212 lines
8.5 KiB
TeX
212 lines
8.5 KiB
TeX
\subsection{Serial Console}
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One core part of any computer systems is it's way to get human input. On older
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systems, and even today on server machines, this is done via a serial console.
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On this serial console characters are transmitted in serial, which means bit
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by bit over the same line. The voltage levels used in these systems vary from
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5V to 3.3V or +-10V. The most common standard for these voltage levels is the
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former RS-232\footnote{RS... Recommended Standard}, or as it should be called
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now, TIA-\footnote{TIA...Telecommunications Industry
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Association}/EIA-\footnote{EIA.. Electronic Industries Alliance}232.\cite{rs232}
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Voltage-levels, as per TIA-/EIA-232 standard, are not practical to handle over
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short
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distances however, so other voltages are used on most interface chips
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and need to be converted.
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\subsubsection{16550 UART}
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The 16550 UART\footnote{Uinversal Asynchronous Receiver and Transmitter} is a
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very common interface chip for serial communications. It produces 5V logic
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levels as output on TX and needs the same as input on RX. Though common for a
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UART, these voltage levels need to be converted to TIA-/EIA-232 levels for a
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more common interface.
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The 16550 UART is in it's core a 16450 UART, but has been given a FIFO
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\footnote{First-In First-Out} buffer. It needs three address lines, and 8
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data lines, which can be seen in Figure \ref{fig:16550_pinout}
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\begin{figure}[H]
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\centering
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\includesvg[height=.3\textheight, angle=0]{pics/pc16550d_pinout.svg}
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\caption{PC-16550D Pinout\cite{pc16550}}
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\label{fig:16550_pinout}
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\end{figure}
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In Figure \ref{fig:16550_pinout} the most important lanes are the SIN and
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SOUT lanes, as they contain the serial data to and from the 16550 UART.
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\subsubsection{MAX-232}
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To convert the voltage levels of the 16550 UART to levels compliant with
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TIA-/EIA-232 levels the MAX-232 is used. It has two transmitters and two
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receivers and generates the needed voltage levels via an internal voltage
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pump\cite{max232}.
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\subsubsection{Schematics}
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Based on the descriptions in the datasheets, the schematic in figure
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\ref{fig:schem_uart} was developed.
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\begin{figure}[H]
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\centering
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\includegraphics[height=.65\textheight, angle=-90]{schem_pdf/16550.pdf}
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\caption{The schematic of the UART Module}
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\label{fig:schem_uart}
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\end{figure}
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\paragraph{Element Description}
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The quartz oszillator Y1 is the clock source for the Baud Rate generation and
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was chosen with 1.8432 MHz for availability reasons and because it is the lowest
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ozillator from which all common baud rates can still be derived
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\cite{pc16550}.
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Resistors R1 and R2 are for stability and functionality of the Oszillator
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nescessary as per datasheet. The resulting frequency can be measured via
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J1 as can be seen in Figure \ref{fig:uartquartz}. C1 is used to
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stabilize the
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voltage for the 16550 UART and is common practice. Via JP1 the UART can be
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transformed into a USRT, where the receiver is synchronized to the transmitter
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via a clock line. This mode has, however, not been tested, and the clock needs
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to be 16 times the receiver clock rate\cite{pc16550}. The final output of the
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16550 UART can be used and measured via J2, as shown in Figure \ref{fig:uart232}
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. Before the UART on J2 can be used however, the Jumpers JP2 and JP3 need to be
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removed, as otherwise the MAX-232 will short out with the incoming signal.
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Capacitors C4, C6, C7 and C8 are for the voltage pump as defined in the
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datasheet\cite{max232}. R4 and R5 have been suggested by the supervisor in
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order to avoid damage to the MAX-232. The RJ-45 plug is used to transmit the
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TIA-/EIA-232 signal, rather than the more common D-SUB connector, because the
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RJ-45 connector fits on a 2.54mm grid. The Pinout of the RJ-45 plug can be seen
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in Figure \ref{fig:rs232rj45}. C5 has the same functionality for the
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MAX-232 as the C1 has to the 16550-UART.
