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<title>Project IceStorm – LOGIC Tile Documentation</title>
</head><body>
<h1>Project IceStorm – LOGIC Tile Documentation</h1>
<p>
<i><a href=".">Project IceStorm</a> aims at documenting the bitstream format of Lattice iCE40
FPGAs and providing simple tools for analyzing and creating bitstream files.
This is work in progress.</i>
</p>
<h2>Span-4 and Span-12 Wires</h2>
<p>
The <i>span-4</i> and <i>span-12</i> wires are the main interconnect resource in iCE40 FPGAs. They "span" (have a length of)
4 or 12 cells in horizontal or vertical direction.
</p>
<p>
All routing resources in iCE40 are directional tristate buffers. The bits marked <span style="font-family:monospace">routing</span>
use the all-zeros config pattern for tristate, while the bits marked <span style="font-family:monospace">buffer</span> have
a dedicated buffer-enable bit, which is 1 in all non-tristate configurations.
</p>
<h3 style="clear:both">Span-4 Horizontal</h3>
<p><a href="sp4h.svg"><img alt="Span-4 Horizontal" style="float:right; padding:1em; padding-top:0; border:0" height="200" src="sp4h.svg"></a></p>
<p>
The image on the right shows the <i>horizontal span-4</i> wires of a logic or ram cell (click to enlarge).
</p>
<p>
On the left side of the cell there are 48 connections named <span style="font-family:monospace">sp4_h_l_0</span> to <span style="font-family:monospace">sp4_h_l_47</span>. The lower 36 of those
wires are connected to <span style="font-family:monospace">sp4_h_r_12</span> to <span style="font-family:monospace">sp4_h_r_47</span> on the right side of the cell. (IceStorm normalizes this
wire names to <span style="font-family:monospace">sp4_h_r_0</span> to <span style="font-family:monospace">sp4_h_r_35</span>. Note: the Lattice tools use a different normalization scheme
for this wire names.) The wires connecting the left and right horizontal span-4 ports are pairwise crossed-out.
</p>
<p>
The wires <span style="font-family:monospace">sp4_h_l_36</span> to <span style="font-family:monospace">sp4_h_l_47</span> terminate in the cell as do the wires <span style="font-family:monospace">sp4_h_r_0</span> to <span style="font-family:monospace">sp4_h_r_11</span>.
</p>
<p>
This wires "span" 4 cells, i.e. they connect 5 cells if you count the cells on
both ends of the wire.
</p>
<p>
For example, the wire <span style="font-family:monospace">sp4_h_r_0</span> in cell (x, y) has the following names:
</p>
<table class="ctab">
<tr><th>Cell Coordinates</th><th>sp4_h_l_* wire name</th><th>sp4_h_r_* wire name</th></tr>
<tr><td>x, y</td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">sp4_h_r_0</span></td></tr>
<tr><td>x+1, y</td><td><span style="font-family:monospace">sp4_h_l_0</span></td><td><span style="font-family:monospace">sp4_h_r_13</span></td></tr>
<tr><td>x+2, y</td><td><span style="font-family:monospace">sp4_h_l_13</span></td><td><span style="font-family:monospace">sp4_h_r_24</span></td></tr>
<tr><td>x+3, y</td><td><span style="font-family:monospace">sp4_h_l_24</span></td><td><span style="font-family:monospace">sp4_h_r_37</span></td></tr>
<tr><td>x+4, y</td><td><span style="font-family:monospace">sp4_h_l_37</span></td><td><span style="font-family:monospace">-</span></td></tr>
</table>
<h3 style="clear:both">Span-4 Vertical</h3>
<p><a href="sp4v.svg"><img alt="Span-4 Vertical" style="float:right; padding:1em; padding-top:0; border:0" height="200" src="sp4v.svg"></a></p>
<p>
The image on the right shows the <i>vertical span-4</i> wires of a logic or ram cell (click to enlarge).
</p>
<p>
Similar to the horizontal span-4 wires there are 48 connections on the top (<span style="font-family:monospace">sp4_v_t_0</span> to <span style="font-family:monospace">sp4_v_t_47</span>) and
48 connections on the bottom (<span style="font-family:monospace">sp4_v_b_0</span> to <span style="font-family:monospace">sp4_v_b_47</span>). The wires <span style="font-family:monospace">sp4_v_t_0</span> to <span style="font-family:monospace">sp4_v_t_35</span>
are connected to <span style="font-family:monospace">sp4_v_b_12</span> to <span style="font-family:monospace">sp4_v_b_47</span> (with pairwise crossing out). Wire names are normalized
to <span style="font-family:monospace">sp4_v_b_12</span> to <span style="font-family:monospace">sp4_v_b_47</span>.
