Add explanatory note about C_DEBUGEN

Author: Brandon Matthews

src/peripheral/dcb.rs

@@ -34,8 +34,14 @@ impl DCB {
         // unset bit 24 / TRCENA
         unsafe { self.demcr.modify(|w| w & !DCB_DEMCR_TRCENA); }
 
-    /// Is there a debugger attached?
-    pub fn is_debugger_attached(&self) -> bool {
-        self.dhcsr.read() & 0x1 == 1
+    /// Is there a debugger attached? (see note)
+    ///
+    /// N.B. This function is [reported not to
+    /// work](http://web.archive.org/web/20180821191012/https://community.nxp.com/thread/424925#comment-782843)
+    /// on Cortex-M0 devices. Per the ARM v6-M Architecture Reference Manual, "Access to the DHCSR
+    /// from software running on the processor is IMPLEMENTATION DEFINED". Indeed, from the
+    /// [Cortex-M0+ r0p1 Technical Reference Manual](http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0484c/BABJHEIG.html), "Note Software cannot access the debug registers."
+    pub fn is_debugger_attached() -> bool {
+        unsafe { (*Self::ptr()).dhcsr.read() & 0x1 == 1 }
     }
 }