Echoing the others in this thread who are calling for USB-C. Micro-USB cables are annoying and failure-prone.
I would further request that power delivery is supported, to as many watts as possible, so that we can power larger LED arrays, servos, steppers, small heaters, etc., without having to use separate power bricks. These days, you can get gallium nitride USB-C supplies that are very small and deliver 100 watts for $40. If the application's power demands can be supported by the USB-C port on whatever laptop or workstation you're working on, then you have an all-in one solution that can be used to program, debug, and run the device.
Some method of determining power delivery status would also be ideal. For example, if I'm developing on a big array, and my workstation only delivers 20 watts, I may want to clamp the max brightness, but then kick it up to maximum when more wattage is available.
If power delivery has to be through a secondary board, that would be fine with me, but if it can be reasonably integrated into a Teensy without driving the cost through the roof it would be perfect.
The current 4.0 and 4.1 form factors are both ideal for various use cases, so I would keep both.
This is sort of off topic for the main thread, but In terms of USB-C PD, I just put in an order for my first USB-C PD gear (wall charger, battery, and PD trigger to get access to the power).
I have this project to measure how much power things take. This was sparked by learning how to power my Olympus and Panasonic cameras using dummy batteries, and I wanted to figure out the maximum power the camera used at any one time. In addition, I like to do costume props with WS2812B leds (i.e. neopixels) and it is always handy to figure out how much power a display takes (though I'm on the low end of the power scale, since I generally don't do more than 100 or so LEDs). Normal amp/volt meters will tell you what power is being used at the current time, but it doesn't give you the max power.
I do have two meters that are a little better than just tell me the current value, and they can do graphs on my cell phone via bluetooth.
On one of the meters, has an option to save the values as a CSV file, which I can import to my computer. But that meter is limited to 5v/USB input (the cameras run on 2 cell li-po batteries, so I need 7.2v and up, with the cameras running fine on 9v). But even with a CSV file, I have to manually coordinate doing the item I want to measure (for example how much power does it take to record a 4K video) and then figuring out where the spike for the action was.
I recently got another meter that is more flexible in that it can measure micro USB-b, USB-C, or 5.5mm x 2.1mm inputs/outputs. It also does a bluetooth display on the phone, but the graph is not all that detailed. I can't make the graph larger or have lines that tell me the various mAmps and mWatts, so I have to guess. Also there appears no way to save a file to the PC.
So I have a Teensy 4.1 in a feather shield using the Adafruit INA219 featherwing and an Adafruit OLED featherwing display that I've been working on. I display both the current volts, amps, and watts, as well as the maximum values. I have a button to clear the maximum values, so I can clear the state before doing the action I want to measure.
I have an A/C -> D/C power converter that I can dial in specific voltages, batteries that can produce 9v directly, and I have various usb micro-B boost cables that may/may not produce enough watts. Part of the desire with doing the measurement is some of the USB boost circuits don't provide enough power, and I wanted to avoid them in the future, once I know how much power is taken.
While I don't own them, two of the latest Olympus cameras (E-m1 mark III and E-m1x) can be powered by USB-C PD, and I wanted to borrow them, and I wanted to eventually measure them as well. Since they don't use the same battery as my current cameras, I couldn't just use the dummy batteries I have.
I also upgraded my cell phone to one that uses USB C, and I wanted to upgrade to a USB battery that can do quick charges via USB-PD.
In my looking around today, I didn't see any programmatic methods of determining which of the USB-C PD voltages are available. There may be chips out there, but I didn't know the search terms. So I bought one of the cheap devices that have a LED and a button to display/set the voltage.
While I would likely buy such a device, I suspect it won't go into the Teensy proper. But it may be useful as a stand-alone device that feeds the Teensy and talks I2C/SPI/uart about the USB-C PD state.
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While I'm grousing, I should mention, none of the INA219 designs that I glanced at seem to have given much thought to the ways people might use them. In particular, it would be nice if the device had both power/ground for each of the input power from the power source and power/ground to the device that uses the power. The two grounds should be connected on the PCB, and connected to the ground of the I2C device. It would be nice if the device had standard connectors, such as 5.5mm x 2.1mm male/female power ports. And whatever connection should have thick enough wires to meet the actual voltage/amps that is specified.