PFW-1523 Implement M79 timer for PrusaLink

The idea is to have the host ping the printer periodically with a M79 to
enable certain features/UI. Using the usb_timer is not a good solution
for this as it depends on seeing a 'G' character

The LCD code, or whatever code is implementing the new functionality
will need to include host.h and check whether M79_timer_get_status()
returns 0 (timer not running) or 1 (timer is running).

I created a new file for the code host.cpp which we can use to expand
host related functionality and not clutter Marlin_main.cpp further.

Change in memory:
Flash: +104 bytes
SRAM: +5 bytes

CMakeLists.txt

@@ -182,6 +182,7 @@ set(FW_SOURCES
     Filament_sensor.cpp
     first_lay_cal.cpp
     heatbed_pwm.cpp
+    host.cpp
     la10compat.cpp
     language.c
     lcd.cpp

Firmware/Configuration_adv.h

@@ -66,6 +66,9 @@
 */
 #define AUTO_REPORT
 
+// Keepalive period which is restarted with M79
+#define M79_TIMEOUT 30 * 1000 // ms
+
 //===========================================================================
 //=============================Mechanical Settings===========================
 //===========================================================================

Firmware/Marlin_main.cpp

@@ -68,6 +68,7 @@
 #include "backlight.h"
 
 #include "planner.h"
+#include "host.h"
 #include "stepper.h"
 #include "temperature.h"
 #include "fancheck.h"
@@ -5850,6 +5851,20 @@ Sigma_Exit:
         }
         break;
     }
+
+    /*!
+    ### M79 - TODO
+    Restart the printer-host enable keepalive timer. While the timer has not expired, the printer will enable host specific features.
+    #### Usage
+
+        M79
+    #### Parameters
+       None
+    */
+    case 79:
+        M79_timer_restart();
+        break;
+
     /*!
   ### M104 - Set hotend temperature <a href="https://reprap.org/wiki/G-code#M104:_Set_Extruder_Temperature">M104: Set Extruder Temperature</a>
   #### Usage
@@ -6306,7 +6321,6 @@ Sigma_Exit:
 	case 113:
 		if (code_seen('S')) {
 			host_keepalive_interval = code_value_uint8();
-//			NOMORE(host_keepalive_interval, 60);
 		}
 		else {
 			SERIAL_ECHO_START;
@@ -9424,6 +9438,7 @@ void manage_inactivity(bool ignore_stepper_queue/*=false*/) //default argument s
   host_autoreport();
 #endif //AUTO_REPORT
   host_keepalive();
+  M79_timer_update_status();
 }
 
 void kill(const char *full_screen_message) {

Firmware/host.cpp

@@ -0,0 +1,17 @@
+#include "Configuration_adv.h"
+#include "host.h"
+#include "Timer.h"
+
+static LongTimer M79_timer;
+
+void M79_timer_restart() {
+    M79_timer.start();
+}
+
+bool M79_timer_get_status() {
+    return M79_timer.running();
+}
+
+void M79_timer_update_status() {
+    M79_timer.expired(M79_TIMEOUT);
+}

Firmware/host.h

@@ -0,0 +1,12 @@
+#pragma once
+
+/// Restart the M79 timer
+void M79_timer_restart();
+
+/// Get the current M79 timer status
+/// @returns true if running, false otherwise
+bool M79_timer_get_status();
+
+/// Checks if the timer period has expired. If the timer
+/// has expired, the timer is stopped
+void M79_timer_update_status();