Add note on M4 clock freq

refs.bib

@@ -547,3 +547,10 @@
   pages         = {43--50},
   note          = {\url{http://joye.site88.net/papers/BJ03isog.pdf}},
 }
+
+@misc{STM32F407,
+  author        = {{STMicroelelectronics}},
+  title         = {{RM0090 reference manual}},
+  year          = 2019,
+  note          = {\url{https://www.st.com/content/ccc/resource/technical/document/reference_manual/3d/6d/5a/66/b4/99/40/d4/DM00031020.pdf/files/DM00031020.pdf/jcr:content/translations/en.DM00031020.pdf}},
+}
\ No newline at end of file

results.tex

@@ -9,6 +9,9 @@ and the ARM STM32F407 (Cortex-M4).
 On the Intel processors, all measurements were done with Turbo Boost disabled, 
 all Hyper-Threading cores shut down, and with the CPU clocked at the maximum
 nominal frequency.
+The STM32F407 device was run at its default settings,
+as listed in the datasheet~\cite{STM32F407}
+(i.e.~clocked from the internal 16\unit{MHz} internal RC-oscillator).
 We list the benchmarking results in Table~\ref{tab:benchmarks}. As expected,
 none of our implementations exceed the performance of Curve25519.