Lightweight status screen for ST7920

- This status screen uses the ST7920 character generator to greatly reduce SPI traffic and MCU load when updating the status screen.

- Has been tested with the RepRapDiscount Full Graphics Smart Controller but should work with any LCD that uses an ST7920 or fully compatible controller.

Marlin/status_screen_lite_ST7920_spi.h

@@ -0,0 +1,39 @@
+/**
+ * Lightweight Status Screen for the RepRapDiscount Full
+ * Graphics Smart Controller (ST7920-based 128x64 LCD)
+ *
+ * (c) 2017 Aleph Objects, Inc.
+ *
+ * The code in this page is free software: you can
+ * redistribute it and/or modify it under the terms of the GNU
+ * General Public License (GNU GPL) as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option)
+ * any later version.  The code is distributed WITHOUT ANY WARRANTY;
+ * without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE.  See the GNU GPL for more details.
+ *
+ */
+
+#include "status_screen_lite_ST7920_class.h"
+
+void ST7920_Lite_Status_Screen::cs() {
+  ST7920_CS();
+  current_bits.synced = false;
+}
+
+void ST7920_Lite_Status_Screen::ncs() {
+  ST7920_NCS();
+  current_bits.synced = false;
+}
+
+void ST7920_Lite_Status_Screen::sync_cmd() {
+  ST7920_SET_CMD();
+}
+
+void ST7920_Lite_Status_Screen::sync_dat() {
+  ST7920_SET_DAT();
+}
+
+void ST7920_Lite_Status_Screen::write_byte(const uint8_t data) {
+  ST7920_WRITE_BYTE(data);
+}