</p>
<p>
But in addition to that, each cell also has access to <span style="font-family:monospace">sp4_v_b_0</span> to <span style="font-family:monospace">sp4_v_b_47</span> of its right neighbour.
This are the wires <span style="font-family:monospace">sp4_r_v_b_0</span> to <span style="font-family:monospace">sp4_r_v_b_47</span>. So over all a single vertical span-4 wire
connects 9 cells. For example, the wire <span style="font-family:monospace">sp4_v_b_0</span> in cell (x, y) has the following names:
</p>
<table class="ctab">
<tr><th>Cell Coordinates</th><th>sp4_v_t_* wire name</th><th>sp4_v_b_* wire name</th><th>sp4_r_v_b_* wire name</th></tr>
<tr><td>x, y</td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">sp4_v_b_0</span></td><td><span style="font-family:monospace">-</span></td></tr>
<tr><td>x, y-1</td><td><span style="font-family:monospace">sp4_v_t_0</span></td><td><span style="font-family:monospace">sp4_v_b_13</span></td><td><span style="font-family:monospace">-</span></td></tr>
<tr><td>x, y-2</td><td><span style="font-family:monospace">sp4_v_t_13</span></td><td><span style="font-family:monospace">sp4_v_b_24</span></td><td><span style="font-family:monospace">-</span></td></tr>
<tr><td>x, y-3</td><td><span style="font-family:monospace">sp4_v_t_24</span></td><td><span style="font-family:monospace">sp4_v_b_37</span></td><td><span style="font-family:monospace">-</span></td></tr>
<tr><td>x, y-4</td><td><span style="font-family:monospace">sp4_v_t_37</span></td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">-</span></td></tr>
<tr><td>x-1, y</td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">sp4_r_v_b_0</span></td></tr>
<tr><td>x-1, y-1</td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">sp4_r_v_b_13</span></td></tr>
<tr><td>x-1, y-2</td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">sp4_r_v_b_24</span></td></tr>
<tr><td>x-1, y-3</td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">-</span></td><td><span style="font-family:monospace">sp4_r_v_b_37</span></td></tr>
</table>
<h3 style="clear:both">Span-12 Horizontal and Vertical</h3>
<p>
Similar to the span-4 wires there are also longer horizontal and vertical span-12 wires.
</p>
<p>
There are 24 connections <span style="font-family:monospace">sp12_v_t_0</span> to <span style="font-family:monospace">sp12_v_t_23</span> on the top of the
cell and 24 connections <span style="font-family:monospace">sp12_v_b_0</span> to <span style="font-family:monospace">sp12_v_b_23</span> on the bottom of the
cell. The wires <span style="font-family:monospace">sp12_v_t_0</span> to <span style="font-family:monospace">sp12_v_t_21</span> are connected to
<span style="font-family:monospace">sp12_v_b_2</span> to <span style="font-family:monospace">sp12_v_b_23</span> (with pairwise crossing out). The connections
<span style="font-family:monospace">sp12_v_b_0</span>, <span style="font-family:monospace">sp12_v_b_1</span>, <span style="font-family:monospace">sp12_v_t_22</span>, and <span style="font-family:monospace">sp12_v_t_23</span>
terminate in the cell. Wire names are normalized to <span style="font-family:monospace">sp12_v_b_2</span> to <span style="font-family:monospace">sp12_v_b_23</span>.
</p>
<p>
There are also 24 connections <span style="font-family:monospace">sp12_h_l_0</span> to <span style="font-family:monospace">sp12_h_l_23</span> on the left of the
cell and 24 connections <span style="font-family:monospace">sp12_h_r_0</span> to <span style="font-family:monospace">sp12_h_r_23</span> on the right of the
cell. The wires <span style="font-family:monospace">sp12_h_l_0</span> to <span style="font-family:monospace">sp12_h_l_21</span> are connected to
<span style="font-family:monospace">sp12_h_r_2</span> to <span style="font-family:monospace">sp12_h_r_23</span> (with pairwise crossing out). The connections
<span style="font-family:monospace">sp12_h_r_0</span>, <span style="font-family:monospace">sp12_h_r_1</span>, <span style="font-family:monospace">sp12_h_l_22</span>, and <span style="font-family:monospace">sp12_h_l_23</span>
terminate in the cell. Wire names are normalized to <span style="font-family:monospace">sp12_v_r_2</span> to <span style="font-family:monospace">sp12_h_r_23</span>.
