PCF8574T Module IO Expansion Module For Arduino - on Spark

Hardware
1

First off - this has to be said - i’m utterly new to Particle/Spark development, and my knowledge of code in general is quite limited, so please bear that in mind :smiley:

Right then - the project that I have in mind requires a multitude of digital IO pins for a load of switches and LEDs - 8 digital IO just isn’t going to cut it. I’ve stumbled across this particular bit of kit PCF8574T Module IO Expansion Module For Arduino and it touts itself as a stackable 8-pin IO expansion running off of i2c.

The only example code that I have been able to find (towards the bottom of that page, although that code is replicated elsewhere on the internet) sets P0-P7 on the expansion board to a high signal, and it’s doing that through a Wire.write(0xff)

I, on the other hand, could do with P0-P7 being initialised as individual pins (in both input and output modes) - has anyone had any experience with this particular IO board, and has anyone done individual pin addressing on it?

Any assistance in this one will be sincerely appreciated.

Regards,

Steve

2

You can also use the analog pins as digital pins if you are ok with that idea.

What exact problem are you having with the module?

3

I’m sorry if I am not making much sense - I’ve been awake now for 23 hours now with no sign of sleep in sight as yet - damn you, Insomnia >_<

Yeah, I was planning on using some of the analog pins as digital pins through a voltage latch in code (0-1.5v = 0 // 1.51v-3.3v = 1 or something similar) but I’m still going to need more pins ! Too many inputs, perhaps, but the number of physical switches and individual LEDs in the project require individual discrete inputs.

< edit > Yeah, I now know that I can do a digitalRead(A1) or whatnot as well, which would make life simpler when it comes to hooking switches up to analog pins - but the base issue remains the same - not enough IO pins :smiley: < /edit >

I’ve found the datasheet for the PCF8574/A chipset, and I have figured out that each expansion module is addressable (and set through the DIP switches) but what I seem to be missing somewhere is the addressing for the individual pins (the only two known commands I can find at the moment are 0xff (for all pins thrown HIGH) and 0x00 (for all pins dropped LOW)

Am I missing something fundamental and basic in my sleep-deprived state?

4

@SteveBlackLight, from the looks of it, each bit in an 8-bit byte represents a pin. By adding and subtracting bit values to a byte variable, you build the value you need to write to each pin on the board. In coding, it is called bit masking. For example:

#define PIN1  0B00000001  // binary value of the first pin
#define PIN2  0B00000010  // binary value of the second pin
#define PIN3  0B00000100  // third pin
#define PIN4  0B00001000  // fourth pin
#define PIN5  0B00010000  // fifth pin
#define PIN6  0B00100000  // sixth pin
#define PIN7  0B01000000  // seventh pin
#define PIN8  0B10000000  // eighth pin

To write pins 1, 5 and 8 at the same time:

byte OUT = PIN1 + PIN5 + PIN8;
...
Wire.write(OUT);
...

To turn of just pin 5 but leave the others on:

OUT = OUT - PIN5;
...
Wire.write(OUT);
..

A better form would use logical operators (AND, OR, XOR) instead of add and subtract but this is to illustrate the point. :smiley:

1 Like
5

@peekay123 - After getting a couple of hours sleep and “recharging the batteries” a little, I’ve just seen your post, and now I have some brainpower, your suggestion looks more than likely.

Many thanks to both yourself and @kennethlimcp for putting some brainpower into this for me :slight_smile:

2 Likes
6

you should also be able to read from the module to find what its set as, you then change the value to turn on or off each bit.