1. IPC-2221 6.1.3.1 Digital Circuits Digital circuits are composed of electronic components that can provide state information (1 or 0), as a function of the performance of the overall circuit. Normally, logic integrated circuits are used to perform this function; however, discrete components may also be used sometimes to provide digital responses.

2. IPC-2221 6.1.3.1 Digital Circuits Integrated circuit devices use a variety of logic families. Each family has its own parameters regarding the speed of the digital transmission, as well as the temperature rise characteristics necessary to provide the performance. In general, a single board usually uses the same logic family in order to facilitate a single set of design rules for conductor length for signal driving restrictions. Some of the more common logic families are:

3. IPC-2221 6.1.3.1 Digital Circuits TTL - Transistor Transistor Logic MOS - Metal Oxide Semiconductor Logic CMOS - Complimentary Metal Oxide Semiconductor Logic ECL - Emitter Coupled Logic GaAs - Gallium Arsenide Logic In certain high-speed applications, specific conductor routing rules may apply. A typical example is serial routing between signal source, loads and terminators. Rating branches (stubs) may also have specified criteria.

4. IPC-2221 6.1.3.1 Digital Circuits Digital signals can be roughly placed in four classes of criticality. These classes are: 1. Non-Critical Signals - are not sensitive to coupling between them. Examples are between the lines of a data bus or between the lines of an address bus where they are sampled long after they are settled. 2. Semi-Critical Signals - are those where coupling must be kept low enough to avoid false triggering, such as reset lines and level triggering strobe lines.

5. IPC-2221 6.1.3.1 Digital Circuits 3. Critical Signals - have waveforms that must be monotonic through the voltage thresholds of the receiving device. These are normally clocking signals and any glitch while the wave form is in transition may cause a double clocking of the circuit. A non-critical signal has a waveform that need not be monotonous and may even make multiple transitions between the voltage thresholds before it settles.

6. IPC-2221 6.1.3.1 Digital Circuits Obviously it must settle before the receiving device acts upon the data, e.g., the data input to a flip-flop may be a non-critical but the clock signal is most probably a critical signal. Asynchronous signals, although they may (or may not) be non-critical signals, should not be mixed with critical signals since there is a real possibility of the asynchronous signals inducing noise on the critical signals during the clock transitions. Clock signals that do not have a common master frequency should also not be routed together for similar reasons.

7. IPC-2221 6.1.3.1 Digital Circuits 4. Super-Critical Signals - are those in applications such as clocks or strobes for A/D and D/A converters, signals in Phase Locked Loops, etc. In these types of applications phase lock jitters and crosstalk, causing errors, noise and timing jitters, will show up in the application's output performance.

8. IPC-2221 6.1.3.1 Digital Circuits It is only a question of the amount of disturbance within the required performance specification. This class of signal is essentially the same as an analog coupling situation. In other words, it is completely linear (the total noise is the sum of the individual noise elements; no averaging of canceling out can be assumed).