74HC595
The 74HC595 is an 8-bit serial-in, parallel-out shift register with a storage register and tri-state outputs. It lets a Pico control more output pins while using only a few GPIO signals.
Part images
What it is
The 74HC595 is an 8-bit serial-in, parallel-out shift register with a storage register and tri-state outputs. It lets a Pico control more output pins while using only a few GPIO signals.
How students use it
Students use it when one Pico pin at a time is not enough, such as driving rows of LEDs, a 7-segment display, a 4-digit display, or an LED matrix through a few control lines.
Pins and power
DS serial data, SHcp shift-register clock, STcp storage/latch clock, MR active-low reset, OE active-low output enable, Q0-Q7 parallel outputs, Q7' serial output, VCC, GND.
Use a logic supply compatible with the circuit. With Pico projects, keep the logic side 3.3V-safe unless a level-shifting design is intentionally used.
SunFounder notes that data shifts on the rising edge of SHcp and transfers to the storage register on the rising edge of STcp. OE must be low for outputs to drive the bus, and MR must be high to avoid resetting the register.
Voltage and safety
Do not power the chip at 5V and connect its outputs or inputs directly to Pico GPIO unless the circuit has been checked for 3.3V compatibility.
The chip expands outputs; it does not increase safe current. Respect per-pin and total current limits.
Module internals
Main component: 74HC595 8-bit shift register / storage register IC.
A useful 74HC595 output circuit usually adds current-limiting resistors for LEDs and may need transistor drivers when the load current is too high for the chip.
Datasheet notes
The TI CD74HC595 datasheet covers supply range, input thresholds, output current limits, timing, and the exact pinout. Check current limits before trying to drive many LEDs directly.
Common libraries
No special MicroPython library is required. Students usually use machine.Pin plus bit-banging for DS/SHcp/STcp, or SPI-style output for more advanced projects.
Common mistakes
Leaving OE floating, pulling MR low by accident, swapping SHcp and STcp, forgetting current-limiting resistors for LEDs, and expecting Q0-Q7 to update before the latch clock.