Memory Timing

By RickM
Jun 18, 2007
  1. Hello,
    Thank you for taking the time to read this post.

    Could anyone tell me what ( CL-RCD-RP-RAS ) associated with Memory Timing stand for ? Also the numbers associated to each.
    Thank you.
  2. Tedster

    Tedster Techspot old timer..... Posts: 6,002   +15

    RAS - Row Address Strobe or Row Address Select
    CAS - Column Address Strobe or Column Address Select

    tRAS - Active to precharge delay; this is the delay between the precharge and activation of a row

    tRCD - RAS to CAS Delay; the time required between RAS and CAS access

    tCL - (or CL) CAS Latency

    tRP - RAS Precharge; the time required to switch from one row to the next row, for example, switch internal memory banks

    tCLK – ClocK; the length of a clock cycle

    Command Rate - the delay between Chip Select (CS), or when an IC is selected and the time commands can be issued to the IC

    Memory architecture is like a spreadsheet with row upon row and column upon column, with each row being one bank. For the CPU to access memory, it first must determine which row or bank in the memory is to be accessed and then activate that row with the RAS signal. Once activated, the row can be accessed over and over, until the data is exhausted. This is why tRAS has little effect on overall system performance but could impact system stability if set incorrectly.

    tRCD is the delay from the time a row is activated to when the cell (or column) is activated via the CAS signal and data can be written to or read from a memory cell. When memory is accessed sequentially, the row is already active and tRCD will not have much impact. However, if memory is not accessed in a linear fashion, the current active row must be deactivated and then a new row selected/activated. In such an example, low tRCD's can improve performance. However, like any other memory timing, putting this too low for the module can cause in instability.

    CAS Latency
    Certainly, one of the most important timings is the CAS Latency, which is also the one most people understand. Since data is often accessed sequentially (same row), the CPU need only select the next column in the row to get the next piece of data. In other words, CAS Latency is the delay between the CAS signal and the availability of valid data on the data pins (DQ). The latency between column accesses (CAS) then plays an important role in the performance of the memory. The lower the latency, the better the performance. However, the memory modules must be able to support low-latency settings.

    tRP is the time required to terminate one row access and begin the next row access. tRP might also be seen as the delay required between deactivating the current row and selecting the next row. So in conjunction with tRCD, the time required (or clock cycles required) to switch banks (or rows) and select the next cell for reading, writing, or refreshing is a combination of tRP and tRCD.

    tRAS is the time required before (or delay needed) between the active and precharge commands. In other words, how long the memory must wait before the next memory access can begin.

    This is simply the clock used for the memory. Note that because frequency is 1/t, if memory were running at 100Mhz, the timing of the memory would be 1/100Mhz, or 10nS.

    Command Rate
    The Command Rate is the time needed between the chip select signal and when commands can be issued to the RAM module IC. Typically, these are either 1 clock or 2.

    Latency - The time from when a request is made to when it is answered; the total time required before data can be written to or read from the memory.
  3. RickM

    RickM TS Member Topic Starter Posts: 27

    Thank you very much for the information TED... I appreciate it very much and is very helpful.

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