Transcript IPC-2221 6.1.3.1 Digital Circuits

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.
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:
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.
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.
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.
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.
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.
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).