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\title{Simple UART}
\author{S\'ebastien Bourdeauducq}
\date{December 2009}
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\section{Specifications}
The UART is based on a very simple design from Das Labor. Its purpose is basically to provide a debug console.

The UART operates with 8 bits per character, no parity, and 1 stop bit. The default baudrate is configured during synthesis and can be modified at runtime using the divisor register.

The divisor is computed as follows :
\begin{equation*}
\text{divisor} = \frac{\text{Clock frequency (Hz)}}{16 \cdot \text{Bitrate (bps)}}
\end{equation*}

\section{Registers}
\begin{tabularx}{\textwidth}{|l|l|l|X|}
\hline
\bf{Offset} & \bf{Read/Write} & \bf{Default} & \bf{Description} \\
\hline
0x0 & RW & 0x00 & Data register. Received bytes and bytes to transmit are read/written from/to this register. \\
\hline
0x4 & RW & for default bitrate & Divisor register (for bitrate selection). \\
\hline
\end{tabularx}\\

\section{Interrupts}
The core has two active-high edge-sensitive interrupts outputs.

The ``RX'' interrupt is sent whenever a new character is received. The CPU should then read the data register immediately. If a new character is sent before the CPU has had time to read it, the first character will be lost.

The ``TX'' interrupt is sent as soon as the UART finished transmitting a character. When the CPU has written to the data register, it must wait for the interrupt before writing again.

\section{Using the core}
Connect the CSR signals and the interrupts to the system bus and the interrupt controller. The \verb!uart_txd! and \verb!uart_rxd! signals should go to the FPGA pads. You must also provide the desired default baudrate and the system clock frequency in Hz using the parameters.

\section*{Copyright notice}
Copyright \copyright 2007-2009 S\'ebastien Bourdeauducq. \\
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the LICENSE.FDL file at the root of the Milkymist source distribution.

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