refreshable

In electronics, refreshable refers to dynamic memory that loses stored data unless its contents are continuously rewritten. The most common example is DRAM (dynamic random-access memory), where each bit is stored as charge in a capacitor. Unlike static RAM, which holds data as long as power remains on, DRAM capacitors leak charge over time, typically within milliseconds. To retain data, a refresh circuit must periodically read and rewrite every cell before the charge decays below the threshold needed for reliable detection.

Refresh cycles occur automatically and transparently to the user. A dedicated controller scans through memory rows at intervals set by the device's refresh rate, commonly specified as the number of refreshes required per second across the total memory array. A 16 millisecond refresh interval on an 8 megabit DRAM, for example, means the entire array must be cycled roughly every 16 ms. Modern DRAM refresh rates range from 7.8 microseconds to several milliseconds per row, depending on the technology node and specified retention time. During a refresh cycle, the memory controller temporarily blocks read and write access, creating small latency spikes.

The advantage of refreshable memory is density and cost. Because DRAM stores charge rather than maintaining transistor logic states, it requires only one transistor and one capacitor per bit, making it far denser and cheaper than static RAM per unit capacity. This trade-off between refresh overhead and density is why DRAM dominates main system memory. Modern CPUs and controllers handle refresh transparently, but in older systems or specialized embedded applications, refresh timing had to be managed manually.

Temperature and process variation affect refresh requirements. Higher temperatures increase leakage, shortening safe intervals between refreshes. Process variations across a wafer create cells with different retention characteristics, so refresh rates must accommodate the weakest cells. This is why DRAM specifications include maximum allowed refresh interval; exceed it and data corruption becomes probable. Some enterprise and automotive-grade DRAM offers extended retention to reduce refresh frequency, trading power consumption against density.

The term "refreshable" also appears in other contexts: refreshable braille displays use electromagnets to raise and lower pins for tactile reading, and some displays or sensors require periodic calibration or data refresh. In memory contexts, however, refreshable specifically denotes the periodic rewrite requirement of dynamic charge-based storage. Understanding refresh constraints is essential when working with memory power budgets, latency-sensitive systems, or devices operating in extreme temperatures where refresh intervals tighten further.