revD minispec

[whitequark]

Summarizing the broad design points regarding revD in one place:

1. Four I/O banks (ABCD) instead of two (AB)
2. Placement of AB and CD so that addons developed for revC work on revD side by side (at least if width allows). That means relative position of AB and CD is the same as AB on revA-C.
3. No LVDS bank.
4. Goal: we should provide I2C and power to ports. Proposal#1: put I2C on NC pins in the 20-pin connector, put GND/3V3/GND/5V on a pin header next to 20-pin connector. Proposal#2: put SDA/SCL/GND/3V3/GND/5V on a pin header next to 20-pin connector, leave NC pins as NC.
5. Goal: I2C on ports should not impact reliability due to faults such as stuck bus. Proposal: use 1:4 I2C level translating passthrough switch such as PCA9545A with dedicated reset pin.
6. Problem: we run out of I2C addresses (definitely on PCA6408, possibly on DAC081 without change of package). Proposal#1: change PCA6408 part and DAC081 package. Proposal#2: expand passthrough switch from (5) to 1:8, use identical parts to revC (simplifies production!). Proposal#3: Change only DAC081 package to allow four addresses.
7. Placement/layout sketch: https://marcan.st/transf/Screenshot_20190510_145158.png
8. Proposal: replace ADC with INA233 to allow for precise current and voltage limiting (Replace ADC with INA233 #165). Existing 0.68 ohm resistor BOM line could be used for the shunt. Vsense range is also expanded to 0-36 V.
9. Proposal addendum to (8): Connect INA233 ALERT pin directly to LDO EN pin, adding a resistor between FX2 GPIO and LDO EN. This way, any voltage or current fault condition will lead to immediate port shutdown, significantly improving safety.
10. Consideration: if there is free board space, some sort of low pin count RAM would help with streaming workloads. Only a "nice to have" and should be dropped if troublesome at all. The RAM chip should be PSRAM, either QSPI or HyperRAM.
11. Consideration: if there is still free board space left after (10), perhaps on underside, add a DNP footprint for SPI Flash connected in parallel to RAM.
12. Proposal: expose Vref DAC output on extension connector from (4) to accomodate @electroniceel's advanced IO protection board.

[electroniceel]

Placement of AB and CD so that addons developed for revC work on revD side by side (at least if
width allows). That means relative position of AB and CD is the same as AB on revA-C.

This could be solved by an adapter board which can be plugged into revD and which provides connectors with the same width as revC.

For this to work revD has to be wider and the position of the new connectors must not intersect the position of the revC ones.

[whitequark]

Huh? Why use an adapter if we can just not have one?

[electroniceel]

It might make layout easier if we want/need to use a different layout than what marcan proposed.
Such an adapter board would be dirt cheap and I don't think many users with revC addons would mind.

[whitequark]

But it needs to be produced, packaged, sold, assembled, etc. And then you have to not lose or forget it.

The only thing better than solving a problem cheaply is to not have it in the first place.

[electroniceel]

Another point: my I/O protection board will work easier and better if it has access to the IO voltage DAC output. I suggest to put that on the addon pins too. We could replace one of the GNDs proposed above, there are enough GND pins on the port header already.

[whitequark]

We are not going to change any GND pins on the connectors to non-GND.

[whitequark]

Oh sorry, you meant one new GND pin. That could work.

[whitequark]

The reason I suggested using this many GND pins on the extension connectors is to make shorts between adjacent pins a little less destructive (short to GND generally stresses the LDO, short from 5V to 3V3 will stress all logic on 3V3 too).

[electroniceel]

yes, the new header. Not change the existing 20 pin header.
I suggest using female connectors for the new header. It is harder to short these.

[whitequark]

That makes sense. I think they should somehow be keyed, too.

[whitequark]

Another point: my I/O protection board will work easier and better if it has access to the IO voltage DAC output.

I'm confused. Isn't that just Vio already?

[electroniceel]

What is the use case for this keying? If you plug in single wires, a keying won't matter. If you plug in full addon boards, these won't fit into the 20 pin header the wrong way, so the other connector fits too.

I think of regular female pin header, something like this: https://lcsc.com/product-detail/Pin-Header-Female-Header_HOAUC-2685Y-106CNG1SNA01_C350307.html
Getting keyed ones might be more expensive.

[whitequark]

I guess it's fine. If it's not centered wrt 20 pin header, then plugging it the wrong way wouldn't result in electrical contact even if you try to plug in the board one pin off and it skews.