palmPi

A SMALL PORTABLE PI ZERO WITH DISPLAY

NOTE: This project is not complete yet.

Yes, this project violates my cost criteria. There are some things you can do to get the cost down. You can source the screen from China: it listed for $17.01 recently. I don't recommend making your own breakout board: it wouldn't save much, and soldering that connector is a bear. I know, I've done it. You can save a few dollars by sourcing the battery charging and boost converters from China. And you can salvage the battery from somewhere. Even then the project is pretty expensive compared to the cost of a Zero.

Why did I do it then? I wanted to get some experience with the DPI interface in preparation for an upcoming project that uses old laptop screens. I also wanted a small computer so I can write code while on public transport. And yes, I do realize that an Android tablet connected to a Zero (using serial gadget mode) is far cheaper. But that's not as fun as building your own, right? At least for certain values of 'fun'.

This project uses this great tutorial as a starting point. You should read it. Seriously, it's way better than anything I could write.

PARTS

Adafruit 800x480 5" screen #1596

Adafruit TFT Friend LCD breakout #1932

Adafruit PowerBoost 500 #1944

Lithium battery

double-sided foam tape

a small switch for power

MODS

The Adafruit TFT Friend needs the following modifications: The 3.3V regulator (by 5VIN) must be removed, the 10K resistor next to it must also be removed, and the 22 ohm resistor (by CLK) must be shorted out. I also scraped off a bit of the solder mask below the panel connector for a better ground connection.

WIRING

For all connections that aren't ground or 3.3V, 22 ohm resistors are soldered to the Zero, then wire is run from the resistors to the TFT Friend. These are important: they're series terminations for the high-frequency signal lines. The wiring is then as follows:

FILES

The files subpage has the config files you need. All are placed in the /boot directory, except for lcd-overlay which goes in /boot/overlays.

BATTERY LIFE

With a 2000 mAH lithium battery, about 4 hours. You can use the following values to calculate run-times for different configurations. NOTE: These are measured at a supply voltage of 5.35V, which is a little high. You will get slightly different results depending on your voltage.

68 mA: Pi Zero by itself, at the command prompt, under-clocked to 700MHz, measured at Pi 5V input

27 mA: Bluetooth dongle, Cambridge Silicon Radio ID 0x0a12, measured at Pi 5V input

95 mA: Panel backlight (at 25mA setting, about half bright), at 3.7V, measured at backlight converter 5V input

73 mA: LCD panel, measured at Pi 5V input

When panel and backlight converter are disabled their current drops below 1mA.

BLUETOOTH SETUP

A Bluetooth keyboard can be used for input. If you use a combo WiFi/Bluetooth dongle, you can also have networking.

Here is a good howto on getting a Bluetooth keyboard working.

This site has been tested to display correctly using Epiphany on the Raspberry Pi