Design demands, features request, opinion for several details

This is my comprehension and demand several details of phone design.

More flexible and customizable device by design more users will buy and use it.

Generally items below was sorted in descending priority inside units too.

1.Hardware related details mostly

1.1 Absolutely necessary

1.1.1 Full miniPCIe connector (with PCIe and USB) for cellular modem.

Flexibility to set any modem card at any time.
The user can change his mind and remove card at all.
The designers of phone not need to make final solution about modem type at early stage of design and later can change vendor at final stages.
Or even sell mix of modem options to end user, very useful to fulfill different national service providers requirement worldwide.

(Future open modem design (FreeMT(FreeMobileTermination)) can use (OsmocomBB or srsLTE with) XTRX card installed and additional small PA to conform cellular standards).

IMHO Seems to most cellular modem modules use usb2 as main interface and non miniPCIe compliant audio PCM input/output pins (45,47.49,51), but this pins was reserved in miniPCIe standard, and probably will not interfere.

1.1.2 Removable rear cover to access all changeable parts inside (modem, SD/MMC, switches, battery, other modules and cards)

This allow simple access to many thing inside (and probably to resolder strap resistors)

Made as simple flat sheet (metal, polycarbonate, CFRP, etc.) screw up with six or more screws to side frame.
This simplicity allow easy customization in future, even end user can produce and/of modify cover to it needs
for attaching special clip or equipment.

1.1.3 Hardware Kill Switches for periphery as many as possible

DIP switches switch panel looks fine for this purpose. (If phone will be equipped with stylus it will help).
With two exceptions for high operational load switches for mic and cam:
For mic need near (locally) placed switch for nail action.
For cam most effective mechanical sliding cover embedded in case (this is not eliminate need for electrical HKS (DIP switch)).
All other moderate operational load switches should be fine with DIP switches.

Using ordinary DIP switches can improve repair ability of the phone.

1.1.4 Industrial temperature range -40…+85C
Mostly concerned electronic components with probably exception for display (not critical part) and Li battery (for temperature below -10…-20С suggested to use external battery with different chemistry)

1.1.5 SIM Slot size: Mini-SIM (2FF)

1.1.6 Two DDR IC with x16 data bus width to form 32 bit bus width (to maximize transfer speed) with signals routed to as maximum pads (address lines at first) as possible (DDR4 pinout is standardized) so anyone can upgrade/downgrade to desired amount of RAM by resoldering.

1.1.7 At least 1GB RAM


1.2.1 Selectable boot devices (one of two options selectable by HW switch (without case disassembly)):

  • SD/MMC (external slot (primary) first try, internal slot second try)

1.2.2 Half miniPCIe connector (with PCIe and without USB) for WiFi or WiFi+Bluetooth card.

1.2.3 3.5mm audio socket with nokia pinout (male: from housing to tip: gnd, mic, right, left) (! not apple: mic, gnd, right, left)

1.2.4 At least one UART (pads on PCB) (without level translators) for boot and debug and working(login) console.

1.3. Desired

1.3.1 Separate module/chip for Bluetooth.

But usb hub is necessary if Bluetooth card will be separate module/chip (i.e. not in combined WiFi+Bluetooth mini PCIe card) with USB interface.
Mapping USB for this:
USB Port which support Serial download connected to external connector.
Another USB port of i.MX8M connected to hub.
Hub downstream ports:

  • Full miniPCIe connector
  • Half miniPCIe connector
  • Bluetooth
  • Smth. else
    And of course hub is USB 2.0 (more not needed in this config) Removable Bluetooth module

1.3.2 (1.2.1 expansion) Selectable boot devices (one of three options selectable by HW switch (without case disassembly)):

  • SD/MMC (external slot (primary) first try, internal slot second try)
  • Second NAND/NOR i.MX8M BOOT_MODEx pins connect to DIPswitch (internal) (2 bits)
Used mainly to select serial download or internal boot mode while in fabrication/debugging.
Experienced end user also can use it to extend phone functions. All i.MX8M SAI1_RXDx and SAI1_TXDx (BOOT_CFG[*]) (16 bits) pins connect to resistor (which is not populated)
Exception can be made to signals connected to output of boot device selection logic (1.2.1, 1.3.2)
For end user can latter change and/or “make permanent” boot device selection.

1.3.4 Mounting pads for LEDs (according to miniPCIe standard) connected for both miniPCIe connectors.
User can solder LEDs or wires if needed. To debug modem card.

1.3.5 At least mounting pads for 0402 resistor connected to W_DISABLE# (and GND) for both miniPCIe connectors.
If user want to separate HKS (i.e. (probably) power off) and “logical off” there will be option (soldering and extra switch of course required)
May be needed for debugging of not expected behavior of particular modem card.
Maybe one of extra reserved switches on DIP switch panel of HKSs can be used here.

1.3.6 Extra DIP switches on DIP switches panel (HKS panel).
Couple of switches probably left unused while selecting available for buy DIP switches.
User can use it for future updates of HW functionality or use for own purpose (e.g. control XTRX).

1.3.7 (1.1.7 expansion) at least 4 GB RAM

1.3.8 Several status LEDs on front (or side)

1.3.9 Dedicated buzzer
with output to side (not to front or rear side)

1.3.10 Vibrating alert motor

1.3.11 (1.2.4 expansion) At least one UART (no need level translators) for boot and debug and working(login) console with connector (something like PicoBlade) on PCB.

1.3.12 If there will enough space on top of system PCB place extra contact pads (to solder/tap) form various i.MX8M interfaces:
about 8 GPIO, SPI, extra UART, SPDIF, SAI, etc.
No one know when this really can be used, but sometimes it is very useful redundancy :wink:

1.3.13 As many test/tap pads on PCB connected to various subsystems of the phone as possible
User can later debug his own extension of SW and/or HW of the phone.