</p>
<h2>Local Tracks</h2>
<p>
The <i>local tracks</i> are the gateway to the logic cell inputs. Signals from the span-wires
and the logic cell outputs of the eight neighbour cells can be routed to the local tracks and
signals from the local tracks can be routed to the logic cell inputs.
</p>
<p>
Each logic tile has 32 local tracks. They are organized in 4 groups of 8 wires each:
<span style="font-family:monospace">local_g0_0</span> to <span style="font-family:monospace">local_g3_7</span>.
</p>
<p>
The span wires, global signals, and neighbour outputs can be routed to the local tracks. But not
all of those signals can be routed to all of the local tracks. Instead there is a different
mix of 16 signals for each local track.
</p>
<p>
The buffer driving the local track has 5 configuration bits. One enable bit and 4 bits that select
the input wire. For example for <span style="font-family:monospace">local_g0_0</span> (copy&paste from the bitstream doku):
</p>
<table class="ctab">
<tr><th style="width:5em">B0[14]</th><th style="width:5em">B1[14]</th><th style="width:5em">B1[15]</th><th style="width:5em">B1[16]</th> <th style="width:5em">B1[17]</th>
<th style="width:5em">Function</th><th style="width:15em">Source-Net</th><th style="width:15em">Destination-Net</th></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_r_v_b_24</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp12_h_r_8</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">neigh_op_bot_0</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_v_b_16</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_r_v_b_35</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp12_h_r_16</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">neigh_op_top_0</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_h_r_0</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">lutff_0/out</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_v_b_0</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">neigh_op_lft_0</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_h_r_8</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">neigh_op_bnr_0</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_v_b_8</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp12_h_r_0</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">sp4_h_r_16</span></td><td><span style="font-family:monospace">local_g0_0</span></td></tr>
</table>
<p>
Then the signals on the local tracks can be routed to the input pins of the logic cells. Like before,
not every local track can be routed to every logic cell input pin. Instead there is a different mix
of 16 local track for each logic cell input. For example for <span style="font-family:monospace">lutff_0/in_0</span>:
</p>
<table class="ctab">
<tr><th style="width:5em">B0[26]</th><th style="width:5em">B1[26]</th><th style="width:5em">B1[27]</th><th style="width:5em">B1[28]</th><th style="width:5em">B1[29]</th>
<th style="width:5em">Function</th><th style="width:15em">Source-Net</th><th style="width:15em">Destination-Net</th></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g0_0</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g2_0</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g1_1</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g3_1</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g0_2</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g2_2</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g1_3</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g3_3</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g0_4</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g2_4</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g1_5</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g3_5</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g0_6</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g2_6</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g1_7</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">buffer</td><td><span style="font-family:monospace">local_g3_7</span></td><td><span style="font-family:monospace">lutff_0/in_0</span></td></tr>
</table>
<p>
The 8 global nets on the iCE40 can be routed to the local track via the <span style="font-family:monospace">glb2local_0</span> to <span style="font-family:monospace">glb2local_3</span>
nets using a similar two-stage process. The logic block clock-enable and set-reset inputs can be driven
directly from one of 4 global nets or from one of 4 local tracks. The logic block clock input can be driven
from any of the global nets and from a few local tracks. See the bitstream documentation for details.
</p>
<h2>Logic Block</h2>
<p>
Each logic tile has a logic block containing 8 logic cells. Each logic cell contains a 4-input LUT, a carry
unit and a flip-flop. Clock, clock enable, and set/reset inputs are shared along the 8 logic cells. So is the
bit that configures positive/negative edge for the flip flops. But the three configuration bits that specify if
the flip flop should be used, if it is set or reset by the set/reset input, and if the set/reset is synchronous
or asynchronous exist for each logic cell individually.
</p>
<p>
Each LUT <i>i</i> has four input wires <span style="font-family:monospace">lutff_<i>i</i>/in_0</span> to <span style="font-family:monospace">lutff_<i>i</i>/in_3</span>. Input
<span style="font-family:monospace">lutff_<i>i</i>/in_3</span> can be configured to be driven by the carry output of the previous logic cell,
or by <span style="font-family:monospace">carry_in_mux</span> in case of <i>i</i>=0. Input <span style="font-family:monospace">lutff_<i>i</i>/in_2</span> can be configured to
be driven by the output of the previous LUT for <i>i</i>>0 (LUT cascade). The LUT uses its 4 input signals to
calculate <span style="font-family:monospace">lutff_<i>i</i>/lout</span>. The signal is then passed through the built-in FF
and becomes <span style="font-family:monospace">lutff_<i>i</i>/out</span>. With the exception of LUT cascades, only the signal
after the FF is visible from outside the logic block.
