Double Data Rate - Relation of Bandwidth and Frequency

Relation of Bandwidth and Frequency

Describing the bandwidth of a double-pumped bus can be confusing. Each clock edge is referred to as a "beat", with two beats (one upbeat and one downbeat) per cycle. Technically, the hertz is a unit of cycles per second, but many people refer to the number of transfers per second. Careful usage generally talks about "500 MHz, double data rate" or "1000 MT/s", but people will refer casually to a "1000 MHz bus", even though no signal cycles faster than 500 MHz.

DDR SDRAM popularized the technique of referring to the bus bandwidth in megabytes per second, the product of the transfer rate and the bus width in bytes. DDR SDRAM operating with a 100 MHz clock is called DDR-200 (after its 200 MT/s data transfer rate), and a 64 bit (8 byte) wide DIMM operated at that data rate is called PC-1600, after its 1600 MB/s peak (theoretical) bandwidth. Likewise, 800 MHz clock DDR3-1600 is called PC3-12800.

Some examples of popular designations of DDR modules:

Note that DDR SDRAM only uses double-data-rate signalling on the data lines. Address and control signals are still sent to the DRAM once per clock cycle (to be precise, on the rising edge of the clock), and timing parameters such as CAS latency are specified in clock cycles. (Some less common DRAM interfaces, notably LPDDR2, GDDR5 and XDR DRAM, send commands and addresses using double data rate.)