🧰 The Glue Logic Toolbox on Digital Logic Notebook

One Inverter

Mon, 01 Jan 0001 00:00:00 +0000

One Inverter#

The problem is small and extremely common: a design needs exactly one inverter — to flip an active-low signal to active-high, correct a polarity, or generate a complement — and nothing else. Dropping a 14-pin hex inverter to use one-sixth of it is wasteful on a modern board.

The Part#

Reach for a single-gate (“tiny logic”) inverter: the 74LVC1G04 in a tiny SOT-23-5 or smaller package, running at low voltage with 5 V-tolerant inputs. On an all-5 V board the classic 74HC04 hex inverter still works if the extra five gates can be spared or reused elsewhere. If the signal is also slow or noisy, use the Schmitt-trigger version (74LVC1G14 / 74HC14) and get debounce-grade edge cleaning in the same part.

Debounce

Mon, 01 Jan 0001 00:00:00 +0000

Debounce#

A mechanical switch or button does not make one clean transition. Its contacts — like the relay contacts that first exposed the problem — bounce for a few milliseconds on every closure, producing a burst of edges that a fast logic input or an interrupt reads as many presses.

The Part#

The classic answer is a 74HC14 hex Schmitt-trigger inverter (or the single-gate 74LVC1G14) with a small RC filter on its input. The resistor and capacitor slow the signal so the bounce is smeared into a gentle ramp; the Schmitt input’s hysteresis then turns that ramp into exactly one clean edge, ignoring the wobble in between.

More Outputs

Mon, 01 Jan 0001 00:00:00 +0000

More Outputs#

A microcontroller or FPGA has run out of pins, but the design still needs to drive many things — a bank of LEDs, seven-segment digits, relays, or status lines. The trick is to trade a few pins and a little time for as many outputs as needed.

The Part#

The workhorse is the 74HC595, an 8-bit serial-in, parallel-out shift register with a latched output. Three signals — data, shift clock, and latch — feed eight bits in serially, and the latch then presents all eight at the outputs at once (so they change cleanly together rather than rippling). Its serial-output pin feeds the next ‘595’s input, so devices daisy-chain: three pins can drive 8, 16, 24, or more outputs, limited mainly by how long the shifting takes.

More Inputs

Mon, 01 Jan 0001 00:00:00 +0000

More Inputs#

The mirror of the output problem: a design needs to read more switches, buttons, or digital sensors than there are pins to spare. The same shift-register trick works in reverse — sample many inputs, then clock them out over a few wires.

The Part#

The go-to is the 74HC165, an 8-bit parallel-in, serial-out shift register. A load pulse captures all eight inputs at once into the register; the controller then clocks those bits out serially on a single data line. Like the 74HC595 it daisy-chains — each ‘165’s serial output feeds the next’s input — so three pins can read 8, 16, or more inputs. The ‘595 for outputs and the ‘165 for inputs are the standard complementary pair.

Switch Analog Signals

Mon, 01 Jan 0001 00:00:00 +0000

Switch Analog Signals#

A logic gate cannot pass an analog signal, and a relay is bulky and slow for routing a small voltage. The need here is to steer an analog signal — an audio line, a sensor voltage, a reference — on and off, or from one path to another, under digital control.

The Part#

The classic answer is the 4066, a quad bilateral (analog) switch. Each switch is a CMOS transmission gate — an NMOS and a PMOS transistor in parallel — that, when its control input is high, conducts in either direction and passes whatever analog voltage sits between the supply rails. When the control is low, the path is open. It is a genuine switch for signals, controlled by a logic level.

One of Eight Sensors

Mon, 01 Jan 0001 00:00:00 +0000

One of Eight Sensors#

A design has one ADC input but eight analog sensors to read (or one signal path that must select among eight sources). Rather than eight converters, route each sensor to the single ADC in turn.

The Part#

The standard part is the 4051, an 8-channel analog multiplexer/demultiplexer. Three address lines select which of the eight channels connects to the common pin, and because it is built from transmission gates it passes analog voltages in either direction — so the same part can gather eight inputs to one ADC or fan one source out to eight destinations. Its relatives cover other groupings: the 4052 is a dual 4-channel mux, the 4053 a triple 2-channel. The 74HC4051 is the faster high-speed-CMOS version.

Level Shifting

Mon, 01 Jan 0001 00:00:00 +0000

Level Shifting#

Modern boards are rarely a single voltage. A 3.3 V microcontroller must talk to a 5 V sensor, a 1.8 V memory, or an I²C bus shared across domains, and connecting mismatched levels directly either fails to register a logic high or over-volts an input. Level shifting moves a signal safely from one voltage domain to another — and the right part depends entirely on the direction and the drive type.

Reset Delay

Mon, 01 Jan 0001 00:00:00 +0000

Reset Delay#

At power-up, a chip must be held in reset until its supply is stable, then released cleanly. Do this wrong and the device wakes into a garbage state, latches up, or resets erratically as the rail sags. The need is a reset that asserts while power is coming up and releases only once the supply is good — ideally with a defined delay.

The Right Answer: a Supervisor IC#

A dedicated reset supervisor (MAX809, MCP100/MCP809, TPS3839, and many others) is the proper part. It watches the supply voltage against a precise threshold, holds reset asserted whenever the rail is below it, and releases after a fixed delay once the rail is good — and, crucially, re-asserts on a brown-out if the supply dips later. Many include a watchdog input as well. This is what production designs use, because it handles the cases a naïve delay cannot.