Labs/final/report.tex

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\title{Final Project Report}
\author{Danila Fedorin}
\begin{document}
\maketitle
\tableofcontents
\pagebreak
\section{General Design and Considerations}
The goal of this assignment was to create a 256-byte SRAM memory unit. In order
to minimize wire delays, I chose to split each bit into \textbf{4 columns of 64 SRAM cells
each}. This was motivated by the following factors:
\begin{itemize}
    \item \emph{Larger} columns were eliminated due to the high cost of interconnect.
        Even large write blocks were not able to charge the ``far ends'' of the wire
        at shorter clock cycles. Increasing wire width did not help; although resistance
        decreased, the capacitance increased, leading to small net gains. Thus, I made
        the decision to shrink the columns as much as possible. However...
    \item \emph{Smaller} columns became a routing challenge. Even with a 4-column split,
        to properly connect each cell of the SRAM column, the SRAM cells themselves need
        to accommodate an additional three \textsc{Wl} lines. Due to the pitch requirements
        on metals three and four, this is the upper limit (for reasonably sized cells).
        Alternatives included splitting the decoder into pieces, but for large numbers
        of columns, this meant that the decoder signal traveled through significant amounts
        of wire, and was thus slower.
\end{itemize}

For each of the 4 64-bit columns, I attached separate read and write blocks. However,
my placement of the write block was unorthodox. I observed that, although the write block
is perfectly capable of quickly manipulating the bitlines close to it, the changes
to the wires take too long to propagate through to the end. I addressed this with two separate
changes:

\begin{itemize}
    \item I added \textbf{additional precharge transistors} along the column, a total of 4.
        Each was sized at $10\lambda$, much like the SRAM transistors themselves. When the clock
        was low, these PMOS transistors became transparent, and helped precharge the bitlines faster.
        Doing so helped avid hysteresis. However, this did not help with writing during high clock,
        so...
    \item I also \textbf{placed the write block in the middle of the column}. This increased the distance
        between my furthest SRAM cell and the read block (since the write block now contributed to wire
        length). However, this made it significantly easier to drive the entire length of the wire,
        which was my main bottleneck. This was because the maximum distance from the write
        block to any cell in the column was halved. Since my read circuit continued to work in this
        configuration, I did not place it in the middle of the column, as that would needlessly
        increase the length of the wires. 
\end{itemize}
%
This led to the configuration shown in Figure \ref{fig:top-design}. To simulate this design, I \textbf{tested three configurations}:
\begin{enumerate}
    \item A memory cell at the very top of my column, which is the furthest spot from both the read and write.
        This is the simulation in the figure.
    \item A memory cell in the middle of my column, in the same place as the write block. Since the write block
        has brief ``false starts'', this test was to ensure that the read block can still pick up data
        despite the write block's misfires.
    \item A memory cell at the very bottom of my column. This area has additional capacitance from the read block;
        it thus takes longer to charge up, and tends to be the first spot where writes fail.
circuit. 
%
\end{enumerate}
I also split the wire into 4 equally-sized fragments, each with resistance $\frac{R}{4}$ and
capacitance $\frac{C}{4}$. Between each fragment, I added the aforementioned $10\lambda$ precharge
transistors, as well as 16 always-off $5\lambda$ transistors, which simulated the remaining memory cells.
I also placed \textsc{Din}, \textsc{Ad0}, and \textsc{Rwt} behind the default-sized flip-flops
attached to the clock to simulate something like a pipeline stage. My overall design is shown
in Figure \ref{fig:top-design-sim}.

\pagebreak
\begin{figure}[h]
    \centering
    \includegraphics[width=\linewidth]{toplevel_design.png}
    \caption{Top-level design for a single bit.}
    \label{fig:top-design}
\end{figure}

My SRAM cell ended up being $30\lambda$ units tall when arrayed. With
a total of 64 cells in a single column, this led to a wire length of $1920\lambda$.
However, since my write block was now included in the column, I added another $300\lambda$
of length to this number, to a total of roughly $2200\lambda$.

\begin{figure}
    \centering
    \includegraphics[width=0.6\linewidth]{toplevel.png}
    \caption{Architecture of top-level simulation.}
    \label{fig:top-design-sim}
\end{figure}

