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Comment by kens | Hacker News Reader

Author here for all your chip questions...

Any particular favorite book or course recommendations for learning about vintage (or current) IC design? I’m not looking to go into the field, just programmer who thinks being able to puzzle out things like die shots is really cool.

I stick to ICs from the 1970s and 1980s; after that, things get too hard to understand. Books from that era provide a reasonable description.

For digital circuits, Mead and Conway's Introduction to VLSI Systems is a good source of information. It's available online: https://ai.eecs.umich.edu/people/conway/VLSI/VLSIText/VLSITe... (the link seems to be down right now, though.)

For analog circuits, the book "The Art of Analog Layout" is good if you can find a cheap used copy. The designer of the 555 timer wrote an interesting book "Designing Analog Chips" which I found very interesting. It is available online: http://www.designinganalogchips.com/

There's also an online tutorial on IC reverse engineering: http://siliconzoo.org/tutorial.html

Thanks so much!

Wow, thanks for the book links. I remember the VLSI book from my uni curriculum.

What kind of chip layout/validation tools and methods would have been used for this chip? The layout does not appear to be as dense or uniform as other chip images that I've seen. Is the large spacing between elements more indicative of a prototype that would have been refined later in production?

The chip was designed in 1971, so layout would have been drawing it on a large piece of paper and then cutting the masks out of sheets of plastic (Rubylith).

The layout looks typical for that era. Keep in mind that this is an analog chip, so resistors take up a lot of space. Also, with just 24 transistors, you don't need to squeeze every bit out of the layout. I've looked at other versions of the 555 timer and the layout isn't much better. Even the CMOS 555 has a lot of wasted space.

Thanks for posting. Love your stuff!

Do you have a layman's explanation for what the 555 timer is used for and how it works?

It's such a simple IC that it's hard to see what it can do. But if you stick some resistors and capacitors around it in just the right ways, it can do so much. Google "555 examples" for thousands of example schematics. Wikipedia has a few examples: https://en.wikipedia.org/wiki/555_timer_IC#Modes

The basic idea of the 555 timer is to generate a pulse of a fixed timing, from microseconds to hours. But it is a very flexible chip so it has hundreds of applications, operating as anything from a timer or latch to a voltage-controlled oscillator or modulator. As a result, the chip was very widely used.

How it works is that when you charge a capacitor through a resistor, the capacitor will charge relatively slowly. The rate depends on the value of the components. What the 555 chip does is monitors the voltage level on the capacitor. When it reaches an upper level, the chip discharges the capacitor. When the voltage reaches a lower level, the chip starts charging the capacitor. Thus you get oscillations with the desired timing.

Think of it kind of like filling your sink with water. When the sink gets 2/3 full, you pull the plug from the drain. When the water level drops to 1/3, you put the plug back in the drain. This will give you periodic oscillations. The timing depends on the size of the sink (capacitor) and how much you open the tap (resistor).

This is a fantastic explanation. Thank you!

How do you get such a wide range of times with such a tiny capacitor? I imagine there is some magic with multiplying this effect across multiple caps?

For a short delay, you use a small resistor and a small capacitor. For a long delay, you use a large resistor and a large capacitor. The result is that it supports "timing from microseconds through hours." The timing resistors and capacitors are external, so you can select whatever values you want for your application.

It's not necessarily tiny. You could use an electrolytic capacitor with a capacity of a few millifarads, for example. However, the tolerance of larger capacitors is larger (usually 20%, and up to 50%) so that quickly becomes expensive. It's more likely that you'll generate a 10 second or less pulse with the 555 and then make it longer using (for example) T flip-flops. A single 744040 ripple counter IC for example can divide the output by up to 4096.

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