</p>
<p>
The carry unit calculates <span style="font-family:monospace">lutff_<i>i</i>/cout</span> = <span style="font-family:monospace">lutff_<i>i</i>/in_1</span> + <span style="font-family:monospace">lutff_<i>i</i>/in_2</span> + <span style="font-family:monospace">lutff_<i>(i-1)</i>/cout</span> > 1. In case of <i>i</i>=0, <span style="font-family:monospace">carry_in_mux</span> is used as third input. <span style="font-family:monospace">carry_in_mux</span> can be configured to be constant 0, 1 or the <span style="font-family:monospace">lutff_7/cout</span> signal from the logic tile below.
</p>
<p>
Part of the functionality described above is documented as part of the routing
bitstream documentation (see the buffers for <span style="font-family:monospace">lutff_</span> inputs). The <span style="font-family:monospace">NegClk</span>
bit switches all 8 FFs in the tile to negative edge mode. The <span style="font-family:monospace">CarryInSet</span>
bit drives the <span style="font-family:monospace">carry_in_mux</span> high (it defaults to low when not driven via the buffer from
<span style="font-family:monospace">carry_in</span>).
</p>
<p>
The remaining functions of the logic cell are configured via the <span style="font-family:monospace">LC_<i>i</i></span> bits. This
are 20 bit per logic cell. We have arbitrarily labeled those bits as follows:
</p>
<table class="ctab">
<tr><th>Label</th><th>LC_0</th><th>LC_1</th><th>LC_2</th><th>LC_3</th><th>LC_4</th><th>LC_5</th><th>LC_6</th><th>LC_7</th></tr>
<tr><td>LC_<i>i</i>[0]</td><td>B0[36]</td><td>B2[36]</td><td>B4[36]</td><td>B6[36]</td><td>B8[36]</td><td>B10[36]</td><td>B12[36]</td><td>B14[36]</td></tr>
<tr><td>LC_<i>i</i>[1]</td><td>B0[37]</td><td>B2[37]</td><td>B4[37]</td><td>B6[37]</td><td>B8[37]</td><td>B10[37]</td><td>B12[37]</td><td>B14[37]</td></tr>
<tr><td>LC_<i>i</i>[2]</td><td>B0[38]</td><td>B2[38]</td><td>B4[38]</td><td>B6[38]</td><td>B8[38]</td><td>B10[38]</td><td>B12[38]</td><td>B14[38]</td></tr>
<tr><td>LC_<i>i</i>[3]</td><td>B0[39]</td><td>B2[39]</td><td>B4[39]</td><td>B6[39]</td><td>B8[39]</td><td>B10[39]</td><td>B12[39]</td><td>B14[39]</td></tr>
<tr><td>LC_<i>i</i>[4]</td><td>B0[40]</td><td>B2[40]</td><td>B4[40]</td><td>B6[40]</td><td>B8[40]</td><td>B10[40]</td><td>B12[40]</td><td>B14[40]</td></tr>
<tr><td>LC_<i>i</i>[5]</td><td>B0[41]</td><td>B2[41]</td><td>B4[41]</td><td>B6[41]</td><td>B8[41]</td><td>B10[41]</td><td>B12[41]</td><td>B14[41]</td></tr>
<tr><td>LC_<i>i</i>[6]</td><td>B0[42]</td><td>B2[42]</td><td>B4[42]</td><td>B6[42]</td><td>B8[42]</td><td>B10[42]</td><td>B12[42]</td><td>B14[42]</td></tr>
<tr><td>LC_<i>i</i>[7]</td><td>B0[43]</td><td>B2[43]</td><td>B4[43]</td><td>B6[43]</td><td>B8[43]</td><td>B10[43]</td><td>B12[43]</td><td>B14[43]</td></tr>
<tr><td>LC_<i>i</i>[8]</td><td>B0[44]</td><td>B2[44]</td><td>B4[44]</td><td>B6[44]</td><td>B8[44]</td><td>B10[44]</td><td>B12[44]</td><td>B14[44]</td></tr>
<tr><td>LC_<i>i</i>[9]</td><td>B0[45]</td><td>B2[45]</td><td>B4[45]</td><td>B6[45]</td><td>B8[45]</td><td>B10[45]</td><td>B12[45]</td><td>B14[45]</td></tr>
<tr><td>LC_<i>i</i>[10]</td><td>B1[36]</td><td>B3[36]</td><td>B5[36]</td><td>B7[36]</td><td>B9[36]</td><td>B11[36]</td><td>B13[36]</td><td>B15[36]</td></tr>
<tr><td>LC_<i>i</i>[11]</td><td>B1[37]</td><td>B3[37]</td><td>B5[37]</td><td>B7[37]</td><td>B9[37]</td><td>B11[37]</td><td>B13[37]</td><td>B15[37]</td></tr>
<tr><td>LC_<i>i</i>[12]</td><td>B1[38]</td><td>B3[38]</td><td>B5[38]</td><td>B7[38]</td><td>B9[38]</td><td>B11[38]</td><td>B13[38]</td><td>B15[38]</td></tr>
