74HC595 serial-to-parallel chip

22 September 2021
The Texas Instruments SN74HC595 is a chip that I have experimented with in the past but so far had not used in anything other than that experimental circuit, and the previous article did not include a few critical details. This article is an update that includes these details, presented here so that I do not need to go trawling through data-sheets to find them, which in turn was spun-out from one of the many projects that got put on the back-burner due to the current pandemic.

Demonstratiomn circuit

Chip overview

The 74HC595 serial-to-parallel chip is summerised in the flow diagram below. On the rising edge of the clock (not shown) a bit is shifted in from Serial In, shifting the existing bits up one. On the rising edge of Load, bits are copied from the shift register into the output buffer. If Enable is asserted the output buffer is output to the parallel output pins, and if it is deasserted the parallel output pins are set to high-impedence. Note that if the clock and Load pins are tied together the output buffer receives the pre-shift values, so the output is always one clock cycle behind the shift buffer.

Data-flow summary

Chip pin-out

The pin-out for the SN74HC595N which is the DIP variant of the chip is summerised below; all the other 16-pin variants such as SOIC and TSSOP use the same pin-out. Unlike a lot of other integrated circuits most of the output pins are contiguous and on the same side of the chip. I have also come across the CD74HC595 which from a glance over the data-sheet has the same pinning.

V_cc	Q_a	Serial In	Enable	Load Clock	Serial Clock	Serial Clear	Overflow
16	15	14	13	12	11	10	9
SN74HC595
1	2	3	4	5	6	7	8
Q_b	Q_c	Q_d	Q_e	Q_f	Q_g	Q_h	Ground

Note that the pin names given here differ to those given in the data-sheet, and these are detailed in the table below.

Pin	Description
Serial In	New lowest-bit to to shift in
Serial Clock	Rising-edge shift trigger
Serial Clear	Active-low shift register clear
Load Clock	Rising-edge storage trigger
Q_a-h	Parallel data out
Enable	Active-low output enable
Overflow	MSB of shift buffer
V_cc	Power supply
Ground	Power supply

Remarks

The previous article of almost two years ago omitted some important chip details, which resulted in me having to trawl through the data-sheets rather than just looking back at the article. Having done this for a project earlier in the year I wrote up the details, but since that project has effectively been abandoned I forgot about the write-up and as a result ended up trawling through the data-sheets for a third time. For ease of finding in the future I decided to give these details their own mini-article. The circumstances of the past 18 months has left many projects in various stages of incompleteness, and as a result have lost track of a lot of information.