Suggested edits to the interrupts chapter

mdbook/src/15-interrupts/nvic-and-interrupt-priority.md

@@ -30,6 +30,10 @@ Preemption allows processors to respond very quickly to critical events.  For ex
 
 If an equal-priority or lower-priority interrupt occurs during an ISR, it will be "pended": the NVIC will remember the new interrupt and run its ISR sometime after the current ISR completes.  When an ISR function returns the NVIC looks to see if, while the ISR was running, other interrupts have happened that need to be handled. If so, the NVIC checks the interrupt table and calls the highest-priority ISR vectored there. Otherwise, the CPU returns to the running program.
 
+Note that in most Cortex-M-based systems, *higher-priority* interrupts will be pended by the NVIC 
+*even* if all interrupts are disabled by a critical section (discussed later in this chapter), 
+though their execution will be delayed until the critical section ends.
+
 In embedded Rust, we can program the NVIC using the [`cortex-m`] crate, which provides methods to
 enable and disable (called `unmask` and `mask`) interrupts, set interrupt priorities, and trigger
 interrupts from software. Frameworks such as [RTIC] can handle NVIC configuration for you, taking

mdbook/src/15-interrupts/sharing-data-with-globals.md

@@ -4,8 +4,8 @@
 > *[Interrupts Is Threads]* by James Munns, which contains more discussion about this
 > topic.
 
-As I mentioned in the last section, when an interrupt occurs we aren't passed any arguments and
-cannot return any result. This makes it hard for our program interact with peripherals and other
+As I mentioned previously, when an interrupt occurs we aren't passed any arguments and
+cannot return any result. This makes it hard for our program to interact with peripherals and other
 main program state. Before worrying about this bare-metal embedded problem, it is likely worth
 thinking about threads in "std" Rust.
 

mdbook/src/15-interrupts/the-challenge.md

@@ -4,7 +4,7 @@ Let's make the MB2 into a siren! But not just any siren — an
 interrupt-driven siren. That way we can turn the siren on
 and the rest of our program can run on, ignoring it.
 
-Make your siren sweep the pitch from 220Hz to 440Hz and back
+Make your siren sweep the pitch from 440Hz to 660Hz and back
 over a one-second period. The main program should start the
 siren, then print a ten-second countdown from 10 to 1, then
 stop the siren and print "launch!". The main program should