Purpose of DRAM refresh cycles Why is a refresh cycle necessary in dynamic RAM (DRAM)?

Difficulty: Easy

Correct Answer: The refresh cycle keeps the charge on the capacitor cells.

Explanation:


Introduction / Context:
Unlike SRAM, DRAM uses capacitors to store bits. Due to leakage, the stored charge decays, so periodic refresh is required to maintain data validity. Understanding what refresh actually does prevents misconceptions about DRAM behavior.


Given Data / Assumptions:

  • DRAM cell = capacitor that slowly discharges.
  • Refresh consists of reading and writing back the same data.
  • No flip-flops are used inside the DRAM array for data retention.


Concept / Approach:

During refresh, the sense amplifiers detect the small residual charge and then restore a full charge to represent a 1 (or zero level as required), effectively “recharging” each capacitor so that data persists for the specified retention time.


Step-by-Step Solution:

Recognize the physical mechanism: leaking capacitors.Determine refresh purpose: restore the correct charge level.Thus, refresh keeps (replenishes) the charge on the capacitor cells.


Verification / Alternative check:

DRAM timing diagrams show refresh cycles that access rows even without CPU requests, ensuring each row is periodically restored.


Why Other Options Are Wrong:

  • Flip-flop references (A and B) are inapplicable; DRAM cells are not flip-flops.
  • Discharges the capacitors (C) is the opposite of the goal—refresh recharges them.


Common Pitfalls:

  • Believing ordinary accesses always cover all rows. Controllers must guarantee refresh across the full array within the retention window.


Final Answer:

The refresh cycle keeps the charge on the capacitor cells.

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