implement decoder sizing logic

docs/designv3.md

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+Changes:
+Separate wen/ren (should have defined behavior if both are high)
+All SR latches, FFs, etc should have a reset signal
+Write drivers should be inverters, not just single pull-down transistors
+Write mux should be transmission gate rather than NMOS
+Unify write/read mux
+4-way banking, with control logic in the middle
+Everything manually routed except non-scaling control logic
+Divided WL + divided BL architecture
+Top and bottom banks
+During R/W, pick either top or bottom bank based on MSB of addr
+Always read from both left+right halves of array
+Col circuitry shared between top/bottom
+latch input/output/inout pins
+testbench should read/write every addr in pex sim and check internal state of sram bitcells
+Write wordline pulse width controlled by inverter chain
+Wordline gating happens after decoder
+
+Open questions:
+Why should there be a separate read/write mux
+How to size predecoders
+How to partition addr bits
+How to layout predecoders
+Where to place rbl
+Do we need to support R90 variants?
+Write assist circuitry?
+Before turning on wl_en, must ensure predecoders/decoder have finished decoding. How should we figure out when to turn on wl_en? How should we minimize the time between when wl_en actually turns on and the theoretical earliest time at which wl_en could turn on? Do we assume that the delay through the driver chain for wl_en to all final stage gates is approximately equal to the decoder delay?
+
+Bitcell organization:
+4 banks: ul, ur, ll, lr
+Words split across left and right banks
+Only one of upper and lower banks activated at any time
+Control logic inputs:
+ce (chip enable)
+we (write enable)
+clk
+reset
+Control logic outputs:
+sae
+pc_b
+colsel[i]/colsel_b[i]
+wlen (rwl)
+wrdrven
+
+Read sequence of operations:
+Latch addr/din
+Disable precharge, sae
+Decode address (turn on appropriate read mux config)
+Turn on wlen pulse
+Turn off wlen pulse once replica bitline reaches VDD/2
+Send sae to trigger sense amp
+Send sense amp outputs to DFFs/latches
+Enable precharge
+
+Write sequence of operations:
+Latch addr/din
+Disable precharge, sae
+Decode address (turn on appropriate write mux config)
+Turn on wrdrven/wlen pulse such that both worldline and bitline are driven at same time
+Turn off wlen/wrdrven pulse after some inverter chain delay
+Enable precharge
+
+Control logic implementation:
+SR latch for pc_b
+Set at positive edge of clock
+Reset inverter chain after sae goes high OR immediately after wlen goes low
+SR latch for sae
+Set when wlen goes low during read operation
+Reset at positive edge of clock
+
+```verilog
+module controlv3 (
+input ce, we, clk, reset,
+output sae, pc_b, wlen, wrdrven,
+inout rbl
+)
+  wire clkp;
+  edge_detector clk_edge_detector(.clk(clk & ce), .clkp(clkp));
+
+  sr_latch pc_latch(.s(sae #4 | reset), .r(clkp), .q(pc_b));
+  sr_latch sae_latch(.s(we_b & wlen_decoder), .r(clkp | reset), .q(sae));
+
+  wire wlen_set, wlen_rst;
+  assign wlen_set = clkp;
+  assign wlen_rst = we ? clkp #4 : rbl_b;
+  sr_latch wlen_latch(.s(wlen_set), .r(wlen_rst | reset), .q(wlen));
+
+  wire wrdrven_rst;
+  decoder_replica decoder_replica(.a(wlen_rst), .o(wlen_decoder));
+  sr_latch wrdrven_latch(.s(clkp & we), .r(wlen_decoder | reset), .q(wrdrven));
+
+endmodule
+```