G7X battery 4th terminal for communication - Hotwire! Hardware Mods, Accessories and Insights - CHDK Forum
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G7X battery 4th terminal for communication

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Offline Recyclojunk64

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G7X battery 4th terminal for communication
« on: 28 / December / 2020, 09:22:04 »
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Hi all,


So I've noticed that the batteries for my G7X cameras have 4 terminals instead of 3. Applying a piece of tape over this it gives a battery communication error indicating that there is some communication going on between the camera and battery over this terminal. If this could be used as a serial input/output terminal it could be very useful for many things such as for my StereoSync project https://chdk.setepontos.com/index.php?topic=14195


I did some searching on this forum all I came across was this post: https://chdk.setepontos.com/index.php?topic=13451.msg137584#msg137584


Unfortunately my digital oscilloscope is currently broken so I just probed them with my analog scope for now, the result is attached. Scale is 0.1ms and 2V
Each bit is about 20us in length. The camera seems to transmit many packets (with different bits each time) for about a second before displaying the battery communication error.


I will try and borrow a digital oscilloscope soon or set it up with a logic analyzer to see what communication is going on here. Perhaps it could be 1-Wire serial protocol?


Any ideas of how to go about reading and writing to this pin with CHDK?
« Last Edit: 28 / December / 2020, 09:24:58 by Recyclojunk64 »
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Offline reyalp

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Re: G7X battery 4th terminal for communication
« Reply #1 on: 28 / December / 2020, 14:46:23 »
You can find various things related to the the battery authentication in the the firmware (the string are relatively obvious), but it's an area we've mostly avoided for reasons discussed here https://chdk.setepontos.com/index.php?topic=12542.msg130630#msg130630 I believe third party sellers have now widely bypassed the battery validation for that generation of camera, so possibly less relevant now.

Note on some cameras at least one battery related task runs on the Omar core: https://chdk.setepontos.com/index.php?topic=11316.msg142038#msg142038

I'm not clear why you need this rather than one of the existing inputs, I guess you're expecting / hoping for some kind of 2 way communication? FWIW, on g7x you should be able to do that with HDMI power control and hot plug detect. Alternately, the HDMI interface also includes i2c if you want to reverse engineer that. I'd expect that to be a lot of work to repurpose for general communication, but it would be broadly useful and potentially applicable to many cameras if you figured it out.

Or you could include a pi zero in your rig and control it all with PTP :)
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Offline Recyclojunk64

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Re: G7X battery 4th terminal for communication
« Reply #2 on: 28 / December / 2020, 23:09:03 »
Thanks, yep after 2 way communication. I started looking into this before I found out about the HDMI power control output which could also work. I guess the advantage of this is that everything is nicely self contained into a dummy battery without cables plugged into the side (which would force the cameras to be spaced out more depending on how small the HDMI connector is). And the bitrate will be faster and more reliable as it is presumably a proper serial port rather than bitbanging with interrupts as with the HDMI power control and hot plug detect (or what I was doing before with the battery temp terminal and AF LED).

However the disadvantage is that only a few cameras have this 4th terminal, whereas all of the non-ancient ones have mini/micro HDMI. Do you know if the HDMI power control output and the hot plug detect input can work on all of the HDMI cameras or only the newer ones? If it can also work for S100 and S110 then that would make this a more attractive option than putting in the work to get the battery 4th terminal communication working but only being able to use it on the G7X and M series cameras.

HDMI I2C sounds like a good idea I will have a look into that and start another thread if make any progress with it.

