Cameras: External Trigger on GS: Please check voltage

[njhollinghurst]

Another one like #3146

https://www.raspberrypi.com/documentation/accessories/camera.html#external-trigger-on-the-gs-camera

According to the IMX296 datasheet, XTRIG should be limited to OVDD+0.3V where OVDD=1.8V; but the documentation suggests it's safe to apply 3.3V to XTRIG.

The GPIO->XTRIG connection on the camera board uses a level shifter, but if R11 is removed as instructed then there will be neither level shifter nor pull-up, so it is hard to see how this can safely be driven from a Pico.

Asked on forum here https://forums.raspberrypi.com/viewtopic.php?t=358276#p2148485

[njhollinghurst]

It's definitely dodgy, the question is what is the most elegant circuit to recommend in the doc. Need to decide whether to recommend a level shifter or if another resistor divider will suffice (maybe with smaller resistances, e.g. 680R or 1K, as it's now the Pico that's providing the current).

By the way, Pico can't conveniently do PWM + Open Drain (except using PIO), so an OD/Pull-up style is no good here.

@naushir @6by9 might have comments.

[Edit: Not sure why the doc calls for a Pico here rather than a Pi. Just for variety I suppose.]

[aallan]

[Edit: Not sure why the doc calls for a Pico here rather than a Pi. Just for variety I suppose.]

That's what Simon's summer intern used I think?

[simon-martin-rpi]

Speaking to Nick. We keep the docs the same, i.e. remove R11. This guy needs to add a resistor divider to his output to reduce 3.3V to 1.8V. I suggest 18K to GND and 15K input resistor. Since the whole task needs soldering, I guess they will be Ok with it. I hope to respin this board and put a proper connector on it which accepts any voltage at high impedance one day. We have a lot of units with the wrong circuit to sell first though.

[njhollinghurst]

Got it. So we advise removing R11 in all cases (not just if Q2 fitted), then can use a divider (need to change picture).
Will we need to revise the documentation for the new new board version?

[njhollinghurst]

Nitpick: The current text is also wrong where it says "This connects GP0 to XTR", since this is GPIO1.

Perhaps the entire paragraph should be something like: "You will need to remove R11, shown in red below, to enable external trigger input."

Somewhere or other we should state clearly that it's a 1.8V signal.

[njhollinghurst]

I'll attempt a PR for this.

Still wondering about resistor values in #3146 -- it would be paradoxical for us to recommend much larger resistances for a driven signal than for an idle pull-up -- so I might revisit that as well. Will do some testing first!

[aallan]

I'll attempt a PR for this.

Thanks @njhollinghurst!

[njhollinghurst]

This guy needs to add a resistor divider to his output to reduce 3.3V to 1.8V. I suggest 18K to GND and 15K input resistor.

@simon-martin-rpi Those resistor values seem high compared to the (entirely unrelated) pull-up we suggest here. Can I reduce them to (say) 1.8K and 1.5K? That would draw 1mA from the RP2040.