fix spim is limit up to 64KB each transfer

Author: hathach

libraries/SPI/SPI.cpp

@@ -193,28 +193,34 @@ void SPIClass::setClockDivider(uint32_t div)
 
 void SPIClass::transfer(const void *tx_buf, void *rx_buf, size_t count)
 {
-  nrfx_spim_xfer_desc_t xfer_desc =
+  const uint8_t* tx_buf8 = (const uint8_t*) tx_buf;
+  uint8_t* rx_buf8 = (uint8_t*) rx_buf;
+
+  while (count)
   {
-    .p_tx_buffer = (uint8_t*) tx_buf,
-    .tx_length   = tx_buf ? count : 0,
-    .p_rx_buffer = (uint8_t*) rx_buf,
-    .rx_length   = rx_buf ? count : 0,
-  };
+    // each transfer can only up to 64KB (16-bit) bytes
+    const size_t xfer_len = min(count, UINT16_MAX);
+
+    nrfx_spim_xfer_desc_t xfer_desc =
+    {
+      .p_tx_buffer = tx_buf8,
+      .tx_length   = tx_buf8 ? xfer_len : 0,
+
+      .p_rx_buffer = rx_buf8,
+      .rx_length   = rx_buf8 ? xfer_len : 0,
+    };
 
-  nrfx_spim_xfer(&_spim, &xfer_desc, 0);
+    nrfx_spim_xfer(&_spim, &xfer_desc, 0);
+
+    count -= xfer_len;
+    if (tx_buf8) tx_buf8 += xfer_len;
+    if (rx_buf8) rx_buf8 += xfer_len;
+  }
 }
 
 void SPIClass::transfer(void *buf, size_t count)
 {
-  nrfx_spim_xfer_desc_t xfer_desc =
-  {
-    .p_tx_buffer = (uint8_t*) buf,
-    .tx_length   = count,
-    .p_rx_buffer = (uint8_t*) buf,
-    .rx_length   = count,
-  };
-
-  nrfx_spim_xfer(&_spim, &xfer_desc, 0);
+  transfer(buf, buf, count);
 }
 
 byte SPIClass::transfer(uint8_t data)