Designing a Low-Power PLA for Battery-Operated Projects

If you’ve ever tried to squeeze weeks of run time out of a tiny coin cell, you know that every milliamp counts. In the age of IoT gadgets that sit on a shelf or a wrist for months without a charge, a low‑power programmable logic array (PLA) can be the quiet hero that makes the difference between “just works” and “needs a new battery every week.” In today’s post for PAL Flash Logic I’ll walk you through a practical way to build a PLA that drinks as little power as possible, using simple steps you can apply to any battery‑operated project.

Why a PLA, and Why Low Power?

A PLA is a small, configurable logic block that can implement any sum‑of‑products function you need. It sits between a microcontroller and the world, handling fast, deterministic decisions without waking the CPU. That alone can save power, but the PLA itself can become a hidden drain if you’re not careful. Flash‑based PLAs are especially attractive for battery projects because they retain their configuration without power, so you never have to keep a configuration flash alive.

Step 1: Define the Real Logic, Not the Guesswork

Start by writing down exactly what you need the PLA to do. In my recent garden‑moisture sensor, the logic was simple:

If soil is dry and battery voltage is above 2.8 V, turn on the pump.
If the pump has been on for more than 30 seconds, turn it off.
If the battery drops below 2.5 V, shut everything down.

Write these as Boolean equations. Keep them as small as possible; each extra term adds a product term in the PLA and more switching activity. Use Karnaugh maps or a quick online minimizer to reduce the expressions. The fewer product terms, the fewer internal nodes will toggle, and the lower the dynamic power.

Step 2: Choose a Flash‑Based PLA with Power‑Saving Features

Not all PLAs are created equal. Look for devices that:

Use flash memory cells for configuration (no SRAM that needs a hold voltage).
Offer high‑threshold (high‑Vt) transistors for non‑critical paths.
Provide built‑in clock‑gating or enable pins.

A good example is the Microchip ATF1504AS. It stores its logic in flash, has a separate power‑down pin, and lets you disable unused rows. The datasheet even lists a “low‑power mode” where the internal oscillator is turned off. Selecting a part with these features saves you from adding external power‑gating circuitry later.

Step 3: Power‑Gate the PLA When It’s Not Needed

Even a low‑power PLA can waste energy if it sits idle with its inputs toggling. Use an enable pin (often called OE or EN) to turn the whole block off when the logic is not required. In the garden sensor, the PLA only needs to be active when the moisture sensor changes state or when the battery voltage is being monitored. Connect the enable pin to a simple comparator that watches the battery level; when the voltage falls below 2.5 V, the enable goes low and the PLA powers down.

Step 4: Reduce Switching Activity on the Inputs

Dynamic power is proportional to the number of transitions on the inputs. Add small RC low‑pass filters or Schmitt triggers to clean up noisy sensor signals. For the moisture probe, a 10 kΩ pull‑down together with a 0.1 µF capacitor smooths out the rapid spikes caused by water droplets, so the PLA sees a clean high or low instead of a jittery waveform.

Step 5: Use the Right Supply Voltage

Flash‑based PLAs often support a range of supply voltages, typically 2.7 V to 5.5 V. Running at the lower end reduces both static and dynamic power. In my garden project I ran the PLA at 3.0 V, which is comfortably above the minimum for reliable operation but still low enough to keep the current draw under 10 µA in standby.

Step 6: Simulate Power Consumption Early

Before you solder the board, run a quick power analysis in your favorite simulator (LTspice, PSpice, or even the free KiCad simulator). Set up a testbench that toggles the inputs at the expected rates and watch the average current. If you see more than a few microamps in idle, go back and check for stray pull‑ups or unnecessary clock signals.

Step 7: Layout for Low Power

When you move to PCB design, keep the PLA’s power pins short and close to the decoupling capacitor. A 0.1 µF ceramic right next to the VCC pin can shave off a lot of noise‑induced switching. Also, route the enable line away from high‑frequency traces; a clean enable signal prevents accidental wake‑ups.

Personal Anecdote: The Day My Battery Died at Noon

I remember the first time I tried a PLA without any power‑gating. The garden sensor worked great for a few hours, then the battery died right before lunch. I was standing in the yard, holding a half‑filled water can, wondering why the pump never turned on again. A quick look at the current trace showed the PLA was still toggling its inputs even when the pump was off. Adding the enable pin and a simple voltage comparator solved the problem in under an hour. Now the same sensor runs for three weeks on a single CR2032 coin cell. That little fix taught me that a PLA is only as low‑power as the way you control it.

Quick Checklist for a Low‑Power PLA Design

Minimize logic terms – fewer product terms, less switching.
Pick a flash‑based device – no SRAM hold current.
Use enable/disable pins – power‑gate when idle.
Filter noisy inputs – reduce unnecessary transitions.
Run at the lowest safe voltage – lower static draw.
Simulate early – catch hidden currents before hardware.
Place decoupling close – keep supply clean and stable.

Follow these steps and you’ll have a PLA that behaves like a well‑trained cat: it only moves when it needs to, and otherwise stays still, conserving every drop of juice.

Happy designing, and may your batteries last as long as your ideas!