Due to the small foot print, I will convert the Pro Micro to 3.3v by replacing the regulator chip from 5V to 3.3V to avoid the voltage level shifter circuits. I plan to mount a daughter board on top of the pro micro for the 16 Mb serial flash.
Now figuring out ways to avoid damaging the 3.3V serial flash if you need to re-flash the boot loader. Seems the 5V USBasp programmer can be modified to program in 3.3V mode. That may be a solution. Thanks to @serisman. Alternatively I am testing if it’s ok not to take the 5V power supply from the USBasp and keep using the 3.3V battery during boot loader flashing. That will solve the VCC issue but the TTL voltage level of SCK, MISO, MOSI, Reset, will still reach 5V during programming. That may be too close to the absolute max limit of signaling voltage level of the serial flash (VCC +0.4 V) , VCC max 4.6 V
Basically you remove the voltage regulator from the Pro micro and make use of the 3.3V regulator on the display board.
You can take the USB power from the RAW pin like @serisman did (easier to solder) but because of the onboard diode the voltage will be 4.5V. So I prefer to take VUSB from the top pad of J1 which is 5V to charge the lipo. In any case make sure the bottom pad of J1 is not connected.
@Mr.Blinky, That is clever . I never thought of using the 3.3V regulator on the oled to power the Pro Micro. But I already replaced the regulator on the Pro micro board with a 3.3V one I got. I want the oled to be replaceable easily, so I can swap it with different colored oled. So I don’t want any wire soldered onto that Oled circuit board directly. I will keep this idea for future. Thanks
Thanks for the diagrams… Those look more useful than the wall of text that I posted.
Keep in mind that the TP4056 is a linear based li-ion charger and doesn’t really benefit from a higher input voltage (as long as it is high enough, which 4.5V should be). If anything, having a diode drop the 5V down to 4.5V is actually beneficial as it keep the TP4056 IC running cooler and therefore able to sustain a higher current for longer. In fact a lot of the unprotected TP4056 modules even have a dropper resistor incorporated on the input (~ 0.4 ohm) to drop the input voltage down a bit (0.4V @ 1A) and keep the TP4056 running cooler. The protected boards don’t seem to have the dropper resistor, and therefore a diode on the input is fine.
Thanks for the detailed discription of the TP4056. I didn’t really look at the details of the charger chip. Was just thinking that 0.3V difference may be a small margin to fully charge the lipo at 4.2V in some cases.
This is the schematics of what I plan to make - I will name it “ArduBaby”.
Replacing the on board 5V regulator with a 3.3V one works but has a drawback that the board will always be powered up even during charging of the battery.
So, modified the schematics to use an external regulator like what @Mr.Blinky and @serisman suggested.
But the additional external 3V regulator takes up additional space. To make the board smaller, I am trying to replace the 3.3V regulator with a divide to drop 0.7V from the 3.7-4.2V to get 3.0 to 3.7V.
Don’t have a look on the watch then It’s fine to offer some other possibilies. A ultra portable console. I like. Can be kept in the pocket very easy. Maybe not good for any game but alot will works fine on it.
Actually I thought the watch looked super interesting. My comment was about playing games. I find the Arduboy itself ALMOST at the edge of too small to actually use for an extended period. If you had a custom app in mind that just needed to fit in your pocket then the possibilities indeed become a lot more intereting.