<tr><td>LC_<i>i</i>[13]</td><td>B1[39]</td><td>B3[39]</td><td>B5[39]</td><td>B7[39]</td><td>B9[39]</td><td>B11[39]</td><td>B13[39]</td><td>B15[39]</td></tr>
<tr><td>LC_<i>i</i>[14]</td><td>B1[40]</td><td>B3[40]</td><td>B5[40]</td><td>B7[40]</td><td>B9[40]</td><td>B11[40]</td><td>B13[40]</td><td>B15[40]</td></tr>
<tr><td>LC_<i>i</i>[15]</td><td>B1[41]</td><td>B3[41]</td><td>B5[41]</td><td>B7[41]</td><td>B9[41]</td><td>B11[41]</td><td>B13[41]</td><td>B15[41]</td></tr>
<tr><td>LC_<i>i</i>[16]</td><td>B1[42]</td><td>B3[42]</td><td>B5[42]</td><td>B7[42]</td><td>B9[42]</td><td>B11[42]</td><td>B13[42]</td><td>B15[42]</td></tr>
<tr><td>LC_<i>i</i>[17]</td><td>B1[43]</td><td>B3[43]</td><td>B5[43]</td><td>B7[43]</td><td>B9[43]</td><td>B11[43]</td><td>B13[43]</td><td>B15[43]</td></tr>
<tr><td>LC_<i>i</i>[18]</td><td>B1[44]</td><td>B3[44]</td><td>B5[44]</td><td>B7[44]</td><td>B9[44]</td><td>B11[44]</td><td>B13[44]</td><td>B15[44]</td></tr>
<tr><td>LC_<i>i</i>[19]</td><td>B1[45]</td><td>B3[45]</td><td>B5[45]</td><td>B7[45]</td><td>B9[45]</td><td>B11[45]</td><td>B13[45]</td><td>B15[45]</td></tr>
</table>
<p>
<span style="font-family:monospace">LC_<i>i</i>[8]</span> is the <span style="font-family:monospace">CarryEnable</span> bit. This bit must be set if the carry logic is used.
</p>
<p>
<span style="font-family:monospace">LC_<i>i</i>[9]</span> is the <span style="font-family:monospace">DffEnable</span> bit. It enables the output flip-flop for the LUT.
</p>
<p>
<span style="font-family:monospace">LC_<i>i</i>[18]</span> is the <span style="font-family:monospace">Set_NoReset</span> bit. When this bit is set then the set/reset signal will set, not reset the flip-flop.
</p>
<p>
<span style="font-family:monospace">LC_<i>i</i>[19]</span> is the <span style="font-family:monospace">AsyncSetReset</span> bit. When this bit is set then the set/reset signal is asynchronous to the clock.
</p>
<p>
The LUT implements the following truth table:
</p>
<table class="ctab">
<tr><th>in_3</th><th>in_2</th><th>in_1</th><th>in_0</th><th>lout</th></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[4]</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[14]</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[15]</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[5]</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[6]</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[16]</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[17]</span></td></tr>
<tr><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[7]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[3]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[13]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[12]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[2]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[1]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[11]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">0</td><td><span style="font-family:monospace">LC_<i>i</i>[10]</span></td></tr>
<tr><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td style="text-align:center">1</td><td><span style="font-family:monospace">LC_<i>i</i>[0]</span></td></tr>
</table>
<p>
LUT inputs that are not connected to anything are driven low. The set/reset
signal is also driven low if not connected to any other driver, and the clock
enable signal is driven high when left unconnected.
</p>
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