\pagebreak
\section{Performance Results}
I was able to clock my design at $1.9\textit{ns}$.
%
Two factors lead to these upper limits. 
%
\begin{itemize}
    \item \textit{Write capacitance} makes it increasingly difficult to overwrite the value
        in the cell. Clocking my design any faster leads my cell to \textit{almost} flip, but not resolve correctly.
        I have found no way to work around these limits once my wire was properly sized, and my
        write block was placed in the middle of the column.
    \item \textit{Flop, decoder, and read delays} are the major limitation when both the inputs
        and the outputs of the circuit are connected to flip flops. The most significant
        instance of this issue is my write block: both \textsc{Din} and \textsc{Rwt} arrive
        around $300\textit{ps}$ into the cycle. This means two things: a) if the previous
        operation was ``read'', then the block does not start writing until halfway into
        the positive phase of the clock and b) if the data being written is different
        from the data in the previous cycle, for half the time, the write block will write
        the old data (until the flip flop switches).
\end{itemize}

\section{Components}
\subsection{Decoder}
\subsubsection{In My Own Words}
The decoder in this design is \textit{almost} the exact same one as we were given in lecture.
It computes all combinations of two consecutive bits using a \textsc{Nand} gate; for
each combination, there are 4 adjacent two-bit combinations,
leading to a 4 \textsc{Nor} gates connected to each \textsc{Nand}. There are now
16 combinations of 4 adjacent bits; each combination of the lower 4 bits
needs to be compared with each of the 16 combinations of the upper 4 bits,
leading to 16 \textsc{Nand} gates connected to each \textsc{Nor}. This
results in 256 unique \textsc{Wl} wires. Finally, these need to be attached
to the clock, so that cells aren't open randomly. This is done using an \textsc{And}
gate (a \textsc{Nand} followed by an inverter).

I adjusted this design to account for the address signals that need to be fed
into the write blocks. Which of the read/write columns is triggered
depends on the upper two bits of the address (since we have 4 columns). I modeled
this by increasing the fanout on the first \textsc{Nand} gate from 1 to 4.
This is pessimistic; each 2-bit combination would only feed into one write block,
whose trigger gate is normally sized.

\begin{figure}[h]
    \centering
    \includegraphics[width=\linewidth]{decoder.png}
    \caption{Decoder model used in project.}
    \label{fig:decoder}
\end{figure}

% TODO: Domino logic
% TODO: More inverters?

\pagebreak
\subsection{Read Block}
\subsubsection{In My Own Words}
The read block uses a \emph{sense amplifier} to detect small changes on the bitlines,
which it then translates into a zero-or-one output. The changes in the wires are below
the threshold of what could be considered digital logic; all the sense amplifier
designs I've come across rely on metastability, a state in which even tiny fluctuations
can significantly alter the outcome\footnote{My favorite analogy is a pencil balanced on its tip.
Technically, it's stable; however, even a small air current -- one you can't feel -- can knock it over.}.
The \textsc{Trigger} signal, which depends on the clock and \textsc{Rwt}, puts the amplifier
into a metastable state. From there, the connected bitlines cause it to resolve one way
or another. Finally, if one of the wires resolves, a value is written into the keeper circuit
at the end, which ensures that the value that was read continues to be expressed until
the next read operation.

\subsubsection{Details}
For my read block, I used a different sense amplifier design. The design based
on the two \textsc{Nand3} gates was easy to understand and build, but was less
sensitive, and tended to behave strangely under pressure. This led to difficulties
with debugging (the output would, for instance, flip completely at certain
wire widths), and was seemingly random. Instead, I used
an \textbf{improved latch-based sense amplifier design} from \cite{210039}. % TODO: cite
The design I used is shown in Figure \ref{fig:latch-amp}.
I left it sized at $40\lambda$, since larger amplifiers seem to take longer
to trigger and exit metastability.

The read block is not a particular bottleneck in this design. The main concern
was to handle the \textbf{``false start'' activation of the write block}. Because the \textsc{Rwt}
input is behind a latch, it takes nearly $300\textit{ps}$ to pull up or down after
the initial clock. Thus, if a write occurred during a previous cycle, the write block will
activate for a short period of time before the read block does. The memory cell
will overpower this initial misfire\footnote{According to my additional simulations, this is true even when the memory cell is close to the write block.}, but in this case, both \textsc{Bt} and \textsc{Bf}
will be below \textsc{Vdd}. The ``improved sense amplifier'' seems to handle this
case better than the one based on two \textsc{Nand} gates. 

