN52 and will be able to play around.

Let me know if you have any further questions about RN52 or BlueSaab in general.

[czabel]

Great, thanks for the link. Though, if that's what I have to do I might as well just buy the eval board. It's a little more expensive than the Sparkfun board, but I'll be able to just plug right in. We'll cross that bridge when we come to it though.

I've submitted a tech support case, hopefully someone can dig a little deeper and figure out how best to use that line. I did see that this module was not originally made by Microchip though, so that may be a futile effort.

If all else fails, I imagine we'd be able to apply an external bias to the mic and have it work... That just adds complexity and requires a new board rev though.

[czabel]

Quick update, I've got some information from the support staff at Microchip:

The MIC_BIAS line is regulated to provide anywhere from 0.199 to 1.299 mA at 2V nominal. The voltage is constant, and is specified to be +/- 10% of nominal.

When asked about the specified "large resistor to ground" in the datasheet, he replied that should imply a value of 100k, but it might be up to 1Meg in some cases. Either way, doesn't really matter, as the mic needs upwards of 4.5V to operate properly.

So the way I see it, we have two options: A new board revision with an external bias circuit, or we add an electret mic to the BOM and tell people to swap out the Saab Mic assembly for the new one.

Any thoughts?

[Karlis]

Nice work!

Not being "captain hardware" either option works for me. It would be nice to give both of the options a POC try first. I'd let Seth chime in on the hardware change to see what can be done. As for the electret mic - any ideas which one to pick? I could then give that a try myself.

[Seth]

I’m ok with adding a 4.5v power supply line for mic_bias, trick might be where to put it...I’m sure Saab owners would rather just plug something in rather than replace the mic...but maybe it’s gonna come down to cost vs ease...?

Seth

On Tue, Jul 16, 2019 at 16:29 Karlis [via BlueSaab Forum] <[hidden email]> wrote:

Nice work!

Not being "captain hardware" either option works for me. It would be nice to give both of the options a POC try first. I'd let Seth chime in on the hardware change to see what can be done. As for the electret mic - any ideas which one to pick? I could then give that a try myself.

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[czabel]

If a mic swap is desired, it might be interesting to build a mic PCB to match the existing Saab mic setup, that way it's just -Unplug Saab mic -> plug in new mic, and you're off to the races. This would also allow us to add the RC circuit that the bare electret desires on that PCB instead of a new board rev. Keeping it in that nice little Saab box would certainly simplify the install for end users, and you'll already have to get in there to run new wires.

If we don't want to swap, and instead want to add a bias circuit to the mainboard, I'd be happy to try and
 help with that as well - I've got a decent amount of experience in Eagle, and really enjoy creating board layouts. It's also a tiny circuit and would work with 5V (I'm assuming there's 5V somewhere on this board?).

As for an electret to try out, give this one a shot: DigiKey Part Number 102-5628-ND. The datasheet supplies a circuit to use, where Vs will be MIC_BIAS. Set MIC_BIAS to the minimum value before connecting, and move up from there. It shouldn't be many steps before you hit the 500uA required for operation. Let us know how you do! If that doesn't work, the RN52 eval board has a more complicated circuit we could try.

[Seth]

I kind of like the idea of replacing the Saab mic pcb with our own and new mic…BUT…where is it going to get power from (4.5v in this case)? There might be 12v somewhere near the footwell where the mic connector is…but also remember that this entire setup requires running wires from the front of the car to the trunk to bluesaab…

I know there’s a demand/desire for handsfree using bluesaab (RN52)…but I just don’t know if the amount of work required to make it happen is going to be worth the end result. But we can try.

I no longer have my Saab, so Karlis or someone else is gonna have to be the guinea pig :)

You’re more than welcome to try a stab at a pcb with Eagle, but having started with Eagle, then to Altium Designer, the pcbs were made in Circuit Maker (baby Altium). I HIGHLY recommend it; it blows eagle out of the water (once you get used to it of course).