1.3.14 Stylus (passive stick without any electronics)
reside inside phone case along long side.


1.4.1 Two Full miniPCIe connectors.

1.4.2 Connector to external cellular antenna (move it to required section if modem not equipped with standard U.FL connector(s))

1.4.3 Dedicated power socket to connect external battery

  • with charging capability for LiPo
  • Or/and power adapter.

1.4.4 Good 96k or 192K 20+ bit DAC for external 3.5 audio output and low THD+N headphone amp.

1.4.5 3.5 mm microphone/line in dedicated socket

1.4.6 16GB RAM (8GB max supported by i.MX8M)

1.4.7 Good ADC for line input

1.4.8 Several unassigned push-buttons

Below some sort of reference/reminder about i.MX8M features related to topic.

i.MX8M interfaces (not all available types):

  • Two PCIe
  • Two USB
  • Two uSDHC
  • One Giga Eth
  • Four UARTs
  • None SATA

Exact mapping could be viewed latter, because in i.MX8M every intance of same interface has specific differences
(related to boot process), but more deep exploration will be meaning after i.MX8M will be finaly chosen.

Boot beahavior of i.MX8M is not very clear from current revision (Rev. 0, 01/2018) of Reference Manual
Some of notable things:
6.1.11 SD/MMC manufacture mode boot (before entering USB serial download mode) Manufacture boot port = USDHC1 (not clear how)
6.1.9 Recovery devices = USDHC2 (not clear how)

2. Mechanical design notes

The 5.5-inches screen size most probably correspond to overall case size 158x78 mm.
Necessary area dimensions for Full miniPCIe Card is 30x56.05mm (with connector)
Necessary area dimensions for Half miniPCIe Card is 30x31.90mm (with connector)
Highness (from system board) is 5mm (for both)
So long size of Full mini PCIe Card fit to short size of case with more than 10 mm remainder each side.
More than enough for case side_frame.

Estimation of case thickness.
2 mm screen
1 mm system board
5 mm miniPCIe
1 mm rear cover
1 mm clearance (total)

10 mm max

10 mm thickness is nice for me. And it is not big pay (if any) for flexibility to set any modem card at any time.
And more thickness could be used to install battery with more capacity.

Probably in this design main and complex base element will be side_frame.
Screen, system board, rear cover, and some others elements will be mounted on side_frame.

3.Aluminum alloy case vs Antenna performance.

This is not clear area.
But my preferences is antenna gain and overall performance is more important than metallic case.
In case of custumizable (metallic or plastic) rear cover (and attaching accessories to rear cover) question become much complex.

4. Certification (FCC) and timeline.

Most probably for every cellular modem which will planned to used with will require certification procedure.
Hence maybe better sell (formally) not the phone, but assembled kit, to fit timeline,
with end user responsibility for using cellular modem(s).

And it seems FCC will never certify device with open mobile termination software stack :frowning:
I will glad to mistake here.

List of related forum topics corresponding to paragraphs

+ mean topicstarter request is similar
- mean topicstarter request is dissimilar
~ mean relaxed relation


























1.3.8 and 1.4.8




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I have my reservations about too much plastic as cover.
I think other materials are better for the environment and more robust and also for that price plastic would feel a bit too cheap in my opinion.

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I don’t know.
Energy to produce one 1kg of plastic from crude oil is about 17-31 kWh.
Aluminum requires 60 kWh
Magnesium - 80 kwH.

As long as you don’t throw them into a garbage, plastics are having the least impact on the environment. And the electromagnetic properties of plastic can be tuned so that it does not impair connectivity. Metal case - not so much. What other materials are there besides plastic and metal? Wood? Bamboo? Paper? Ivory? Leather? Stone? Glass? Rubber?

One up for plastic from me. Or carbon fibers.


I’m fine with plastic as a case material. From an engineering perspective, it’s perfect for phones - it’s radio transparent (which metal is not) so antenna design is much easier, and it can take an impact without shattering into a zillion pieces (what kind of idiot thought that making an easily dropped object out of glass was a good idea).

Also, plastic doesn’t necessarily feel cheap. Nokia’s Windows Phone lineup was made of plastic, and they felt and looked amazing.


While radio design is a valid concern, I hope for something else than plastic. Metal feels good, if you drop it it may get a dent but it won’t break. Given the thickness of the casing, phones made out of plastic get micro cracks over time even if you don’t drop them (my experience with that material).
If the phone’s not destined to land on trash after a year like most phones due to their lack of updates, the increased energy needed shouldn’t be the deciding factor. Better have something that feels good and is durable.

Hi @Raven,

I understand your point. Still, I keep in mind the internet-famous warrior : the Nokia 3310


It was made of sturdy plastic, and could last very long without cracks even after many drops. An elder of mine kept it during 15 years, and it still looked almost brand new :slight_smile:

As for the feeling, I guess it is impossible to satisfy anyone. I feel much more at ease with a Nokia 3310 or a Nexus 5 in my hands than with a iPhone made of steel (and wet soap, seemingly).

I’d say devices back then were way thicker, than phones nowadays, and were sometimes also also lighter or at least more more compact.
Or maybe the 3310 was using the good kind of plastic, I don’t know.
Thank you for bringing up the N5, I didn’t want to mention it as an example in my previous post, but if you take a closer look at the sides of the N5 you’ll likely notice a lot of micro cracks around the buttons. At least mine has it, and about 2-3 of my friends N5s. So my aversion for plastic cases is also due to the N5.