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\begin{figure}[H]
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\begin{tikzpicture}
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\begin{axis}[
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ylabel=Quartz Voltage,
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xlabel=Time,
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grid=both,
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minor tick num=5,
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width=\textwidth,
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height=0.5\textheight]
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\addplot table [x=t, y=c1, col sep=comma, mark=none] {meas/20200319uartquartz.csv};
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\end{axis}
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\end{tikzpicture}
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\caption{Measurement of the 1.8432 MHz Output on J1}
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\label{fig:uartquartz}
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\end{figure}
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\begin{figure}[H]
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\begin{tikzpicture}
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\begin{axis}[
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ylabel=Lane Voltage,
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xlabel=Time,
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grid=both,
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minor tick num=5,
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width=\textwidth,
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height=0.5\textheight]
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\addplot table [x=t, y=c1, col sep=comma, mark=none] {meas/20200218ttluart.csv};
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\addplot table [x=t, y=c2, col sep=comma, mark=none] {meas/20200218ttluart.csv};
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\legend{TIA-/EIA-232 level,UART level}
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\end{axis}
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\end{tikzpicture}
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\caption{Measurement of a character transmission before and after MAX-232}
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\label{fig:uart232}
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\end{figure}
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\begin{figure}[H]
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\centering
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\includegraphics[width=\textwidth, angle=0]{pics/rj45-consoleport-iface-500.png}
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\caption{Pinout of the RJ-45 Plug; Src: \url{https://www.wti.com/}}
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\label{fig:rs232rj45}
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\end{figure}
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\subsubsection{Demonstration Software}
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To demonstrate the functionality and prove that the schematic has no underlying
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error, a program which regularly transmits a character was written as well as
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a simple echo program, which transmits all received characters. Both programs
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transmit 8 bit characters without parity at 38400 Baud. The output for program
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one can be seen in Figure \ref{fig:uart232} and the output for program two in
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Figure \ref{fig:232_echo}.
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\begin{figure}[H]
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\begin{tikzpicture}
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\begin{axis}[
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ylabel=Lane Voltage,
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xlabel=Time,
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grid=both,
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minor tick num=5,
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width=\textwidth,
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height=0.5\textheight]
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\addplot table [x=t, y=c1, col sep=comma, mark=none] {meas/20200218echo.csv};
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\addplot table [x=t, y=c2, col sep=comma, mark=none] {meas/20200218echo.csv};
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\legend{RX,TX}
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\end{axis}
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\end{tikzpicture}
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\caption{Measurement of a character echo}
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\label{fig:232_echo}
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\end{figure}
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\paragraph{Transmit code}
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The transmit code regularly transmits the letter capital A via the 16550 UART.
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Before it can do this it needs to perform some initialisations. The
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functions shown in Listing \ref{lst:16550-general} are the read and write
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routines for accessing the 16550 UART. These routines also apply to the echo
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code.
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\lstinputlisting[language=C,frame=trBL,
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breaklines=true, breakautoindent=true, formfeed=\newpage,
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label={lst:16550-general}, caption={Read and write routines for the
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16550 UART},
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columns=flexible, style=cstyle, firstline=0, lastline=69]
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{code/16550/transmit/src/main.c}
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To write to the 16550 UART, you need to perform some setup tasks. After startup,
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it requires a $MR$ for at least 5µs\cite{pc16550}. The baud rate divisor latch
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needs to be set to the specified divisor for the desired
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baud rate, and the character width and parity control
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needs to be set. The $MR$ signal is beeing generated by the AVR on bootup. To
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access the divisor latch, the divisor latch access bit needs to be set and
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after setting up the baud rate divisor latch, it nees to be cleared to allow
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a regular transmission. This process can be seen in Listing \ref{lst:16550-transmit}
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\lstinputlisting[language=C,frame=trBL,
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breaklines=true, breakautoindent=true, formfeed=\newpage,
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label={lst:16550-transmit}, caption={16550 INIT routines and single char transmission},
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columns=flexible, style=cstyle, firstline=71, lastline=106]
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{code/16550/transmit/src/main.c}
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The output of this code on the address, data and control bus as well as on the
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SOUT lane of the 16550 UART can be seen in Figure \ref{fig:16550A}
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\begin{figure}[H]
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\centering
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\includegraphics[width=\textwidth, angle=0]{meas/20200211_first_trans.png}
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\caption{Transmission of character A via the 16550 UART}
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\label{fig:16550A}
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\end{figure}
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\paragraph{Echo code}
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The echo code permanently polls the 16550 UART wether a character has been
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received, and if yes, reads it from the receiver holding register and writes it
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back to the tx holding register. The output of this code can be seen in Figure
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\ref{fig:232_echo}. The initialisation is practically the same as for the
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transmission code, as well as the read and write routines in Listing
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\ref{lst:16550-general}.
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\lstinputlisting[language=C,frame=trBL,
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breaklines=true, breakautoindent=true, formfeed=\newpage,
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label={lst:16550-echo}, caption={16550 character echo},
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columns=flexible, style=cstyle, firstline=76, lastline=109]
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{code/16550/echo/src/main.c}
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\subsubsection{Final Module}
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The final module can be seen in Figure \ref{fig:16550_mod} with the pc16550 UART
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in the center and the MAX-232 above.
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\begin{figure}[H]
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\centering
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\includegraphics[width=\textwidth, angle=0]{pics/uart}
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\caption{The final uart module with the pc16550 uart in the center}
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\label{fig:16550_mod}
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\end{figure}
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