On Jul 16, 2019, at 17:46, czabel [via BlueSaab Forum] <[hidden email]> wrote:

If a mic swap is desired, it might be interesting to build a mic PCB to match the existing Saab mic setup, that way it's just -Unplug Saab mic -> plug in new mic, and you're off to the races. This would also allow us to add the RC circuit that the bare electret desires on that PCB instead of a new board rev. Keeping it in that nice little Saab box would certainly simplify the install for end users, and you'll already have to get in there to run new wires.

If we don't want to swap, and instead want to add a bias circuit to the mainboard, I'd be happy to try and
 help with that as well - I've got a decent amount of experience in Eagle, and really enjoy creating board layouts. It's also a tiny circuit and would work with 5V (I'm assuming there's 5V somewhere on this board?).

As for an electret to try out, give this one a shot: DigiKey Part Number 102-5628-ND. The datasheet supplies a circuit to use, where Vs will be MIC_BIAS. Set MIC_BIAS to the minimum value before connecting, and move up from there. It shouldn't be many steps before you hit the 500uA required for operation. Let us know how you do! If that doesn't work, the RN52 eval board has a more complicated circuit we could try.

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[czabel]

I'm seeing two options for swapping the microphone:
1) If we already have to run two wires for the mic, why not run three? One of them would just then be 5V from BlueSaab.
2) On the BlueSaabMic PCB, we could include a buck converter to drop 12V to 5V for the mic bias. We'd still need to pull 12V from somewhere (what do the "Fasten Seat Belts" lights run on?)

As far as demand goes - Even if it doesn't turn into a proper revision, the best option in that case is just forgoing the main module revision and creating a new mic module. I've just decided that's what I'm going to do in either case.

No more Saab!? That's alright, it happens. I'll check out Circuit Maker, but I've got a pro license for Eagle, and I'm pretty comfortable in it. It also plays nicely with OSHPark.

Karlis, please let me know how you get on with that microphone! After this week I should have some time outside of work to play around, so I'll probably start ordering parts/kits. Fingers crossed Microchip gets back to me about that free sample...

[sbt]

Just a note to add that on the 9³, the mic signal, ground and shield wires are available in connector H16-5, which is for the roof wiring harness and is just above and forward of the CDC connector at rear left of the car.

I guess on the 9⁵, you'd have to tap into the 375 Tel connector in the centre console.

[homerisback]

Hi guys,
Just read latest news and that's good news !
@czabel I'm also familiar with Eagle pro and didn't achieved (maybe took enough time) to be as comfortable with Circuit Maker, so I started a v6.1 from scratch under Eagle and build a bunch of 3 that work very well.
If you want I'd be happy to send you my files as a basis.

No 5V available from existing BlueSaab boards. Adding a 5V regulator on either BlueSaab or Mic board could be done easily but doing it on Mic board would allow to retrofit older boards imho.

@sbt I'm considering searching for H16-5 connector in my 9³ ;) Thanks

Great work anyway !!

[czabel]

No 5V on the board, eh? Then yeah, we'll want to do that on the board... But if we're having to add that on a new mic board, then we don't want to use the Saab mic, and we can use one that will play nicely with the 2V MIC_BIAS line. We'll still need 3 wires to the mic, but I don't see that bring a huge deal.

I'll see about drawing up the new mic board tonight. Should be very simple.

[czabel]

Quick update for those interested:

Microchip does not give out samples of their dev boards/kits, but I did get a discount code. If I have trouble programming on the fully built BlueSaab I'll probably grab a dev kit.

I've also found a microphone that is a physical drop-in replacement in the Saab mic housing, and should also work with the RN52 BIAS line. It's the CMC-9745-130T made by CUI. I just ordered a few to test with at the same time I ordered a set of everything for a BlueSaab 6.1.1. The only downside is there's no decent mic that I could find with pins instead of solder pads, so the soldering and assembly may be a bit fiddly. Though, if nobody really wants to add this, maybe it's not such a big deal! :)

I'll keep updating as I get my BlueSaab built and start to integrate with the microphone.

What are you guys using to interface with everything once it's assembled into a full BlueSaab? Is it still the FTDI UART/USB cable? Do I need a separate one to talk to/program the STM32?