Side-by-side comparison
| Parameter | SRAM | DRAM |
|---|---|---|
| Storage Element | 6-transistor bistable latch | 1 transistor + 1 capacitor |
| Refresh Required | No | Yes — every 64 ms (DDR4) |
| Access Time | <1 ns (SRAM cache) to ~10 ns (standalone) | ~60 ns (CAS latency DDR4) |
| Cell Density | Low — 6T per bit | High — 1T+1C per bit |
| Typical Capacity | KB to a few MB | GB range (DDR4: 4–64 GB) |
| Power Consumption | Low static power; no refresh power | Higher — refresh cycles consume power |
| Cost per bit | Higher (~10x DRAM) | Lower |
| Typical IC / Standard | CY7C1021 (1Mb SRAM), processor caches | DDR4 SDRAM, LPDDR5 (mobile) |
| Interface Complexity | Simple async or sync interface | Complex — requires DRAM controller, refresh timer |
| Use Case | CPU cache, register files, small buffers | Main memory, frame buffers, large data storage |
Key differences
SRAM stores a bit in a six-transistor cross-coupled inverter pair — stable as long as power is on, no refresh needed, access in under 1 ns for on-chip cache. DRAM stores a bit as charge on a femtofarad capacitor; that charge leaks in milliseconds, forcing a refresh cycle every 64 ms which temporarily blocks memory access. SRAM cells are 6x larger, making GB-scale SRAM economically impossible. DDR5 DIMMs achieve 64 GB by exploiting 1T+1C cell density, while a 1 Mb CY7C1021 SRAM chip costs more per bit than an entire 16 GB DDR4 module.
When to use SRAM
Use SRAM (CY7C1021 or on-chip cache) when access time under 10 ns and zero refresh overhead are non-negotiable, such as look-up tables in an FPGA block RAM or the scratchpad memory in a DSP.
When to use DRAM
Use DRAM (DDR4 SDRAM) when the system requires gigabytes of storage at minimum cost and can tolerate a memory controller managing refresh, such as the main RAM in any Linux-running embedded processor.
Recommendation
For exams and placements, choose SRAM when the question involves cache design, low latency, or small capacity. Choose DRAM when capacity exceeds a few MB. Never confuse refresh (DRAM only) with power cycling — SRAM still loses data on power-off.
Exam tip: University papers test why DRAM needs refresh — answer with the leaky capacitor model, state the 64 ms refresh interval for DDR4, and mention that refresh is handled by the memory controller.
Interview tip: Interviewers at semiconductor and embedded companies ask you to compare SRAM and DRAM cell structure — draw the 6T SRAM latch and the 1T+1C DRAM cell side by side and explain the density trade-off.