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Digital counters are integrated circuits
(ICs) that count events in computers and other digital
systems. Because they must remember past states, digital
counters include memory. Generally, digital counters consist
of bistable devices or bistable multivibrators called
flip-flops. The number of flip-flops and the way in which
they are connected determines the number of states and the
sequence of states that digital counters complete in each
full cycle. The way in which devices are clocked determines
whether digital counters are categorized as synchronous or
asynchronous. In synchronous devices, one clock triggers all
of the flip-flops simultaneously. With asynchronous or
ripple counters, an external clock pulse triggers only the
first first-flop. Each successive flip-flop is then clocked
by one of the outputs (Q or Q�) of the previous flip-flop.
Some digital counters can operate either synchronously or
asynchronously. Devices can count in an increasing sequence,
a decreasing sequence, or in either increasing or decreasing
sequences.
Several types of digital counters are available. Binary
coded decimal (BCD) counters use a 4-bit code to represent
each digit of a decimal number by its four-bit binary
equivalent. Decade counters are 4-bit truncated counters
that produce a counting sequence from 00002 (0) to 10012
(9). Biquinary counters are decade counters in which the
most significant bit (MSB) is low for five states and then
high for five states. Mod-n counters use a number of
different states. With non-truncated devices, the modulus
(mod) number is given as 2n, where n is the number of stages
(flip-flops). With truncated counters, n is the total number
of different counting states. Other types of digital
counters include Johnson counters and ring counters. Johnson
counters create sequences that resemble shift registers and
restrict the division ratio to twice the number of
flip-flops. Ring counters feature a loop ring of counter
stages in which only one flip-flop is set at any given time.
Selecting digital counters requires an analysis of logic
families. Transistor-transistor logic (TTL) and related
technologies such as Fairchild advanced Schottky TTL (FAST)
use transistors as digital switches. By contrast, emitter
coupled logic (ECL) uses transistors to steer current
through gates that compute logical functions. Another logic
family, complementary metal-oxide semiconductor (CMOS), uses
a combination of p-type and n-type metal-oxide-semiconductor
field effect transistors (MOSFETs) to implement logic gates
and other digital circuits. Logic families for digital
counters include cross-bar switch technology (CBT), Gallium
arsenide (GaAs), integrated injection logic (I2L) and
silicon on sapphire (SOS). Gunning with transceiver logic (GTL)
and gunning with transceiver logic plus (GTLP) are also
available.
Digital counters are available in a variety of IC package
types and with different numbers of pins and flip-flops.
Basic IC package types for digital counters include ball
grid array (BGA), quad flat package (QFP), single in-line
package (SIP), and dual in-line package (DIP). Many
packaging variants are available. For example, BGA variants
include plastic-ball grid array (PBGA) and tape-ball grid
array (TBGA). QFP variants include low-profile quad flat
package (LQFP) and thin quad flat package (TQFP). DIPs are
available in either ceramic (CDIP) or plastic (PDIP). Other
IC package types include small outline package (SOP), thin
small outline package (TSOP), and shrink small outline
package (SSOP). |
