Introduction / Context:
Dynamic RAMs (DRAMs) achieve very high density per package. One key technique is address multiplexing: using the same physical address pins for both row and column addresses at different times. This requires explicit control strobes to tell the chip when to latch each half of the address. Understanding RAS and CAS clarifies DRAM timing and controller design.
Given Data / Assumptions:
- DRAM core is organized as a 2-D array indexed by row and column.
- Pin count is constrained by package cost/size.
- Timing is managed by external memory controllers asserting RAS/CAS.
Concept / Approach:
To minimize pins, DRAMs present a multiplexed address bus: first the controller drives the row portion and asserts RAS to latch it; then it drives the column portion and asserts CAS to latch it. This splits the full address across time rather than pins, keeping packages compact while accessing any cell in the array.
Step-by-Step Solution:
Controller places RowAddr on address pins and asserts RAS → row is latched.Controller then places ColAddr on the same pins and asserts CAS → column is latched.The intersection cell is selected; read or write proceeds.
Verification / Alternative check:
Examine JEDEC timing: RAS-before-CAS sequences and parameters (tRCD, tRAS) reflect the multiplexed addressing mechanism.
Why Other Options Are Wrong:
the RAS determines operation mode and the CAS enables tristate outputs: Misstates their roles; mode and bus tri-state are handled differently.the RAS latches in the address and the CAS latches in data: CAS latches column address; data transfer is governed by read/write timing, not CAS latching “data.”None of the above: Incorrect because option (a) correctly describes the reason.
Common Pitfalls:
Confusing CAS with data latch rather than column address latch.Ignoring that address multiplexing is primarily for pin-count reduction.
Final Answer:
the address lines are multiplexed to reduce pin count
Discussion & Comments