MAX7219 followup (#7564)

* Add MAX7219_DEBUG to Travis CI testing

* Tweak config and use standard pin naming for MAX7219_DEBUG

* MAX7219: Apply coding standards, use macros, etc.

* Make code work...

Marlin/Configuration_adv.h

@@ -1382,4 +1382,32 @@
 
 #endif // I2C_POSITION_ENCODERS
 
+/**
+ * MAX7219 Debug Matrix
+ *
+ * Add support for a low-cost 8x8 LED Matrix based on the Max7219 chip, which can be used as a status
+ * display. Requires 3 signal wires. Some useful debug options are included to demonstrate its usage.
+ *
+ * Fully assembled MAX7219 boards can be found on the internet for under $2(US).
+ * For example, see https://www.ebay.com/sch/i.html?_nkw=332349290049
+ */
+//#define MAX7219_DEBUG
+#if ENABLED(MAX7219_DEBUG)
+  #define MAX7219_CLK_PIN   64  // 77 on Re-ARM       // Configuration of the 3 pins to control the display
+  #define MAX7219_DIN_PIN   57  // 78 on Re-ARM
+  #define MAX7219_LOAD_PIN  44  // 79 on Re-ARM
+
+  /**
+   * Sample debug features
+   * If you add more debug displays, be careful to avoid conflicts!
+   */
+  #define MAX7219_DEBUG_PRINTER_ALIVE    // Blink corner LED of 8x8 matrix to show that the firmware is functioning
+  #define MAX7219_DEBUG_STEPPER_HEAD  3  // Show the stepper queue head position on this and the next LED matrix row
+  #define MAX7219_DEBUG_STEPPER_TAIL  5  // Show the stepper queue tail position on this and the next LED matrix row
+
+  #define MAX7219_DEBUG_STEPPER_QUEUE 0  // Show the current stepper queue depth on this and the next LED matrix row
+                                         // If you experience stuttering, reboots, etc. this option can reveal how
+                                         // tweaks made to the configuration are affecting the printer in real-time.
+#endif
+
 #endif // CONFIGURATION_ADV_H