OpenRAM/compiler/bitcells/bitcell.py

# See LICENSE for licensing information.
#
# Copyright (c) 2016-2019 Regents of the University of California and The Board
# of Regents for the Oklahoma Agricultural and Mechanical College
# (acting for and on behalf of Oklahoma State University)
# All rights reserved.
#
import design
import debug
import utils
from tech import GDS,layer,parameter,drc
import logical_effort

class bitcell(design.design):
    """
    A single bit cell (6T, 8T, etc.)  This module implements the
    single memory cell used in the design. It is a hand-made cell, so
    the layout and netlist should be available in the technology
    library.
    """

    pin_names = ["bl", "br", "wl", "vdd", "gnd"]
    storage_nets = ['Q', 'Qbar']
    type_list = ["OUTPUT", "OUTPUT", "INPUT", "POWER", "GROUND"] 
    (width,height) = utils.get_libcell_size("cell_6t", GDS["unit"], layer["boundary"])
    pin_map = utils.get_libcell_pins(pin_names, "cell_6t", GDS["unit"])

    def __init__(self, name=""):
        # Ignore the name argument
        design.design.__init__(self, "cell_6t")
        debug.info(2, "Create bitcell")

        self.width = bitcell.width
        self.height = bitcell.height
        self.pin_map = bitcell.pin_map
        self.add_pin_types(self.type_list)
        self.nets_match = self.do_nets_exist(self.storage_nets)
        
    def get_stage_effort(self, load):
        parasitic_delay = 1
        size = 0.5 #This accounts for bitline being drained thought the access TX and internal node
        cin = 3 #Assumes always a minimum sizes inverter. Could be specified in the tech.py file.
        return logical_effort.logical_effort('bitline', size, cin, load, parasitic_delay, False)
 
    def input_load(self):
        """Return the relative capacitance of the access transistor gates"""
        #This is a handmade cell so the value must be entered in the tech.py file or estimated.
        #Calculated in the tech file by summing the widths of all the related gates and dividing by the minimum width.
        
        #FIXME: The graph algorithm will apply this capacitance to the bitline load as they cannot be
        # distinguished currently
        access_tx_cin = parameter["6T_access_size"]/drc["minwidth_tx"]
        return 2*access_tx_cin
 
    def get_all_wl_names(self):
        """ Creates a list of all wordline pin names """
        row_pins = ["wl"]    
        return row_pins
    
    def get_all_bitline_names(self):
        """ Creates a list of all bitline pin names (both bl and br) """
        column_pins = ["bl", "br"]
        return column_pins
    
    def get_all_bl_names(self):
        """ Creates a list of all bl pins names """
        column_pins = ["bl"]
        return column_pins
        
    def get_all_br_names(self):
        """ Creates a list of all br pins names """
        column_pins = ["br"]
        return column_pins
        
    def get_bl_name(self, port=0):
        """Get bl name"""
        debug.check(port==0,"One port for bitcell only.")
        return "bl"
    
    def get_br_name(self, port=0):
        """Get bl name"""
        debug.check(port==0,"One port for bitcell only.")
        return "br"  

    def get_wl_name(self, port=0):
        """Get wl name"""
        debug.check(port==0,"One port for bitcell only.")
        return "wl"  
    
    def analytical_power(self, corner, load):
        """Bitcell power in nW. Only characterizes leakage."""
        from tech import spice
        leakage = spice["bitcell_leakage"]
        dynamic = 0 #temporary
        total_power = self.return_power(dynamic, leakage)
        return total_power
  
    def get_storage_net_names(self):
        """Returns names of storage nodes in bitcell in  [non-inverting, inverting] format."""
        #Checks that they do exist
        if self.nets_match:
            return self.storage_nets
        else:
            debug.info(1,"Storage nodes={} not found in spice file.".format(self.storage_nets))
            return None
    
    def get_wl_cin(self):
        """Return the relative capacitance of the access transistor gates"""
        #This is a handmade cell so the value must be entered in the tech.py file or estimated.
        #Calculated in the tech file by summing the widths of all the related gates and dividing by the minimum width.
        access_tx_cin = parameter["6T_access_size"]/drc["minwidth_tx"]
        return 2*access_tx_cin

    def build_graph(self, graph, inst_name, port_nets):        
        """Adds edges based on inputs/outputs. Overrides base class function."""
        self.add_graph_edges(graph, port_nets)