From 1ba0ed8121f4ccbd9e89f45b84792366d884798a Mon Sep 17 00:00:00 2001
From: Mike La Spina <mike.laspina@shaw.ca>
Date: Mon, 5 Apr 2021 16:34:31 -0500
Subject: [PATCH] Make DELAY_NS round up on AVR (#21546)

---
 Marlin/src/HAL/shared/Delay.h                 | 35 +++++++++++++++++--
 .../dogm/ultralcd_st7920_u8glib_rrd_AVR.cpp   |  2 +-
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/Marlin/src/HAL/shared/Delay.h b/Marlin/src/HAL/shared/Delay.h
index dc1f158b4..f7e01ad25 100644
--- a/Marlin/src/HAL/shared/Delay.h
+++ b/Marlin/src/HAL/shared/Delay.h
@@ -150,8 +150,37 @@ void calibrate_delay_loop();
 
 #endif
 
-// Delay in nanoseconds
-#define DELAY_NS(x) DELAY_CYCLES((x) * ((F_CPU) / 1000000UL) / 1000UL)
-
+/**************************************************************
+ *  Delay in nanoseconds. Requires the F_CPU macro.
+ *  These macros follow avr-libc delay conventions.
+ *
+ * For AVR there are three possible operation modes, due to its
+ * slower clock speeds and thus coarser delay resolution. For
+ * example, when F_CPU = 16000000 the resolution is 62.5ns.
+ *
+ *  Round up (default)
+ *    Round up the delay according to the CPU clock resolution.
+ *    e.g., 100 will give a delay of 2 cycles (125ns).
+ *
+ *  Round down (DELAY_NS_ROUND_DOWN)
+ *    Round down the delay according to the CPU clock resolution.
+ *    e.g., 100 will be rounded down to 1 cycle (62.5ns).
+ *
+ *  Nearest (DELAY_NS_ROUND_CLOSEST)
+ *    Round the delay to the nearest number of clock cycles.
+ *    e.g., 165 will be rounded up to 3 cycles (187.5ns) because
+ *          it's closer to the requested delay than 2 cycle (125ns).
+ */
 
+#ifndef __AVR__
+  #undef DELAY_NS_ROUND_DOWN
+  #undef DELAY_NS_ROUND_CLOSEST
+#endif
 
+#if ENABLED(DELAY_NS_ROUND_DOWN)
+  #define DELAY_NS(x) DELAY_CYCLES((x) * ((F_CPU) / 1000000UL) / 1000UL)          // floor
+#elif ENABLED(DELAY_NS_ROUND_CLOSEST)
+  #define DELAY_NS(x) DELAY_CYCLES(((x) * ((F_CPU) / 1000000UL) + 500) / 1000UL)  // round
+#else
+  #define DELAY_NS(x) DELAY_CYCLES(((x) * ((F_CPU) / 1000000UL) + 999) / 1000UL)  // "ceil"
+#endif
diff --git a/Marlin/src/lcd/dogm/ultralcd_st7920_u8glib_rrd_AVR.cpp b/Marlin/src/lcd/dogm/ultralcd_st7920_u8glib_rrd_AVR.cpp
index 24baae221..4abf91edb 100644
--- a/Marlin/src/lcd/dogm/ultralcd_st7920_u8glib_rrd_AVR.cpp
+++ b/Marlin/src/lcd/dogm/ultralcd_st7920_u8glib_rrd_AVR.cpp
@@ -60,7 +60,7 @@
 #elif F_CPU == 16000000
   #define CPU_ST7920_DELAY_1 DELAY_NS(125)
   #define CPU_ST7920_DELAY_2 DELAY_NS(0)
-  #define CPU_ST7920_DELAY_3 DELAY_NS(125)
+  #define CPU_ST7920_DELAY_3 DELAY_NS(188)
 #else
   #error "No valid condition for delays in 'ultralcd_st7920_u8glib_rrd_AVR.h'"
 #endif