add test and doc for SX1509 io expander

Author: LouWii

Test-SX1509-Sparkfun/README.md

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+# SX1509 I/O Expander
+
+See [MODULES](../MODULES.md) for other modules and for info about the SX1509
+
+Things to know:
+* 3.3V max!
+* I2C address is `0x3E` by default (but might change depending on your breakout board!)
+
+## Schema
+
+![Schema](schema.png)
+
+## Code
+
+The code requires 2 libraries
+* Wire (for I2C communication)
+* SparkFunSX1509 for using the SX1509 itself
+

Test-SX1509-Sparkfun/Sketch.fzz

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+#include <Arduino.h>
+#include <Wire.h> // Include the I2C library (required)
+#include <SparkFunSX1509.h> // Include SX1509 library
+
+SX1509 io; // Create an SX1509 object
+
+const int SX1509_BTN_PIN = 0;
+
+void setup(){
+  Serial.begin(9600);
+
+  // Try to connect to the i/o expander
+  if (!io.begin(0x3E)) {
+    Serial.println("SX1509 not connected");
+  } else {
+    Serial.println("SX1509 connected");
+  }
+
+  // Configure pin 0 as input
+  io.pinMode(SX1509_BTN_PIN, INPUT_PULLUP);
+}
+
+
+void loop(){
+
+  // Use io.digitalRead() to check if an SX1509 input I/O is either LOW or HIGH.
+  if (io.digitalRead(SX1509_BTN_PIN) == LOW) {
+    Serial.println("I/O Expander button pushed");
+    while (io.digitalRead(SX1509_BTN_PIN) == LOW)
+      ; // Wait for button to release (this is blocking the program)
+  }
+
+}