add some basic caching support for chips with write-only registers

Adafruit_BusIO_Register.cpp

@@ -131,6 +131,9 @@ bool Adafruit_BusIO_Register::write(uint32_t value, uint8_t numbytes) {
     return false;
   }
 
+  // store a copy
+  _cached = value;
+
   for (int i = 0; i < numbytes; i++) {
     if (_byteorder == LSBFIRST) {
       _buffer[i] = value & 0xFF;
@@ -166,6 +169,15 @@ uint32_t Adafruit_BusIO_Register::read(void) {
   return value;
 }
 
+/*!
+ *    @brief  Read cached data from last time we wrote to this register
+ *    @return Returns 0xFFFFFFFF on failure, value otherwise
+ */
+uint32_t Adafruit_BusIO_Register::readCached(void) {
+  return _cached;
+}
+
+
 /*!
  *    @brief  Read a buffer of data from the register location
  *    @param  buffer Pointer to data to read into
@@ -195,6 +207,7 @@ bool Adafruit_BusIO_Register::read(uint8_t *buffer, uint8_t len) {
   return false;
 }
 
+
 /*!
  *    @brief  Read 2 bytes of data from the register location
  *    @param  value Pointer to uint16_t variable to read into

Adafruit_BusIO_Register.h

@@ -35,6 +35,7 @@ public:
   bool read(uint8_t *value);
   bool read(uint16_t *value);
   uint32_t read(void);
+  uint32_t readCached(void);
   bool write(uint8_t *buffer, uint8_t len);
   bool write(uint32_t value, uint8_t numbytes = 0);
 
@@ -51,6 +52,7 @@ private:
   uint8_t _width, _addrwidth, _byteorder;
   uint8_t _buffer[4]; // we wont support anything larger than uint32 for
                       // non-buffered read
+  uint32_t _cached = 0;
 };
 
 /*!