As for PTP it could possibly work as well if I used another input for syncing the shutters, though the HDMI does sound like a better idea at this point.
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Offline reyalp

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Re: G7X battery 4th terminal for communication
« Reply #3 on: 29 / December / 2020, 01:18:29 »
Quote from: Recyclojunk64 on 28 / December / 2020, 23:09:03
Do you know if the HDMI power control output and the hot plug detect input can work on all of the HDMI cameras or only the newer ones?
Not all cameras are able to control the HDMI power out. Those with CAM_REMOTE_HDMI_POWER_OVERRIDE definitely can, but most models haven't been tested, and we don't currently have an automated way to detect which ones can. Some discussion of models which are known to be able to do it or not in
https://chdk.setepontos.com/index.php?topic=13451.msg139364#msg139364
https://chdk.setepontos.com/index.php?topic=13451.msg139575#msg139575
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If it can also work for S100 and S110 then that would make this a more attractive option than putting in the work to get the battery 4th terminal communication working but only being able to use it on the G7X and M series cameras.
From the thread above s110 doesn't seem to have HDMI power control, at least not with the simply named function that we use on the other cameras

Note CAM_REMOTE_HDMI_POWER_OVERRIDE doesn't directly refer to the ability to control HDMI power: These cameras turn off HDMI power in rec mode, so need it forced on to use HPD remote. However, there could be cameras which can control the power but don't automatically turn it off in rec.

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As for PTP it could possibly work as well if I used another input for syncing the shutters, though the HDMI does sound like a better idea at this point.
You can do both PTP and hardware remote trigger on USB with usb_force_active and a custom cable but having them separate is certainly simpler.

One other caution: The latency of the non-USB inputs has not been tested that I know of. I'd guess analog AV is likely similar to USB, since it's likely just a simple GPIO, but it requires a much trickier custom connector. HDMI stuff is potentially a lot more complicated, since my impression is the entire HDMI interface is controlled over i2c.
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Offline Recyclojunk64

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Re: G7X battery 4th terminal for communication
« Reply #4 on: 29 / December / 2020, 04:34:45 »
Quote from: reyalp on 29 / December / 2020, 01:18:29
One other caution: The latency of the non-USB inputs has not been tested that I know of. I'd guess analog AV is likely similar to USB, since it's likely just a simple GPIO, but it requires a much trickier custom connector. HDMI stuff is potentially a lot more complicated, since my impression is the entire HDMI interface is controlled over i2c.


If there are latency issues with the HDMI power out then that would be a problem. I will do some experimentation with this once I get a micro HDMI connector for it but doesn't sound very promising at the moment. Especially if it does not work with the S110 cameras as well.


The HDMI I2C option looks the most appealing at the moment especially with its other use cases, I might have a go at getting the I2C output working on one of my old S100 cameras in case I brick it and if it works hopefully the I2C functions will be similar to the G7X camera.
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Offline Recyclojunk64

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Re: G7X battery 4th terminal for communication
« Reply #5 on: 29 / December / 2020, 10:29:50 »
Looks like the I2C on the HDMI connector is done through the HDMI chip, at least for the S100 camera which has an ADV7521NK chip. So it's probably not as straightforward as I was hoping.
According to https://www.analog.com/en/products/adv7521nk.html#product-overview the I2C is done with "On-chip MPU w/ I2C® master performs EDID reading operations" so this would require not only hacking the I2C going to this chip but also getting the on chip MPU here to forward other packets to it's I2C master. Even if possible it might still have latency issues.


I haven't checked what HDMI chip the G7X has in it, I just took the back cover off and it looks like much more disassembly is required to access the mainboard. Even if it is different then that would mean that the same solution is not likely applicable to all HDMI cameras.


Looks like my best options are the HDMI power output if I measure the latency low enough, the battery 4th terminal which I will probably have a go at, or the fall back is a Pi Zero with PTP and a low latency input.

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Offline c_joerg

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Re: G7X battery 4th terminal for communication
« Reply #6 on: 29 / December / 2020, 11:25:34 »
Quote from: Recyclojunk64 on 28 / December / 2020, 23:09:03
AF LED
This is what i'm doing to control an external device. Is not very reliable.
Unfortunately, my cameras do not support HDMI control.
I still hope for another solution....
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