The latch-induced delay in \textsc{Rwt} also causes a strange \textsc{Trigger} signal during write operations
directly following read operations. The trigger signal initialy activates, putting the sense
amplifier into metastability; however, the correct \textsc{Rwt} value arrives before the
sense amp's outputs are compromised. If this became a problem, I would add an additional,
delayed clock signal \emph{after} the sense amplifier, and use an \textsc{And} gate
to delay the read block's output.

\begin{figure}[h]
\centering
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.7\linewidth]{amp.png}
  \caption{The latch-based sense amplifier from \cite{210039}.}
  \label{fig:latch-amp}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.8\linewidth]{read_select.png}
  \caption{The block gathering signals from the four columns.}
  \label{fig:read-collect}
\end{subfigure}
\caption{Read block schematics}
\label{fig:read}
\end{figure}

\pagebreak
\subsection{Write Block}
\subsubsection{In My Own Words}
The write block converts a ``data in'', or \textsc{Din}, signal
into a one-hot representation. It does so by pulling one of the bitlines high, and the other
low. Once the memory cell connects to the bitlines, it takes on the charge provided by the
write block, and is therefore overwritten. In my design, two PMOS transistors for each bitline
are used to pull down; one of the transistors is triggered by the \textsc{Din} signal (which wire
we pull down depends on the signal itself!), and the other by a combination of the clock
and \textsc{Rwt} (we don't want to touch the wires when reading!).

\subsubsection{Details}
My write block was not significantly different from the original design. Under the assumption
that data arrives first, I placed the transistors attached to \textsc{Din} and $\overline{\textsc{Din}}$
close to \textsc{Gnd}, each followed by a transistor attached to the ``write'' signal.
I also configured the write block to only precharge when the clock is low.

I experimented with making the write block pull wires up when writing (during high clock). However,
I did not find this to be of significant use. Since the wires are initially precharged,
there is no more time spent on charging them up; furthermore, the memory cell being written to
does not have enough ``strength'' to pull the wire down enough.

A curiosity of this design is that reads didn't seem to work with hich clock speeds. When enough
time is spent reading the wires, the memory cell in question is able to gradually exhaust the amount
of charge on one of these wires. Since the original, \textsc{Nand}-based sense amplifier required
all inputs to be high to properly function, this led to it eventually ``flipping'' and producing
the wrong output. This was only an issue above $5\textit{ns}$, and only with the original sense amplifier
design, though. I think that both Reed and
Graham experienced this occurrence -- they seemed to post very similar waveforms
to the community Discord group chat.

One thing to note about the write block is that its \textbf{clock input is deliberately delayed} compared
to the ``actual'' clock. This is because of an issue with \textsc{Din}. Since this
input is behind a latch, it takes around $300\textit{ps}$ to arrive after the rising clock
edge. If the previous value of \textsc{Din} was different than its current one, the write
block will start writing the wrong value. This will typically mean that the block cannot properly
perform the write. The delay on the clock input serves to mitigate this issue, by giving more
time for \textsc{Din} to settle before starting to write. To compensate for this delay, I sized
the write block's pull down transistors quite large ($100\lambda$), so that they can pull
the wire down, even starting $300\textit{ps}$ into the cycle. This is why the ``clock'' input
in my diagrams is colored black, unlike every other clocked component. The delay is achieved
by 6 sequenced inverters, two of which are sized 10x larger than the rest.

\begin{figure}[h]
    \centering
    \includegraphics[width=0.65\linewidth]{write.png}
    \caption{Write block used in this project.}
    \label{fig:write}
\end{figure}

\pagebreak
\subsection{Memory Cell}
\subsubsection{In My Own Words}
The memory cell consists of two cross coupled inverters whose outputs
are disconnected from the bitlines by two additional nMOS transistors. When disconnected,
this cell reliably holds its value; one inverter's output turns off the other, and symmetrically,
the ``off'' output of that other inverter keeps the first one on. However, this cell is pretty
small; all of its transistors have size $5\lambda$ is the smallest size that can be properly
connected with a standard $2\lambda\times2\lambda$ via. Thus, when the ``write line'' (signal
connected to the gates of the two outside transistors) is asserted, the charge from the
surrounding bitlines can easily overpower the cell, causing it to switch to a different value.

