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BEAM From the Ground Up is a BEAM Reference Library site.

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."

TTL -- Transistor / Transistor Logic

TTL chips require a fairly narrow range of supply voltage -- 5 volts, +/- 0.5V.

For TTL circuits, a logic "1" is usually defined as a signal of about 2.4 V, while logic "0" is about 0.5 V above ground; any signal between those values is undefined. Also, note that all unused inputs should be tied to ground.

Bipolar TTL logic comes in many flavors (usually numbered in the "74XX" format, occasionally in "54XX" format) including 74LS/ALS types. These all require 1000s of times more power than TTL-compatible CMOS logic (like the 74HCT type, see below). Note that the 74HCT* ICs have the same logic thresholds as other TTL chips, allowing them used in a mixed circuits of TTL and HCT logic.

While we won't use TTL logic in BEAM projects (as a general rule, it's too power-hungry), for completeness, here are some TTL "subfamilies":

74LXX -- low-power TTL (1/10 the speed, 1/10 the power of "regular" TTL)

74HXX -- high-speed TTL (twice as fast, twice as much power)

74SXX -- Schottky TTL (for high-frequency uses)

74LSXX -- combination of low-power & Schottky, same speed as regular TTL, but at 1/5 the power consumption

CMOS -- Complimentary Metal Oxide Silicon

CMOS ICs use much less power than TTL, plus they are fairly forgiving of "slop" in input voltage -- they're happy with anything between 3 (some, like 1381s, go lower) and 12 volts. CMOS, though, is much more susceptible to damage from static electricity (so get that grounding strap out!).

CMOS comes in two flavors, with corresponding numbering schemes: "CD40XX" metal gate CMOS, and "74CXX" silicon gate CMOS (note that this second subfamily borrows its numbering scheme from TTL ICs).

Metal gate CMOS (CD40XX)has a rated working voltage of 3 - 15 V but can be used down to 2V.

Silicon gate CMOS (74CXX) logic has a working voltage range of 2 -6V but can be used to less than 1V. For microwatt power applications you want to use the lowest possible voltage.


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).

For more information...

Craig Maynard has a nice tutorial on logic families here.

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