OpenRAM/docs/source/control_logic.md

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Control Logic and Timing

This page of the documentation explains the control logic and timing of OpenRAM.

Table of Contents

1. Read Timing
2. Write Timing
3. External Control Signals
4. Internal Control Signals
5. Replica Bitline (RBL)
6. Internal Control Signals Diagram (Read)
7. Internal Control Signals Diagram (Write)
8. Clock Distribution

Read Timing

Read Timing Simulation

Write Timing

Write Timing Simulation

External Control Signals

• Behaves as a synchronous register interface
  • Address and data captured on positive edge
  • Data is available after the negative edge (before next positive edge)
  • Clock is used for internal control generation
• Clock cycle
  • Clock high: capture inputs + precharge + decode
  • Clock low: read/write
• Reads and writes on multiple ports to the same address in the same cycle "feed through" but the noise margins of the bitcell must be able to handle this

Internal Control Signals

• Sense Enable (s_en) -- Active high sense amp enable from Replica Bit Line (RB)
  • RBL input: rbl_wl = gated_clk_bar & we_bar
  • Delayed RBL output: pre_s_en = DELAY(rbl_bl)
  • Buffered enable: s_en = BUF(pre_s_en)
• Write Driver Enable (w_en) -- Active high write driver enable
  • w_en = we
  • we is directly from control flops
• Precharge Enable Bar (p_en_bar) -- Active low enable of precharge
  • p_en_bar = !(gated_clk_bar)
  • Active for writes as well to prevent half select destruction
• Word line enable (wl_en) -- Active high word line enable
  • wl_en = BUF(gated_clk_bar)

Replica Bitline (RBL)

• Determines when to start sensing by replicating a bitcell array column
• Replica Bitline Column (RBC) matches transition of column
  • rbl_bl = RBC(rbl_wl)
• Delay Line delays the input signal to match word line driver
  • pre_s_en = DELAY(rbl_bl)
• Buffer drives s_en signal to sense amps

Internal Control Signals Diagram (Read)

Internal Control Signals Diagram (Write)

Clock Distribution

• External clock (clk)
• Buffered clock (clk_buf) drives all DFFs in the design
  • clk_buf = BUF(clk)
• Buffered clock bar (clk_bar) used in control logic
  • clk_bar = INV(clk_buf)
• Gated Clocks (gated_clk and gated_clk_bar) used in control logic
  • This is LOW when disabled
    • gated_clk_bar = cs && clk_bar
  • This is LOW when disabled
    • gated_clk = cs && clk_buf