In an effort to help intrepid hackers who hook their brand-new Raspberry Pi's GPIO pins to standard 5-volt maker fare and wonder why their boards stop working, we present RPio, which does the following:
- Shifts the voltage of 16 of the 17 GPIO pins from 3.3 to a user-selectable voltage (typically 5.0), and back again from the user-selected voltage to a safe 3.3V for the Raspberry Pi. In technical terms, this is bidirectional level translation.
- Provides up to 150mA of 3.3-volt power, which is significantly more than the 50mA of 3.3v power provided by the GPIO pins. It's not enough to drive a robot up a hill, but it is enough to safely power a few ICs while writing data to an SD card.
- Includes four current-limited LEDs, two for status and two for experimenting. One confirms that the board has been successfully connected to the Raspberry Pi. Another indicates that the level-shifted GPIO pins are live. The remaining two are left anode-disconnected with headers ready to connect to GPIOs, for easy light-blinking fun.
Note that almost nobody on Earth, including me, has a Raspberry Pi yet. Thus, these instructions are purely speculative. See Caveats below.
These instructions will refer to the Raspberry Pi as the RPi. The breakout board is the RPio. The north end of the RPio is the short end that has the 26-pin GPIO header matching the RPi's header. The south end is the other short end. The RPi is assumed to be component-side up, with the SD card slot pointing left and the HDMI jack pointing down, toward you.
We're also assuming that you have already soldered either pin or socket headers to the GPIOs on your RPi, and soldered mating connectors to the RPio. The RPi reportedly won't ship with these GPIO pins soldered, so the kind that gets soldered on your board is a decision you get to make.
- Remove all shunts, jumper connectors, wires, etc., from the RPio.
- Turn off your RPi.
- Plug the RPio into the 26-pin GPIO header on the top of your RPi. This board should cover a little less than half of the top of your RPi in the east-west dimension, so that the south side of the board covers the RPi's SD receptacle. If you plug it in any other way, then both your boards will be destroyed, your home will burn down, your friends will stop calling, and your cat will leave you. See Caveats below.
- Turn on your RPi. You should see the LED on the RPio marked "CONNECTED" light up.
- Look at the small jumpers (JP5, "IO-PWR-EXT" and JP6, "VTG-SEL") on the RPio. These two jumpers set the voltage for the I/O pins. If you need a weird voltage, such as 3.14159, then supply it yourself on IO-PWR-EXT, which is the two-pin header. Otherwise, pick 5V by connecting the two VTG-SEL pins labeled 5.0 and VCC, or else pick 3V by connecting VTG-SEL's 3.3 and VCC pins. Think of VCC of the part of the circuit waiting to be powered at the voltage you want, and all the other numbered voltages on the board as potential sources for VCC; it's up to you to connect the right one.
- If you've done the previous step correctly, then the IO-POWER LED should light up.
- If you need immediate gratification, find the two LEDs on the south side of the RPio. Take a jumper wire and connect the header to the left of one of them to the pin marked GP0. Now write some code on your RPi to enable that GPIO pin. The LED should light up.
- If you're wondering why you went through all this, get out a multimeter and measure the voltage of an enabled GPIO pin on the 26-pin header. It will be 3.3 volts. Now measure the same GPIO on the GP pinout of the RPio. Its voltage should match that of VCC. Moreover, when otherwise-incompatible voltages come into the RPio, they will be shifted safely to the 3.3 volts that the RPi needs. This last part is the important part: if you applied 5V to an RPi GPIO, the RPi would probably die. See Caveats below.
- Messing with GPIOs is potentially dangerous to your Raspberry Pi, your possessions, and yourself. Please don't do it unless you know what you're doing and are willing to live with the consequences.
- Level-shifting IC ports are available in powers of two. The RPi has 17 GPIOs. As you may know, 17 is not a power of two; in fact, it's a prime number. We chose to not level-shift the GPIO labeled /SPI CE1, reasoning that as the second SPI chip-select, it's going to be least frequently used. But beware: if you forget that this pin is 3.3 volts and hook it up to a 5-volt I/O source, your RPi will die.
- Board design by Mike Tsao and Dennis Gentry.
- Thanks to the Adafruit Part Finder for making us aware of the TXB0108.
