I3C: 12.5 Mbit/s (SDR, standard), 25 Mbit/s (DDR), 33 Mbit/s (ternary)
I’d say have I3C as a basic option but design for additional PCIe/USB-C pins to have for TOH.
It is possible that the MediaTek smartphone SoC does not include a PCIe interface.
This could be more about the SoC than about the connector… I.e. what connections the SoC provides determines what the phone, including TOH, can use without additional components. Adding more components will have an immediate effect on cost, PCB complexity and device size.
If the SoC provides a decent bus that can be made available to TOH, I think that’s what we’ll get. It will then be up to TOH to add stuff to provide its functions. So, the cost/complexity/size will affect TOH but not the basic phone.
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Sci-Fi corner addition:
- mini DJ Mix controller - I love the portability of my Hercules Starlight, Mixxx is a multiplatform community project DJing software - support of the TOH-DJ could be contributed, internet is full of mini BT controllers for other platforms so this could bring some new guys and attention
- Gaming controller TOH - attach and switch your mobile phone into gaming platform, would be nice to have some game installer and management app ready together with it
- Connectivity Hub/extra-compute/docking-station TOH - transform the phone into a small PC, maybe just HDMI and some USB-C is enough for connecting BT keyboard+mouse+display, maybe there could be more, anything what’d make it usable and powerful
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Oh, yes - DJ Controller! 
I’d like to propose the “Audio TOH”.
Jeah, and if we can add some fancy features to the “Audio TOH”, it would be
(1) Software support for the HighResolution Audio Codes via Bluetooth.
(2) A 3,5 mm Audio Jack
(3) a mini equalizer using a small touch display to change settings and display fancy graphical eq like fancy 2000s animations known from winamp, or classically display signal power per frequency. Oooor hardware fader to setup the equalizer.
(4) Or make a “Multimedia TOH” includin, a micro HDMI port (known from raspi) and DAB+, FM, AM and WB (world wide emergency broadcast freq)
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I’d like one which you can unfold 2 speakers out, which are as big as one smartphone half (so something pretty decent for a smartphone than).
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Hardware support for USB DP Alt mode please. The OS support may take some more time to mature (e.g. Pixel 8 Pro, launched October 2023, it became usable around November 2025)
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Maybe a SPDIF?
20 characters
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More sci-fi (for uncertain times): a TOH (called ‘backburner’), into which one can backup the phone and then use the backburner TOH to generate a ‘burner’ phone for travel (leaving the TOH home). And then afterwards wipe the phone and use the TOH to restore everything.
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2FA TOH - 2nd Factor Authentication by The Other Half for FIDO2 / PIV
Let The Other Half be your 2nd Factor.
Implement a STM32L432, LPC55S69 or any other appropriate secure microcontroller into TOH and connect it with the J2. The solution could be based on Trussed(R) as used in the open source authenticators GitHub - solokeys/solo2: Solo 2 firmware in Rust and GitHub - Nitrokey/nitrokey-3-firmware: Nitrokey 3 firmware .
This solution would be for those that don’t want to use such authenticator keys via NFC or USB with the J2. Of course mechanical engineers could also provide ideas to design a TOH with a removable part for frequent travel between this TOH and your (physical) key ring. This would keep the original idea of a removable 2FA alive.
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LoRa with Meshtastic/Meshcore would initially be a niche product, but it’s growing quite rapidly. Wouldn’t it be amazing if Jolla released the first phone whose functionality could be expanded with something like that simply by adding an other half?
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I had all available ‘the other half’ and loved them a lot. 
According to the link posted by @Jolla, the TOH keyboard sold 1217 units. Someone knows how many Jolla Phone (v1) were sold? I would like to have an idea of the percentage of users who bought the TOH keyboard.
Someone correct me if I’m wrong, but I think the estimations (just estimations) were 50-70K devices.
Also as far as I remember TOHKBD was a community driver effort and Jolla tried to push it with a discount for the phone, but I don’t remember them being involved with TOH other than creating a few other colors.
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How I read that page is that 1217 took part in initial kickstarter. I thought those had been sold after the kickstarter but don’t know for factual
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Pinephone had a LoRa back that fits both the original and the Pro, plus a high power multi functional base unit. Very little software ever developed. The latesr Lilygo T-Deck Pro goes the opposite direction - oled colour touch screen, full companion LoRa plus sim card and phone function and SD card.
Interesting. The LoRa Other Half thing completely passed me by. I know the Lilygo T-Deck Pro. The direction is also interesting. But I’m now claiming, without being able to prove it of course, that it’s easier to teach LoRa functionality to a smartphone than the other way around 
5 posts were merged into an existing topic: Meshtastic on Sailfish OS
That’s right, but only as a companion so you have to carry two devices. The Lilygo Pro only a communicator with two functions -mobile phone if you have network coverage and LoRa if there’s a connection within range. A Lora back would enable carrying just one device. Meshcore also has the App in Aurora store but bluetooth will not connect in the App. It connects in settings to the LoRa companion but one permission cannot be enabled and blocks connecting to the App display. Meshtastic App menu shows a Bluetooth permissions menu which is not in the Sailfish Apps settings menu. Phew.