CAS latency and other memory timings explained

By seanp789
Mar 4, 2006
  1. This quoted from the AMD website and you can visit this LINK for the full article. Useful for both beginers and intermediates alike.

    * 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
    * 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.

    Some of the above terms are more important to system stability and performance than others. However, to understand the whole, it is important to understand the role of each of these settings/signals. Therefore, the numbers 2-3-2-6-T1 refer to CL-tRCD-tRP-tRAS-Command Rate and are measured in clock cycles.

    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.

    This covers much of the basic settings for memory and how they work. As mentioned earlier, it is important to understand what timings your memory will support. Refer to your memory vendor’s website or datasheets for more information.
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