CPEN 412

Static Memory (SRAM)

Updated 2020-01-20

Asynchronous Memory

A typical SRAM cell consists of back-to-back inverter. Note that due to the nature of the cell and logic gate (meta-stablility and regenerative properties), upon powerup the stored bit is random.

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The inverter could either be CMOS (2 MOS + 2 MOS + 2 control MOS = 6T), or using resistive load implementation.

These SRAM cells are arranged in a lattice where individual stored data can be accessed using wordlines and outputted in bit lines.

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Reading from SRAM Cell

The cell select line is activated as response from the CPU address, and turns on the two control MOS and the inverter outputs are fed into a sense amplifier.

We need a sense amplifier because upon read, the change in bitlines is very small due to the fact that all memory cells of a bitline are connected in parallel. The sense amplifier is biased to Vdd/2 and upon a small change, it will be pulled to either 0 or Vdd.

Writing to SRAM Cell

The cell select is asserted HIGH, then the CPU drives data into the data in/out lines (bitlines). The higher power control MOS overpowers the inverter and forces the data into an appropriate stable state.

SRAM Timing

No two chips exhabit identical timing due to difference in temperature, material, capacitance, voltage. For example, a higher voltage can make a circuit work faster because capacitances can be charged and discharged more quickly. Another example is when a digtial pin switches from a 1 to a 0 (discharging capcitance), so there exists a current that flows into the ground. The ground has intrinsically inductance properties, so the Vdd inside the digital chip could drop, making the circuit run slower.

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Key design decisions for read operation:

  1. CE and OE arrive early so it’s not delaying turning on output buffer inside the memory chip.
  2. Get CPU address to chip ASAP; do not wait for AS. This is so the internal decoder in the memory chip can get started immediately.
  3. Make sure CPU wait at least tAA or else it’s going to get erroneous data.

Memory Address Dead Time

There exists a period of time between transitions of different address signals. This is to ensure that we turn one memory device OFF between a another memory device ON to avoid contension. This is done using Address Strobe (AS_L) active low signal. AS_L turns HIGH before the address changes, and turns on a bit after the address changes.

Other Delays

Other delays can come from buffers. In this case, we’re not even considering the propagation delay in the wires.

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The total delay, ttotal, after Dtack is received is at tPAB + tAA + tPDB + tDSU or tPAB + tPAD + tACE + tDSU.

Synchronous Memory

Synchronous memory can have the data clocked, so a clock is needed. SSRAMs have internal counters to automatically increment the last address, allowing burst reading or writing of data (quickly burst different data without changing address).

It is more important to consider timing for synchronous memory system because CPU usally clocks a lot faster than memory access time (1 GHz corresponds to 1ns, and is much faster compared to 10 ns, which is typical for fast SRAM). So either we need to slow CPU down, or have a robust protocol to communicate with the SRAM.

68K Asynchronous Bus Interface

Recall our process-memory interface:

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  1. The processor output the address on the pins A1-A23
  2. It then asserts active low address strobe AS_L
  3. R/W_L indiccates direction of data transfer
  4. UDS or LDS active low signals indicate which byte to transfer.
  5. Data is then transfered.
  6. After memory operation, the memory system responds with a data transfer acknolwedgement signal (DTACK) which means that the memory has done its job and we can move on.

😇 The advantage is that async-memory is flexible and allows for both fast and slow memory to communicate.

😱 The disadvantage is that this DTACK “handshaking” has lots of overheads that puts an upper limit on transfer rates on highspeed systems.


Here is an example of an implenetation fo 68K async SRAM:

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Notice the DTACK active low signal generator.

Timing Analysis

We need to look at both read and write operation and how they look like (they have different timings).

Read Operation

The read operation takes 4 clock cycles. They are further split into 8 states (S0 to S7), this is the fastest we can go. The arrows on the diagram below implies the casal relationships between signals.

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Note: Quartus simulations of the soft core does not simulate tristate of the address bus. So the address is actually asserted a lot earlier. This is good for us becuase the address decoder can start working earlier, reducing delay.

Timing Analysis Annotations

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8 MHz Clock

12.5 MHz Clock

Inserting Wait Statements

The 68k is capable of extending bus cycle indefinitely. We often want to insert wait statements because DTACK* doesn’t arrive in-time.

DTACK* is improtantt for DRAM becuase DRAM is leaky and requires refereshing. During the refresh, no read/write is possible and thus DTACK must be delayed.

DTACK Generator

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For fast SRAM and ROM, we don’t need to delay DTACK*. When AS* goes HIGH, it makes the counter load the preloaded timer value; and when AS* goes LOW and the the counter starts.

Write Operation

The input data must to presented to the memory chip and a the same time WE and CE are activated. OE can be ignored because we don’t care. The memory address must remain valid throughout the write cycle.

In write operation, there are two setup and hold constraints: address setup and hold time, and data setup and hold time.

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The CE will be removed when one of the R/W or UDS/LDS is deactivated, whichever comes first.

Note: the softcore has similar timing diagrams. However, the address hold time is being violated as the address simultaneously changes when UDS/LDS/AS are deactivated. Propagation or capacitance delays may cause UDS/LDS/AS (as result CE) to be delayed and arrive later than address line changes — which could be problematic.