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:
| Names | Frequency | Transfer Rate | Theoretical Bandwidth |
|---|---|---|---|
| DDR-200, PC-1600 | 100 MHz | 200 MT/s | 1.6 GB/s |
| DDR2-800, PC2-6400 | 400 MHz | 800 MT/s | 6.4 GB/s |
| DDR3-1600, PC3-12800 | 800 MHz | 1600 MT/s | 12.8 GB/s |
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.)
Read more about this topic: Double Data Rate
Famous quotes containing the words relation of, relation and/or frequency:
“When needs and means become abstract in quality, abstraction is also a character of the reciprocal relation of individuals to one another. This abstract character, universality, is the character of being recognized and is the moment which makes concrete, i.e. social, the isolated and abstract needs and their ways and means of satisfaction.”
—Georg Wilhelm Friedrich Hegel (1770–1831)
“Our sympathy is cold to the relation of distant misery.”
—Edward Gibbon (1737–1794)
“The frequency of personal questions grows in direct proportion to your increasing girth. . . . No one would ask a man such a personally invasive question as “Is your wife having natural childbirth or is she planning to be knocked out?” But someone might ask that of you. No matter how much you wish for privacy, your pregnancy is a public event to which everyone feels invited.”
—Jean Marzollo (20th century)