driver::keyscanner::ATmega: Revert scanning during the interrupt

Instead of scanning during the interrupt, do so in the main context, and only
use the interrupt for signaling that we need to scan. This resolves a problem
where scanning took too long, and we ended up missing events.

Fixes #812.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/kaleidoscope/driver/keyscanner/ATmega.h

@@ -47,16 +47,16 @@
   constexpr uint8_t kaleidoscope::Device::KeyScannerProps::matrix_row_pins[matrix_rows];                \
   constexpr uint8_t kaleidoscope::Device::KeyScannerProps::matrix_col_pins[matrix_columns];             \
   template<>                                                                                            \
-  volatile uint16_t kaleidoscope::Device::KeyScanner::previousKeyState_[kaleidoscope::Device::KeyScannerProps::matrix_rows] = {}; \
+  uint16_t kaleidoscope::Device::KeyScanner::previousKeyState_[kaleidoscope::Device::KeyScannerProps::matrix_rows] = {}; \
   template<>                                                                                            \
-  volatile uint16_t kaleidoscope::Device::KeyScanner::keyState_[kaleidoscope::Device::KeyScannerProps::matrix_rows] = {};         \
+  uint16_t kaleidoscope::Device::KeyScanner::keyState_[kaleidoscope::Device::KeyScannerProps::matrix_rows] = {};         \
   template<>                                                                                            \
   uint16_t kaleidoscope::Device::KeyScanner::masks_[kaleidoscope::Device::KeyScannerProps::matrix_rows] = {};            \
   template<>                                                                               \
   uint8_t kaleidoscope::Device::KeyScanner::debounce_matrix_[kaleidoscope::Device::KeyScannerProps::matrix_rows][kaleidoscope::Device::KeyScannerProps::matrix_columns] = {}; \
                                                                                            \
   ISR(TIMER1_OVF_vect) {                                                                   \
-    Runtime.device().readMatrix();                                              \
+    Runtime.device().keyScanner().do_scan_ = true;                                         \
   }
 
 namespace kaleidoscope {
@@ -130,6 +130,10 @@ class ATmega: public kaleidoscope::driver::keyscanner::Base<_KeyScannerProps> {
   }
 
   void scanMatrix() {
+    if (do_scan_) {
+      do_scan_ = false;
+      readMatrix();
+    }
     actOnMatrixScan();
   }
 
@@ -192,10 +196,12 @@ class ATmega: public kaleidoscope::driver::keyscanner::Base<_KeyScannerProps> {
                    key_addr.col());
   }
 
+  bool do_scan_;
+
  private:
   typedef _KeyScannerProps KeyScannerProps_;
-  static volatile uint16_t previousKeyState_[_KeyScannerProps::matrix_rows];
-  static volatile uint16_t keyState_[_KeyScannerProps::matrix_rows];
+  static uint16_t previousKeyState_[_KeyScannerProps::matrix_rows];
+  static uint16_t keyState_[_KeyScannerProps::matrix_rows];
   static uint16_t masks_[_KeyScannerProps::matrix_rows];
   static uint8_t debounce_matrix_[_KeyScannerProps::matrix_rows][_KeyScannerProps::matrix_columns];