Explain me the following in detail:

Layout generators are arguably the most important portion of the process. BAG offers a set of functions for drawing transistors, automatically routing metals, drawing vias, etc. to allow the same capability that hand design would produce. The goal of the designer is to use these functions in a generic way that automatically computes where and how to draw connections that will be DRC and LVS clean regardless of the input specifications. The typical process of generating a large complex block is to start with small cells, like an inverter or an active diff amp using AnalogBase. AnalogBase is a class in BAG used for drawing layouts comprised entirely of transistors. Firstly, the user draws a schematic template.

This template holds only a human-readable description of the connections. The schematic will be copied over with actual values filled in by BAG afterward. One thing to note is the presence of seemingly useless transistors, like in the bottom right. These transistors are used by BAG to properly create dummy transistors in the layout, and anything else can be removed if not used in the schematic generator. Additionally, the transistor in the bottom left used for adding stabilization capacitance can also be removed if desired. Example layouts below will not have this transistor. Using the schematic template, the user then decides how many rows of transistors will be in the layout, and assigns a number of transistors to each row. There are a number of helper functions to generate a data structure containing information about which row the transistor is in, the drain/source metal directions, number of fingers, etc. which is called the "initialization step". An example of how one might set up the rows and initialization for the amplifier in schematic is shown in Listing 2.1 and Listing 2.2. Note that these code blocks are only portions of the full layout generator and there are, in general, a small number of extra lines required. This section highlights only the most important portions of a generator.

Listing 2.1:
#***Creating rows for transistors***#

# Rows are ordered from bottom to top
# To use Track Manager, an ordered list of wiring types and their locations must be provided.
# Define two lists, one for the nch rows and one for the pch rows
# The lists are composed of dictionaries, one per row.
# Each dictionary has two list entries ( g and ds ), which are ordered lists of what wire types will be present
# in the g and ds sections of that row. Ordering is from bottom to top of the design.
wire_names = dict (
    nch=[
        # tail row
        dict (
            g=['bias'],
            ds=['bias ']
        ),
        # Input row
        dict (
            g=['sig'],
            ds=['sig']
        )
    ],
    pch=[
        # top row
        dict (
            ds=['sig'],
            g=['bias']
        )
    ]
)
layout_helper.wire_names = wire_names
# Set up the row information
# Row information contains the row properties like width/number of fins, orientation, int ent, etc.
# Storing in a row dictionary/object allows for convenient fetching of data in later functions
row_tail = layout_helper.initialize_rows(row_name='tail',
                                orient='R0',
                                nch_or_pch='nch',
                                )
row_input = layout_helper.initialize_rows(row_name='input',
                                orient='R0',
                                nch_or_pch='nch',
                                )
row_mirror = layout_helper.initialize_rows(row_name='top',
                                orient='R0',
                                nch_or_pch='pch',
                                )

# Define the order of the rows ( bottom to top ) for this analog Base cell
layout_helper.global_rows = [row_tail, row_input, row_mirror]

Listing 2.2:
#***Assigning transistors to a row and initializing***#

################################################################################
# 2:
# Initialize the transistors in the design
# Storing each transistor's information (name, location, row, size, etc) in a
# dictionary object allows for convenient use later in the code, and also
# greatly simplifies the schematic generation
# The initialization sets the transistor's row, width, and source/drain net names
# for proper dummy creation
################################################################################

tail_l = layout_helper.initialize_tx (name='tail_l', row=row_tail,
            fg_spec='bottom_tail',
            deff_net='TAIL')
tail_r = layout_helper.initialize_tx (name='tail_r', row=row_tail,
            fg_spec='bottom_tail',
            deff_net='TAIL')
bias = layout_helper.initialize_tx (name='bias', row=row_tail,
            fg_spec='bottom_bias',
            deff_net='IBIAS_TAIL')
in_left = layout_helper.initialize_tx (name='in_left', row=row_input,
            fg_spec='bottom_in',
            seff_net='TAIL', deff_net='VOUT_N')
in_right = layout_helper.initialize_tx (name='in_right', row=row_input,
            fg_spec='bottom_in',
            seff_net='TAIL', deff_net='VOUT_P')
top_left = layout_helper.initialize_tx (name='top_left', row=row_mirror,
            fg_spec='top',
            deff_net='VOUT_N')
top_right = layout_helper.initialize_tx (name='top_right', row=row_mirror,
            fg_spec='top',
            deff_net='VOUT_P')


After initializing all transistors, the user then must specify their locations in the layout by creating fictitious columns that stacks of transistors will be placed in. Based on the number of fingers each transistor has, technology required spacing and any other spacing (to route dummies, etc.) the user can compute columns based on a number of fingers, and assign the transistors like in Listing 2.3.