\subsubsection{Details}
There are few notable things about my cell design. Even though it was recommended that we only
use metals one and two for the internal wiring, I went up to metal three for cross-connecting
the two internal inverters. This was the only way I found to keep the height of the cell to 
minimum. This limited my routing options somewhat; to compensate, I also used metal three for
the vertical wires, \textsc{Bt} and \textsc{Bf}. This allowed me to use metal four for the
\textsc{Wl} (access) signal. Since this was the only use of metal four, I had enough free
room to route thee additional \textsc{Wl} signals to the remaining three columns. 

My general principle for designing the layout was that, in an 8-bit, 4-column design, \textbf{a single
unit of height costs as much as 64 units of width}. Thus, I was fairly liberal with my layout's
width, but made sure to minimize the height of the design. The most significant bottleneck
was the gate oxide ``poking out'' of the ends of the design. In total, I was able to achieve
a height of $30\lambda$ when arrayed.

Other designs with smaller height were possible, but I found them undesirable. For instance,
Reed's now-famous design used a significant amount of high-level metals to achieve its tiny,
almost square area. This, however, makes routing \textsc{Wl} signals fairly complicated. They either
need to go to yet another layer of metal, or the decoder needs to be split into 4 pieces. The former
is undesirable as per the requirements for this assignment; the latter incurs the cost of additional
decoder hardware between columns, thereby significantly increasing the wire length and signal
delays. Since delays incurred by the flip flops and other signals are already becoming
a significant factor in my design, I thought it would be best to avoid such delays.

Other ideas I am aware of include putting \textit{all} the transistors in a single, horizontal line.
While this certainly succeeds at reducing the height, it incurs all the same issues described
above - it becomes nigh impossible to wire further \textsc{Wl} lines through each column,
unless the decoder is split into bits, in which case the width of the entire assembly drastically increases,
slowing down all signals.

\begin{figure}[h]
    \centering
    \includegraphics[width=0.5\linewidth]{layout_single.png}
    \caption{Electric layout for a single cell.}
    \label{fig:layout-cell}
\end{figure}

\pagebreak
My basic cell is shown in Figure \ref{fig:layout-cell}. The arrayed version (in Figure \ref{fig:layout-arrayed})
merits additional explanation. In my earlier description of the overall design, I mentioned
that I have precharge PMOS transistors. I have integrated these into my layout to accurately model
my design. I also made them $10\lambda$ wide, since this is, at the time of writing,
the size of my 4 precharge transistors. In the bird's eye view (Figure \ref{fig:layout-arrayed-far}),
three things can be observed:
\begin{itemize}
    \item \textit{Additional vertical line:} This line represents the clock signal,
        which must be fed to the precharge transistors. In the full design, there would
        be 5 clock lines (3 shared, and 2 on either side).
    \item \textit{``Empty'' space between nodes:} I left this space because I was not sure
        how wide I would end up making my \textsc{Bt} and \textsc{Bf} wires. I have measured
        the distance to ensure that the design will remain DRC clean with up to \textbf{$8\lambda$-wide bitlines}.
        This appears to be a sweet spot for my design, anyway.
    \item \textit{Moved well contacts:} I have moved my well contacts to the region between
        two columns. By extending the N- and P-wells to this area, I was able to
        share a single contact between two cells, leaving room for prechare transistors
        on both sides of the cell. This was partially inspired by Reed's compact cell design,
        which shared a single contact between two cells\footnote{I am operating based on your
        comment that well contacts for every cell are significantly overkill.}.
\end{itemize}
Figure \ref{fig:layout-arrayed-close} shows a closer view of the design. Due to the additional
space incurred, an entire column is approximately $100\lambda$ wide.

\begin{figure}[h]
\centering
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.7\linewidth]{layout_arrayed.png}
  \caption{Bird's eye view of the arrayed SRAM cells.}
  \label{fig:layout-arrayed-far}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
  \centering
  \includegraphics[width=.8\linewidth]{layout_arrayed_closeup.png}
  \caption{Close up from arrayed SRAM cells.}
  \label{fig:layout-arrayed-close}
\end{subfigure}
\caption{Read block schematics}
\label{fig:layout-arrayed}
\end{figure}

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