Add details on frequency measurement

1 files changed, 13 insertions, 5 deletions

diff --git a/paper/safety-reset-paper.tex b/paper/safety-reset-paper.tex
index c71ec26..a7d4344 100644
--- a/paper/safety-reset-paper.tex
+++ b/paper/safety-reset-paper.tex
@@ -122,7 +122,11 @@ Spectrum (DSSS) modulation carried out on grid frequency through a large control
 After forward error correction and cryptographic verification it re-flashes the meter's main microcontroller over the
 standard JTAG interface.  Note that our modulation technique is \emph{changing the grid frequency itself}.  This is
 fundamentally different in both generation and detection from systems such as traditional PLC that superimpose a signal
-on grid voltage, but leave the underlying grid frequency itself unaffected.
+on grid voltage, but leave the underlying grid frequency itself unaffected. The safety reset controller is an
+off-the-shelf microcontroller much smaller than the one used for the meter's main application controller. It measures
+grid frequency from a voltage waveform acquired using its internal analog-to-digital-converter (ADC) directly connected
+to the mains voltage input through a resistive divider chain. The use of an off-the-shelf microcontroller keeps the
+implementation overhead of our solution very low in both per-unit and engineering cost.
 
 \begin{figure}
     \centering
@@ -340,6 +344,7 @@ while the public internet and mobile networks are still offline and it is unaffe
 telecommunication networks.
 
 \subsection{Characterizing Grid Frequency}
+\label{grid-freq-characterization}
 
 In utility SCADA systems, Phasor Measurement Units (PMUs, also called \emph{synchrophasors}) are used to precisely
 measure grid frequency among other parameters.  This task is much more complicated in practice than it might appear at
@@ -515,10 +520,13 @@ the meter's display after boot-up.
     \label{fig_demo_sig_schema}
 \end{figure}
 
-Since we did not have an aluminium smelter ready, we decided to feed our proof-of-concept reset controller with an
-emulated grid voltage sine wave from a computer's headphone jack. Where in a real application this microcontroller might
-take ADC readings of input mains voltage divided down by a long resistive divider chain, we instead feed the ADC from a
-$\SI{3.5}{\milli\meter}$ audio input. For operational safety, we disconnected the meter microcontroller from its
+To measure grid frequency in our demonstrator, we ported the same code we used in
+Section~\label{grid-freq-characterization} to our demonstrator, again using the voltage measured using the
+microcontroller's internal ADC but using a regular crystal instead of a crystal oven for the microcontroller's system
+clock.  Since we did not have an aluminium smelter ready, we decided to feed our proof-of-concept reset controller with
+an emulated grid voltage sine wave from a computer's headphone jack. Where in a real application this microcontroller
+would  take ADC readings of input mains voltage divided down by a long resistive divider chain, we instead feed the ADC
+from a $\SI{3.5}{\milli\meter}$ audio input. For operational safety, we disconnected the meter microcontroller from its
 grid-referenced capacitive dropper power supply and connected it to our reset controlller's debug USB power supply.
 
 We performed several successful experiments using a signature truncated at 120 bit and a 5 bit DSSS sequence. Taking the