Listing 2.3:
#***Assigning transistors to columns***#

# Calculate positions of transistors
# This uses helper functions to place each transistor within a stack/column of a
# specified starting index and
# width, and with a certain alignment (left, right, centered) within that column
layout_helper.assign_tx_column (tx=bias, offset=col_mid, fg_col=fg_mid, align=0)
layout_helper.assign_tx_column (tx=tail_l, offset=col_stack_left,
    fg_col=fg_stack, align=0)
layout_helper.assign_tx_column (tx=in_left, offset=col_stack_left,
    fg_col=fg_stack, align=0)
layout_helper.assign_tx_column (tx=top_left, offset=col_stack_left,
    fg_col=fg_stack, align=0)
layout_helper.assign_tx_column (tx=tail_r, offset=col_stack_right,
    fg_col=fg_stack, align=0)
layout_helper.assign_tx_column (tx=in_right, offset=col_stack_right,
    fg_col=fg_stack, align=0)
layout_helper.assign_tx_column (tx=top_right, offset=col_stack_right,
    fg_col=fg_stack, align=0)

The final transistor placement step is to set the drain and source orientations. A transistor's source can be routed up or down which a affects where the gate placements are made, and the first diffusion region per transistor can be either source or drain to make alignment simpler. There is also a helper function to automatically compute based on number of fingers which region should be the source or drain based on another transistor the user wishes to align to. This is shown in Listing 2.4.

Listing 2.4:
#***Determining transistor drain/source congurations***#

################################################################################
# 4: Assign the transistor directions (s/d up vs down)
#
# Specify the directions that connections to the source and connections to the drain
# will go (up vs down). Doing so will also determine how the gate is aligned
# (ie will it be aligned to the source or drain)
# See the bootcamp for more details
# The helper functions used here help to abstract away whether the intended
# source/drain diffusion region of a transistor occurs on the even or odd
# columns of that device (BAG always considers the even columns of a
# device to be the 's'). These helper functions allow a user to specify
# whether the even columns should be the transistors effective source or
# effective drain , so that the user does not need to worry about BAG's notation.
################################################################################

# Set tail bias tx to have source on the left most diffusion (arbitrary)
# and source going down
layout_helper.set_tx_directions (tx=bias, seff='d', seff_dir=0)
# Assign the input to be anti-aligned, so that the input source and tail
# drain are vertically aligned
layout_helper.set_tx_directions (tx=in_left, seff='s', seff_dir=0)
layout_helper.set_tx_directions (tx=in_right, seff='s', seff_dir=0)

layout_helper.assign_tx_matched_direction (target_tx=tail_l, source_tx=in_left,
    seff_dir=0, aligned=False)
layout_helper.assign_tx_matched_direction (target_tx=tail_r, source_tx=in_right,
    seff_dir=0, aligned=False)

layout_helper.assign_tx_matched_direction (target_tx=top_left,
    source_tx=in_left, seff_dir=2)
layout_helper.assign_tx_matched_direction (target_tx=top_right,
    source_tx=in_right, seff_dir=2)


Finally, the difficult setup is complete, and the user can call the function self.draw_base() with the information about rows, transistors, etc. set up in the previous steps. BAG will then draw the all of the required polygons for the metal connections to the MOS devices and everything else. An base layout with no wiring done of the schematic.

After drawing the base layout, users can then specify how to connect wires and ports by specifying a metal layer, a wire width and a track. Tracks are an invisible grid that spans the design space used by BAG to place wires properly. Listing 2.5 shows an example command of how to connect elements. Note that this code makes no reference to anything specific nor does it "hardcode" any parameters. Everything is generic to the specified parameters.

Listing 2.5:
#***Drawing wire connections***#

#Connect up bias gates + drain
warr_bias_in = self.connect_to_tracks (
    [tail_l['g'], tail_r['g'], bias['d'], bias['g']],
    tid_tail_gate
)
#connect tail drains to input sources (tail node)
warr_tail = self.connect_to_tracks (
    [tail_l['d'], tail_r['d'], in_right['s'], in_left['s']],
    tid_tail_ds
)

Lastly, the user finalizes connections and adds pins to wires. In the final step, BAG will draw dummy transistors and create straps for the power supplies, automatically routing the dummies' connections to the supply. With a finished layout generator, the user can now arbitrarily change their transistor specifications which will be reflected automatically in the wiring and sizing. Figure 2.6 shows the same generator with various width and number of finger choices. An important note is that the user can do all this without knowing any of the myriad of the layout design rules since BAG handles these complexities internally and abstracts them away from the user.