Path: EDN Asia >> Design Centre >> IC/Board/Systems Design >> Giving old discretes a try
IC/Board/Systems Design Share print

Giving old discretes a try

11 Apr 2016  | Michael Dunn

Share this page with your friends

Nowadays, most digital design work involve little more than patching together a few highly integrated blocks. It seems as if everything is VLSI (and CPLD/FPGA). A lot is, but not everything. "Discrete" logic ICs are still to be found.

Let's briefly review some historical logic basics. SSI (small-scale integration) referred to ICs containing a few gates or flip-flops. MSI (medium-scale integration) was used for things like counters, comparators, decoders, encoders, and other more esoteric functions. LSI (large-sca... you know the rest) was used to denote devices such as memories, calculator chips, keyboard encoders, and eight-bit microprocessors. I'm not quite sure when VLSI (very large...) came into vogue – perhaps with 16bit processors. (I find the claims in Wikipedia suspect.)

In the discrete days, optimising digital designs often meant finding le chip juste, and sometimes a bit of lateral thinking helped repurpose an IC to a function the designers may never have envisioned. There were a couple of standard series of logic parts:

The 7400 series started out as TTL (transistor-transistor logic; an acronym still widely used to describe logic levels). Over the years, it spawned many sub-series, such as 74L, 74S, 74F, and 74LS. When CMOS processes became fast enough, we started seeing 74HC, 74AC, and so on. Many of these parts are still available, though less common parts in less common families have disappeared. Those who lived through the era can still recite a large number of parts by heart: 7400 is a quad NAND gate, 7404 is a hex inverter, 7432 is a quad OR, 7493 is a four-bit counter, 7474 is a dual flip-flop... OK, I'll stop.

The other common series was metal-gate CMOS – the 4000 series. This is also still available, and though it's slow, it has the sometimes useful attribute of working from a very wide supply range (3-18V). There has also been some cross-fertilisation. One may find a 74C00: 4000 technology in 7400 clothing (pinout), or a 74HC4060: 4000 functionality implemented with the faster 74HC silicon-gate technology.

Of course, the introduction of programmable logic (PLAs and PALs) started a revolution in design. This story is entertainingly told in the award-winning book The Soul of a New Machine. But that's another story.

Where is standard discrete logic still used? I would surmise that the most common parts are 8/16/32bit buffers, transceivers, and registers, and the various forms of "tiny" logic – chips rooted in the 7400 past but containing only one or two gates or flip-flops, and coming in small six- or eight-pin packages. These parts minimise size and let you place logic exactly where you need it.

To illustrate some uses of discrete logic, let me describe some examples from past designs.

The first falls into the "lateral thinking" category. I was updating a small datacom protocol converter. The system address bus was 20 bits wide, but one of the new peripheral chips had a four-channel DMA controller that drove only 16 bits. The problem percolated in my head for a while, and then the answer appeared to me. A 74670. This chip is a four-word by four-bit dual-port register file. Very MSI. I hooked it up so the processor could write to the registers as part of DMA configuration. The outputs drove the top four bits of the address when the DMA controller had the bus. The controller had two outputs indicating the channel in use, which were used to address the register file. Talk about a perfect fit.

The 74670 register file block diagram.

Does the 74670 still exist? Digikey and Jameco think so.

A good example of 4000 series CMOS use is from an audio processor design, ca. 1999. Here, in an otherwise all-analogue box, the low power and slow speed of the CMOS chips was an advantage. The main use of the parts was for volume control. I needed to control four signals, and I deemed the use of "LogDACs" preferable to an expensive, inaccurate, and custom four-section pot. The actual volume control was an incremental encoder, and a flip-flop plus a few gates converted those pulses to clock and up/down signals to drive an eight-bit counter. A few more gates and some RC networks generated the addressing and strobes required to write to the DACs. I guess a $1 PIC could have done the job too, but I find this kind of constraints-limited design challenging and rewarding.

My last example is even more archaic – an analogue music synthesiser module I designed for a high school project. Here we have counters, comparators, latches, multiplexers, a number of gates, and, at the heart, a pair of 2101 SRAMs (that's right – 256×4 each). All this logic & memory implemented a VCO and envelope generator with a programmable waveshape. What fun.

So, the next time you need to brush a pretty bit of logic onto your PCB canvas, give the old discretes a try. There's life in them still.

About the author
Michael Dunn has been messing with electronics almost as long as he's been walking. He got his first scope around age 15, and things have been going downhill ever since. The scopes now vie with wine racks, harpsichords, calculators, and 19th century pianos for space. Over the years, he's designed for the automotive, medical, industrial, communications, and consumer industries, as both freelancer and employee, working with analogue, digital, micros, and software.

Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.

Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Control this smart glass with the blink of an eye
K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."

GlobalFoundries extends grants to Singapore students
ARM, Tencent Games team up to improve mobile gaming

News | Products | Design Features | Regional Roundup | Tech Impact