single data rate
A communication signaling technique that transfers data on either the rising edge or the falling edge of the clock signal.
single data rate: one bit per clock pulse, period
Single data rate (SDR) is a digital communication method where one bit of data is transferred during each clock cycle, using either the rising edge (positive transition) or the falling edge (negative transition) of the clock signal, but not both. This straightforward approach contrasts sharply with double data rate (DDR) schemes, which capture data on both edges and thus double throughput at the same clock frequency.
In DRAM and SRAM applications, SDR memory was the standard until DDR variants emerged in the early 2000s. An SDR SDRAM operating at 100 MHz transfers 100 million bits per second per pin; a DDR variant at the same 100 MHz clock frequency achieves 200 million bits per second. SDR interfaces are still found in legacy systems, certain industrial embedded controllers, and specialized low-power applications where simpler timing requirements and lower electromagnetic emission are advantages.
SDR's main strength is design simplicity. Clock recovery circuits need fewer phase-alignment stages, PCB trace routing is less stringent, and signal integrity margins are broader because data transitions occur half as often as in DDR schemes. The penalty is bandwidth: SDR cannot match the throughput-per-megahertz of DDR, DDR2, DDR3, or newer standards. This makes SDR unsuitable for high-performance computing, modern graphics processing, or data-intensive industrial applications.
In contemporary practice, SDR appears primarily in legacy maintenance, retrofit boards, and low-cost embedded systems where processor demands remain modest. Some industrial serial protocols (such as certain UART implementations or older SPI variants) use SDR principles, though they are not typically labeled as such in documentation. Technicians encounter the term most often when reviewing memory specifications for aging equipment or when working with development boards that deliberately use SDR for educational clarity.
The name itself is descriptive but risks confusion: SDR does not mean the data rate is low in absolute terms, merely that it is single-sampled per clock cycle. DDR, DDR2, and subsequent standards all increased effective data rates while keeping clock frequencies modest, reducing power consumption and heat generation relative to the alternative of simply raising clock frequency on an SDR bus.