Expanders — more pins

Why you need them

Your PanelGroup board has 10 free pins for controls, which is plenty for a first experiment but not for a real panel — the armament panel alone has more than 30 switches, knobs and lights. An expander solves this. It's a small chip that adds more pins, a bit like a power strip that turns one wall socket into six.

 PanelGroup board ══ one cable ══▶ expander ──┬── switch
                                              ├── switch
                                              ├── switch
                                              └── … and more

The expander usually sits on the panel's own circuit board, right behind the switches, and connects back to the PanelGroup board with a single cable. For experimenting on your desk, a cheap ready-made module works just as well.

The good news is that very little changes for you. You wire a switch to an expander exactly the way you'd wire it to the board, and in your sketch only its pin address changes.

Which kind?

There are three kinds of expander, and each is good at something different.

	Digital expander	Shift-register chain	Analog expander
The chip	MCP23017	74HC165 (in) · 74HC595 (out)	ADS1115
Plugs into	J_I2C1 or J_I2C2	J_SR	J_I2C1 or J_I2C2
Best for	switches, buttons, lights	encoders, fast needles	knobs and sliders
Pins per chip	16	8	4
How many	8 per board	8 in + 8 out	4 per cable, 8 per board
Board version	any	0.2.0 or later	any

If you're not sure which to pick, a simple rule works for almost every panel. Rotary encoders and fast-moving gauge needles fit best on shift registers, because a spinning encoder has to be read quickly and a fast needle has to be stepped quickly. A digital expander can still read an encoder as long as its interrupt wires are connected, but it moves a needle more slowly. Knobs that turn a potentiometer go on an analog expander, because they need a position rather than on or off. Everything else goes on a digital expander.

You can also mix them freely. One sketch can use the board's own pins, two digital expanders and a shift-register chain all at the same time.

Digital expander (MCP23017)

A digital expander needs three things in your sketch: a name, an introduction to the board, and then pin addresses for each wire.

Give it a name. Up to eight can share one cable, so each one needs its own address between 0x20 and 0x27. You set it with the chip's three address pins, A0 to A2; with all three connected to GND, the address is 0x20. Put this line above setup():

MCP23017 expander1(0x20, Wire);

Introduce it to the board inside setup(). This tells the board the expander exists and which two wires it uses to signal a change:

void setup() {
    Wire.begin();
    PanelGroup::registerExpander(expander1, PB12, PB13);
    PanelGroup::setup();
}

PB12 and PB13 are the expander's interrupt wires, on pins 6 and 7 of J_I2C1. The expander uses them to tap the board on the shoulder the moment a switch changes, so the board doesn't have to keep asking. Several expanders on the same cable can share the same two wires.

Use its pins. The expander's pins are named GPA0 to GPA7 (port A) and GPB0 to GPB7 (port B), and you write them like this:

const PinRef GEAR_HANDLE_PIN = PinRef(expander1, PORT_B, 2);   // GPB2

GPA7 and GPB7 can't read switches

Because of a fault in the chip itself, these two pins only work as outputs. Use them for lights, which leaves 14 pins on each expander for switches. Those switches still need their 10 kΩ pull-up resistor, exactly as on the board — the expander doesn't add one for you.

Shift-register chain (74HC165 / 74HC595)

Shift registers are the easiest kind to use, because there's nothing to set up at all — no name, no introduction to the board. You just write pin addresses, using the chain's built-in name ShiftBus1:

const PinRef MASTER_ARM_PIN  = PinRef(ShiftBus1, 0, 3);   // input chip 0, pin 3
const PinRef CAUTION_LED_PIN = PinRef(ShiftBus1, 0, 3);   // output chip 0, pin 3

The chips are numbered from the board outward, starting at 0, and each chip has pins 0 to 7. There are two kinds of chip — 74HC165s read switches and 74HC595s drive lights — and each kind is counted separately. That's why the two lines above can use the same numbers: one is pin 3 on the first input chip, the other is pin 3 on the first output chip, and they're two different wires.

You also don't need to add any resistors. Shift-register boards built to our standard already have a pull-up on every input.

Analog expander (ADS1115)

An analog expander follows the same pattern as a digital one. First give it a name above setup():

ADS1115 adc1;

Then introduce it to the board inside setup(), along with its address. The address is 0x48 to 0x4B, set by the chip's ADDR pin, so up to four can share one cable:

void setup() {
    Wire.begin();
    PanelGroup::registerADC(adc1, 0x48, Wire);
    PanelGroup::setup();
}

It has four inputs, A0 to A3, and you use them like any other pin:

const PinRef FLOOD_KNOB_PIN = PinRef(adc1, 0);   // input A0

Going further

Using the second cable. To put expanders on J_I2C2, declare the second bus above setup() with TwoWire Wire1(PB11, PB10);, call Wire1.begin() in setup(), and pass Wire1 wherever the examples above say Wire. Its interrupt wires are PA8 and PA15 on board 0.2.0, and PB8 and PB9 on board 0.1.0.

No interrupt wires? Calling PanelGroup::registerExpander(expander1); on its own still works. The board then checks the expander every 20 ms instead of hearing about changes straight away.

The limits are per board. Using both cables doesn't double them — a board can still have at most eight digital and eight analog expanders in total.