Electronic – arduino – A 4.8v reading on a loose pin 0

arduinoexpandergpioinput

I have a setup that demands that I use all my I/O's on my Arduino Uno R3 board – including the serial pins 0 and 1.

So I've initialized those pins as inputs, but when I make a reading on pin 0 it reads high although it's not connected to anything. My multimeter tells me 4.8V steadily on the pin all the time.

All my other pins I've not initialized any serial communication, and I've tried running the board on a battery (so no computer connection).

I have Adafruit's Waveshield connected to it, but as far as I'm concerned it shouldn't interfere with pin 0 in any way.

I've made a simple drawing showing the concept of what I'm doing. It's all there is to it, just the size of 12 buttons and 12 lamps. In order to get enough I/O's, I had to implement an IO expander (mcp23016) on the I2C bus, but it's the same principle as on the drawing.

So when I have everything hooked up, all seems to be good except for the pin 0 on the Arduino board. All other readings (both on the Arduino and the expander) works fine.
The weird thing is: when I measure voltage on pin 0, it gives me 4.8V; on pin 1 it gives me 1V and on the other pins it gives me 0V. So something is obviously different from the serial pins.

Are there any internal pull ups or downs on them or could it be the many 10k resistors I use? Do they need to be smaller or bigger perhaps?

Perhaps it's just a bad soldering, but I've been checking for hours and hours and I really can't see any issue on the hardware part.

concept schematic

Best Answer

If it's set as an input pin then trying to measure the voltage on it is largely unproductive unless, of course you believe it may be still set as an output - then you would need to attach a load to it to see if the voltage it is producing is "real" or just some leakage current from an unconnected input making a high impedance voltmeter show something like 4.8V.

Try connecting a 1k resistor to ground from this pin - does it still show logic 1 or perhaps it's now at logic 0.

Some IO pins will default (on some MCUs) to having an inbuilt high resistance pull-up of the order 100k and this may also be happening. This (on some MCUs) can be programmed to be (a) not there at all, (b) pull-up or (c) pull-down. I believe some MCUs can program a lower resistance or a higher resistance. Maybe this is also happening?

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Electronic – arduino – Problem writing to MCP23017 I/O Expander registers (i2c)

Okay, I can see the changes.

As shown now, the LEDs on the receiving IC will block the data high as they are reversed. The LEDs on the transmitting side aren't a good idea either. If you want LEDs you need to put them from the data line to ground through a resistor, not in series with the connection to cable socket.

Just seen your picture - it looks like you have removed the LEDs, good (I was just about to suggest this.. :-) )
So now it looks as if you have direct connections from IC1 to IC2. If this is the case then if the code (looks reasonable at a glance) and IC wiring are correct then it should work.

If you can confirm with a multimeter that the input pins are seeing a high (or low) voltage and the read value is different, then this would confirm the issue is one or the other of the above. Maybe just apply a known voltage directly and see if you can read that okay)

However, if you are reading different values when the pullups are on/off then that would seem to indicate the read is correct. Try reading a direct voltage and post results, I'm just checking the datasheet for the ICs, will add more shortly.

EDIT - about the pullups:

You can use the internal pullups if you don't mind the line "relaxing" to high when not driven (i.e. default state 1) These are often used for interfacing with open drain buses, or for button to ground, etc to save an external pullup.
If you want to have the lines default state low though, (as is the case for you) you need a pulldown to stop the high impedance floating. Since the IC in question doesn't have internal pulldowns, you need to add them externally.

EDIT - Doh! I've just seen the problem...

In your code you set 1 pin at a time to output and all the rest to inputs. This means that if you have internal pullups on, the undriven pins will default to high! When a pin is set to input, it is high impedance, so effectively it's like disconnecting that end of the line, and the weak pullups will pull the receiving end high.
You need to keep all pins as outputs, and just set one high at a time, this will keep all the pins driven - try this with the pullups on, it should work.
If you know the lines will be driven correctly all the time, you don't need the pullups, but it doesn't hurt to keep them on.

Here is the relevant code (in the intialise registers function):

i2c_start_wait(SLAVE_ADDRESS(0x4E)+I2C_WRITE); // Address Slave 1
    i2c_write(0x00);        // Set memory pointer to the IODIRA register (IODIRA address is 0x00 - see Page 9)
    i2c_write(~(Value));    // Set only one pin at a time as an output and everything else as inputs
    i2c_stop();

Change it to:

i2c_start_wait(SLAVE_ADDRESS(0x4E)+I2C_WRITE); // Address Slave 1
    i2c_write(0x00);        // Set memory pointer to the IODIRA register (IODIRA address is 0x00 - see Page 9)
    i2c_write(0x00);    // Set all pins as outputs
    i2c_stop();

Electronic – Arduino & JY-MCU Bluetooth module

I have one of these modules here somewhere (works fine, once you find the right documentation)
IIRC it has an on board 3.3V regulator, so you can feed it 5V (range 3.6V - 6V) on the VCC line okay.
However, the TX and RX lines are 3.3V, so you must use the same level logic here. If your Arduino pins are 5V logic you need to level shift them accordingly to avoid breaking the new module.

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