Basics Of Integrated Circuits (ICs)

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The basics of integrated circuits Integration is a good thing Integrated circuits (ICs) are, much as their name would suggest, small circuits integrated into a plastic "chip." They provide a handy source of rich functionality in a tiny package. The ICs used in BEAMbots are really fairly simple -- often just consisting of multiple copies of a simple 2- or 3-element circuit, in a small, handy, package. This means that they are simple, easy to learn "circuit blocks," rather than very complex "black boxes".

Think of them as just a natural progression from circuits built from discrete components.

ICs come in a variety of packages (2 are predominant for our uses), and are based on a number of different core technologies (also known as logic families, we'll again be concerned with just 2).

Logic Families For the purposes of BEAMbots, we'll just be concerned with two logic families (IC implementation approaches); note that both have multiple numbering schemes for their "members."

Numbering systems and IC "sub-families" Something I've learned to put up with is the mess that is IC numbering systems. One of the more common numbering systems used for CMOS ICs (most often used in BEAMbots) is borrowed from TTL -- that of 74xxxnnn -- here, "xxx" is AC, ACT, HC, or HCT (subfamily); and "nnn" is the specific chip ID.

So, basically, any given chip will come in a whole mess of variations (one per subfamily); if it's any consolation, we only need to be concerned with 4. For any given chip, you'll need to consider the plusses and minuses of two comparisons -- AC vs HC subfamilies, and AC / HC vs ACT / HCT (essentially, CMOS vs. TTL input levels).

AC vs HC AC / ACT stands for Advanced CMOS Logic (ACL for short). HC / HCT stands for High-speed CMOS Logic (HCL).

The AC and ACT subfamilies are faster than the HC and HCT subfamilies, and draw some more power in some circuits. All chips in the AC* subfamily have lower output resistance than HC* and can sink and source 24 mA at logic levels and up 70 mA (typ) per gate for motor loads. As a result AC* gates can handle more than twice the current of HC* gates (50 - 70 mA vs. 24 mA). Note, though, that while most HC* chips have a 25 mA limit, the HC* driver chips such as the 74HC240 and the 74HC245 (i.e., buffers) can handle 35 mA per device, and a maximum of 75 mA per chip.

The AC & ACT families also draw about twice as much current as the HC & HCT chips (but we speaking here of microamps, so it's usually not a huge deal).

It is occasionally possible to find (high quality) motors that you can drive directly from an HC chip. For intermittent operation, such as you get with a quadcore, you COULD drive very efficient (i.e., very low-current) motors directly. You would definitely need a capacitor (say 0.47 uF) across each motor to keep the noise under control.

CMOS vs. TTL inputs The main difference between AC- and HC-chips vs. ACT- and HCT-chips is that AC / HC chips work with CMOS-level inputs, while ACT / HCT chips work from TTL-level inputs and outputs. AC / HC chips allow a relatively-wide supply-voltage range (2 - 6 V), and works with CMOS levels on input and output (i.e., doesn't like inputs or outputs to be ill-defined, prefers close to 0V for low and Vdd for high). ACT / HCT works only with a supply voltage of 5V +/- 10%, and works with TTL levels on input and output (a valid input is close to 0V for low, and ~2.4 - 5.0 V for high).