Raspberry Pi I/O pins.

DAB,

Not that this relates directly with your experiences with the RPi, but I have noticed a similar thing with a lot of microprocessors.  The default on power-up is to set all GPIO to Tri-State (High-Z) inputs.  This may be what you are seeing here.  This is, in the most general of cases, the safest state for the board (hopefully nothing enabled or driven, but not always the case).

Gene

I personally would expect high impedance as preferable to prevent any risk of damage (e.g. shorting). If the line must be low or high when not driven by the MCU, then an external pull-up or pull-down resistor should do the trick. I suspect some of the voltages you see might be stray voltages capacitively coupled due to the close trace/pin spacing and high impedance of the meter. If not, and lines are driven with strange voltages, that would be rather surprising but may be able to be worked around.

- Gough

I believe the issue is how the gpio pins on the Raspberry Pi  are set at boot.   You could write a script that ran at boot that changes the states so they are the same.  There might be a better way but you would want to be cautious that you don't affect any of the on board devices.

You can view the states of the GPIO pins on your RasPi by running 'gpio readall'

Ex:

$ gpio readall
linedata: processor : 0


linedata: model name : ARMv6-compatible processor rev 7 (v6l)


linedata: BogoMIPS : 697.95


linedata: Features : half thumb fastmult vfp edsp java tls 


linedata: CPU implementer : 0x41


linedata: CPU architecture: 7


linedata: CPU variant : 0x0


linedata: CPU part : 0xb76


linedata: CPU revision : 7


linedata: 


linedata: Hardware : BCM2835


linedata: Revision : 0010


Revision got: Revision : 0010


 +-----+-----+---------+------+---+--B Plus--+---+------+---------+-----+-----+
 | BCM | wPi |   Name  | Mode | V | Physical | V | Mode | Name    | wPi | BCM |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
 |     |     |    3.3v |      |   |  1 || 2  |   |      | 5v      |     |     |
 |   2 |   8 |   SDA.1 | ALT0 | 1 |  3 || 4  |   |      | 5V      |     |     |
 |   3 |   9 |   SCL.1 | ALT0 | 1 |  5 || 6  |   |      | 0v      |     |     |
 |   4 |   7 | GPIO. 7 |   IN | 1 |  7 || 8  | 1 | ALT0 | TxD     | 15  | 14  |
 |     |     |      0v |      |   |  9 || 10 | 1 | ALT0 | RxD     | 16  | 15  |
 |  17 |   0 | GPIO. 0 |   IN | 0 | 11 || 12 | 0 | IN   | GPIO. 1 | 1   | 18  |
 |  27 |   2 | GPIO. 2 |   IN | 0 | 13 || 14 |   |      | 0v      |     |     |
 |  22 |   3 | GPIO. 3 |   IN | 0 | 15 || 16 | 0 | IN   | GPIO. 4 | 4   | 23  |
 |     |     |    3.3v |      |   | 17 || 18 | 0 | IN   | GPIO. 5 | 5   | 24  |
 |  10 |  12 |    MOSI | ALT0 | 0 | 19 || 20 |   |      | 0v      |     |     |
 |   9 |  13 |    MISO | ALT0 | 0 | 21 || 22 | 1 | IN   | GPIO. 6 | 6   | 25  |
 |  11 |  14 |    SCLK | ALT0 | 0 | 23 || 24 | 1 | OUT  | CE0     | 10  | 8   |
 |     |     |      0v |      |   | 25 || 26 | 1 | OUT  | CE1     | 11  | 7   |
 |   0 |  30 |   SDA.0 |   IN | 1 | 27 || 28 | 1 | IN   | SCL.0   | 31  | 1   |
 |   5 |  21 | GPIO.21 |   IN | 1 | 29 || 30 |   |      | 0v      |     |     |
 |   6 |  22 | GPIO.22 |   IN | 1 | 31 || 32 | 0 | IN   | GPIO.26 | 26  | 12  |
 |  13 |  23 | GPIO.23 |   IN | 0 | 33 || 34 |   |      | 0v      |     |     |
 |  19 |  24 | GPIO.24 |   IN | 0 | 35 || 36 | 1 | OUT  | GPIO.27 | 27  | 16  |
 |  26 |  25 | GPIO.25 |   IN | 0 | 37 || 38 | 0 | IN   | GPIO.28 | 28  | 20  |
 |     |     |      0v |      |   | 39 || 40 | 0 | IN   | GPIO.29 | 29  | 21  |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
 | BCM | wPi |   Name  | Mode | V | Physical | V | Mode | Name    | wPi | BCM |
 +-----+-----+---------+------+---+--B Plus--+---+------+---------+-----+-----+

We had the pin set to low, but I still saw about 1.1v drop across my transistor driver from the base to the emitter.

I was expecting there to be no voltage across the junction with the base at ground level.

If you cannot expect the I/O pin to be grounded under that kind of small load, then something is very wrong with their design.

DAB

That was my initial thought, but we were trying to drive the pin low across a transistor base connection.

Even at high impedance, you expect the output to be 0 volts with the pin set to a logic zero.

DAB

Agreed, but we were trying to drive the pin low across a base-emitter junction of a transistor with a 1k ohm resistor from the pin to the base.

I expected zero volts under those conditions.

DAB

Well, low does not necessarily mean off.  A low could mean just low current or low voltage such as 1.1v as you had experienced. You might want to contact the Raspberry Pi folks to see if it is possible to completely disable the pin you are working with.

Have you tried to put a pull-down resistor to ground on the pin in question? 

We had a couple of  the pins pulled down to ground through 1 k ohm resistors, but I do not recall if they were still at ground or not.

DAB

This article shows the electronic setup of the Pi’s pins, and has the voltages documented.

GPIO Electrical Specifications, Raspberry Pi Input and Output Pin Voltage and Current Capability