74HC165 parallel-in shift register

The 74HC595 turned around. The 595 gives you eight outputs from three pins; the 165 gives you eight inputs on three pins.

Eight switches go on D0 to D7, one pulse grabs them all at once, and then they come out of Q7 one at a time as you clock. Aliases: 74165, SN74HC165, parallel-in serial-out, PISO, input shift register.

Pins

Pin	Name	What it does
1	PL	Parallel load, active low. While low, the register copies D0 to D7.
2	CP	Clock. A rising edge shifts the register one place towards Q7.
11 to 14, 3 to 6	D0 to D7	The eight parallel inputs.
10	DS	Serial in: what shifts into stage 0. Chain it to the previous chip's Q7.
9	Q7	Serial out: always shows stage 7.
7	Q7N	The complement of Q7.
15	CE	Clock enable, active low. High stops the clock.
8	GND	Ground.
16	VCC	Supply, 2 V to 6 V.

Truth table

PL	CP	CE	What happens
0	x	x	Load D0 to D7 (continuously, while PL is low)
1	1	x	Hold
1	x	1	Hold
1	0	rising	Shift (CE acts as the clock)
1	rising	0	Shift

After a load, Q7 is already showing D7. Seven more clocks bring out D6 to D0. That is exactly what Arduino's shiftIn(dataPin, clockPin, MSBFIRST) expects, because it reads before each clock:

digitalWrite(PL, LOW);    // grab the switches
digitalWrite(PL, HIGH);
byte switches = shiftIn(Q7, CP, MSBFIRST);

Properties

None.

What the model gets right

The load is asynchronous and continuous, as on the real chip: hold PL low and Q7 follows D7 live, and the clock does nothing. The clock and CE go through one OR gate inside the chip, so a rising CE with CP low shifts too; the model does the same. Propagation delays are the SN74HC165 data sheet's typicals at 4.5 V: 15 ns from the clock to Q7, 20 ns from PL. A clock edge out of a floating or unknown level is not trusted, and leaves the register unknown rather than guessing.

What it does not model

No setup, hold or minimum pulse-width checks. One delay for each path, with no spread with supply voltage or load. The register powers up cleared, where a real one powers up random (which does not matter, because you load it before you read it).

Common mistakes

Forgetting to pull PL low before reading, so the same old byte shifts out. Leaving CE floating (tie it to GND). Reading after the clock instead of before it, which loses D7 and reads a bit from DS at the end.

See it in action

Switch shifter loads an 8-way DIP switch into a 165 and clocks the byte round and round past an LED. Open it at /templates.