RAM Address Decoder Techniques: Comparison of Multiplexer, PLA, and Logic Gate Approaches
An address decoder maps a binary address bus to a single active select line that enables a specific memory cell or block. For RAMs and other memory-mapped peripherals, decoders must be fast, area-efficient, and reliable. This article compares three common implementation techniques—multiplexer-based decoders, programmable logic array (PLA) decoders, and pure logic-gate decoders—analyzing their architectures, pros/cons, performance, and suitable use cases.
How an address decoder works (brief)
An n-to-2^n decoder asserts one of 2^n outputs corresponding to an n-bit input. For RAM, decoders can be built per-chip (chip select generation) or per-row/column inside a memory array. Key metrics: propagation delay, fan-out, silicon area, power consumption, scalability, and ease of routing.
Comparison overview
| Technique | Basic idea | Strengths | Weaknesses | Typical use cases |
|---|---|---|---|---|
| Multiplexer-based | Use multiplexers to route one of many enable signals or to select outputs tied to address patterns | Simple to build from standard MUX primitives; compact for small decodes; predictable delay scaling | MUX depth grows with address width; area and power increase for large n; extra routing for distributed enables | Small decoders, FPGA LUT-mapped designs, microcontroller peripherals |
| PLA (programmable logic array) | Use an array of AND terms (product terms) feeding an OR plane to implement arbitrary minterms of addresses | Highly flexible; can encode complex address ranges and don’t-care conditions efficiently; good for sparse or irregular maps | PLA size (product terms) can explode for dense full decodes; fixed-plane PLAs require careful optimization; slower due to two-level logic fan-in | Address maps with irregular ranges, glue logic combining address and control signals, ASIC blocks with available PLA resources |
| Logic-gate (tree of gates) | Implement decoder as structured gates (NAND/NOR trees, CMOS transmission gates) directly implementing binary decoding logic | Can be optimized for speed (balanced trees), area (shared terms), and low-power; regular structure suits memory-array row/column decoders | Manual design effort increases with complexity; routing congestion for large arrays; less flexible than PLA for irregular patterns | High-performance SRAM row/column decoders, custom ASIC memory macros, timing-critical paths |
Detailed technique descriptions
Multiplexer-based decoders
- Implementation: Use hierarchical multiplexers to select among 2^n inputs or to build one-hot selects by combining MUX outputs with simple logic. In FPGAs, decoders are often synthesized by chaining LUT-based multiplexers.
- Performance: Delay scales roughly with log base-2 of the number of inputs per MUX stage if implemented hierarchically; each MUX stage adds delay.
- Area/power: Moderate for small n; for larger n the number of transistors and switching activity increases, raising dynamic power.
- Design tips:
- Keep MUX fan-in small and use balanced trees.
- Exploit FPGA LUT sizes (e.g., pack 4:1 MUXes into LUTs).
- For partial decodes (address ranges), use smaller multiplexers combined with simple equality checks.
PLA-based decoders
- Implementation: Inputs (and their complements) feed an AND-plane producing product terms for selected address bit combinations; these terms feed an OR-plane to produce one-hot outputs. Can include additional control signals (chip enable, read/write) as inputs to reduce output count.
- Performance: Two-level logic gives minimal literal depth (AND then OR), often resulting in low logical depth but potentially high fan-in on ORs.
- Area/power: Efficient when address space is sparse or when many outputs share product terms. For full dense decodes, product-term count = number of outputs × terms per output can be large.
- Design tips:
- Minimize product terms via Karnaugh maps or logic minimization tools.
- Use don’t-care conditions to reduce terms.
- Share product terms across outputs when possible.
- In ASICs, implement PLAs as logic macros or use dedicated PLA blocks in standard-cell libraries if available.
Logic-gate (tree/structured) decoders
- Implementation: Build standard n-to-2^n decoders via cascaded NAND/NOR/inverter structures or transmission-gate pass networks (useful inside memory arrays). For example, a 3-to-8 decoder can be built from three-input NANDs with proper inversion.
- Performance: Can be optimized for minimal delay by balancing gate loads and using buffered stages; local predecode stages are common (split address into groups, predecode to smaller signals, then final decode) to reduce fan-in and capacitive load.
- Area/power: Predecode reduces final-stage complexity and improves speed at cost of extra area. Transmission-gate implementations inside memories minimize voltage swing and area.
- Design tips:
- Use hierarchical predecoding: split high-order and low-order bits to create smaller local decoders.
- Buffer outputs to drive large wordline capacitances.
- Consider clocked or dynamic techniques for very high-speed SRAM decoders (wordline drivers with boosted voltages).
When to choose each technique
- Multiplexer-based: Choose when using FPGA/LUT fabrics or when decode sizes are small-to-moderate and simplicity is valued. Good for quickly mapping address ranges in glue logic.
- PLA-based: Choose when address maps are irregular, sparse, or involve a mixture of address and control conditions that benefit from shared product terms. Use when a two-level minimized solution yields fewer gates than an equivalent full decode.
- Logic-gate/tree-based: Choose for high-performance memory macro designs, custom ASICs requiring precise timing, or when building internal decoders in RAM arrays where transmission gates and predecode buffers are standard.
Practical examples and sizing guidelines
- Small microcontroller peripheral decode (e.g., <16 regions): Use MUX-based or small gate decoder—lowest effort and fits well in FPGA LUTs.
- Irregular memory map with many sparse regions: Use PLA or logic minimization to reduce total gates and power.
- 32-bit wide SRAM row decoder with thousands of rows: Use hierarchical predecode + gate-based final stage with buffered outputs and wordline drivers; consider dynamic techniques for ultra-high frequency.
Optimization checklist
- Predecode high-order bits to reduce fan-in.
- Share product terms or common subexpressions to save area (PLA advantage).
- Buffer outputs to drive large loads (wordlines).
- Use don’t-care conditions to simplify logic.
- Balance tree depths to minimize critical path delay.
- In FPGAs, map to LUT-friendly primitives (4:1 or 6:1 MUXes).
- Simulate switching activity to estimate dynamic power; consider gating unused regions.
Conclusion
Multiplexer, PLA, and pure logic-gate decoders each offer trade-offs between flexibility, speed, area, and power. For small or FPGA-based decodes, multiplexers are simple and effective. PLAs shine for irregular maps and when shared product terms reduce complexity. Logic-gate implementations—especially with hierarchical predecode—are the go-to for high-performance memory arrays and custom ASICs. Choose based on address density, performance targets, and implementation technology.