docs: Add low-level MMIO coding hints to Code_Overview.md

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Code_Overview.md

@@ -388,8 +388,8 @@ Useful steps:
 3. The first main coding task is to bring up communication support to
    the target board. This is the most difficult step in a new port.
    Once basic communication is working, the remaining steps tend to be
-   much easier. It is typical to use an RS-232 style serial port
-   during initial development as these types of hardware devices are
+   much easier. It is typical to use a UART type serial device during
+   initial development as these types of hardware devices are
    generally easier to enable and control. During this phase, make
    liberal use of helper code from the src/generic/ directory (check
    how src/simulator/Makefile includes the generic C code into the
@@ -416,6 +416,20 @@ Useful steps:
    the micro-controller with the main klippy.py program.
 9. Consider adding build test cases in the test/ directory.
 
+Additional coding tips:
+1. Avoid using "C bitfields" to access IO registers; prefer direct
+   read and write operations of 32bit, 16bit, or 8bit integers. The C
+   language specifications don't clearly specify how the compiler must
+   implement C bitfields (eg, endianness, and bit layout), and it's
+   difficult to determine what IO operations will occur on a C
+   bitfield read or write.
+2. Prefer writing explicit values to IO registers instead of using
+   read-modify-write operations. That is, if updating a field in an IO
+   register where the other fields have known values, then it is
+   preferable to explicitly write the full contents of the register.
+   Explicit writes produce code that is smaller, faster, and easier to
+   debug.
+
 ## Coordinate Systems
 
 Internally, Klipper primarily tracks the position of the toolhead in